Field Book of
Common Rocks
and Minerals

For identifying the Rocks and Minerals of the United States and interpreting their Origins and Meanings

By
Frederic Brewster Loomis
Late Professor of Mineralogy and Geology
in Amherst College

With 47 Colored Specimens and over 100 other Illustrations from Photographs by W. E. Corbin and drawings by the Author

G. P. Putnam’s Sons
New York and London

FIELD BOOK
OF
COMMON ROCKS AND MINERALS

Copyright, 1923, 1948
by
Frederick Brewster Loomis

Twenty-sixth Impression
Revised 1948

All rights reserved. This book, or parts thereof, must not be reproduced in any form without permission.

Made in the United States of America

Dedicated
TO
MY MOTHER
WHO ENCOURAGED ME WHILE A BOY TO GATHER MINERALS, ROCKS AND FOSSILS.

PREFACE

Everyone, who is alert as he wanders about this world, wants to know what he is seeing and what it is all about. Here and there with the aid of capable guides a few have been introduced into the sphere of that wide and fascinating knowledge of Nature which has been so rapidly accumulated during this and the latter part of the last century. It is a full treasure house constantly being enriched, but unfortunately the few who have been initiated have soon acquired a technical language and habit, so that their knowledge and new acquisitions are communicated to but few. The public at large, not having the language nor an interpreter at hand, has come almost at once to a barrier which few have the time or patience to surmount.

Latterly it has become clear that the largest progress cannot be made if the knowledge of any branch of Science is confined to a few only. The most rapid advances have been made where many men are interested and enthusiastic. In no science should there be a difficult barrier between the amateur and the professional student. All Nature is equally open for everyone to study, and there should never be created obstacles as by the use of terminology not easily acquired by anyone. Of late these barriers have been in part broken down and competent students have written guides which anyone can follow, and soon begin to know the plants, trees, birds, insects, etc. So far no one has attempted to make the study of minerals and rocks so direct and simple that everyone can get a start. Most books on minerals, and practically all those on rocks are written for school courses, and to say the least chill any enthusiasm which is naturally aroused by the finding of interesting looking rocks or minerals.

The purpose of this book is first of all to provide a means of identifying minerals and rocks by such methods as are practical without elaborate equipment or previous training: and second to suggest the conditions under which the various minerals and rocks were formed, so that, at the first contact, one may get a conception of the events which have anteceded the mineral or rock which has been found. For this purpose keys have been worked out for determining the rocks and minerals by such obvious features as color, hardness, etc. Each mineral or rock is introduced by a summary of its characters, then the features by which it may be distinguished from any other similar mineral are given, after which its mode of origin and its meanings are considered. For those interested in the composition of the minerals, it is given in chemical symbols with each mineral. Most classifications of minerals are based on the composition, all the sulphides, carbonates, etc., being grouped together, but in this book, because the popular interest and commercial uses are primarily in the metal present, the minerals are grouped in each case about the chief metal, all the minerals of iron being grouped together, for instance.

A few minerals and rocks which are not strictly common have been included such as gems and meteorites; the gems because they are of intense interest to their owners and are often simply perfect examples of a fairly common mineral; and such forms as meteorites because it is important that, if one should run across one, it should be recognized, and so not lost to the world.

The book is freely illustrated, those minerals in which color is important for identification being illustrated in colors, and those which are black, or in which the color is not a determining factor, are shown in either photographic or outline figures.

In the introductory chapter there are explanations of the terms used in describing minerals, and of the systems in which they are grouped. A knowledge of the systems may not be a necessity, but it is a great help in determining minerals, and is very important in understanding why the individual minerals take the varied forms which are characteristic of them. These systems will be better understood after a few minerals have been gathered and examined.

It is hoped the book will help those who have already some knowledge of rocks and minerals, and especially that it will tempt many to begin an acquaintance with the rocks and minerals which are all about them, and are the foundation on which our material progress is built. Rocks and minerals have some advantages over most objects which are collected in that they neither require special preparation before they can be kept, nor do they deteriorate with time.

The author will appreciate corrections or suggestions as to better presentation of the material in this book.

F. B. L.

Amherst, Mass.

CONTENTS

PAGE [Preface] vii CHAPTER [I.—An Introduction] 3 [II.—On the Forms and Properties of Minerals] 10 [III.—The Minerals] 25 [IV.—The Rocks] 170 [V.—Miscellaneous Rocks] 248 [ Bibliography] 270 [ Index] 273

LIST OF PLATES
(AT END OF BOOK)

PAGE [Tourmaline crystals, growing amid feldspar crystals in a cavity in granite, from Paris, Me.] 279 [Plate 1.—Basal forms of the isometric system] 311 [Plate 2.—Basal forms of the tetragonal system. Basal forms of the orthorhombic system] 312 [Plate 3.—Basal forms of the monoclinic system. A cross section of the prism with its edges beveled so that a six-sided prism is formed (pseudo-hexagonal). Basal form of the triclinic system.] 313 [Plate 4.—Basal forms of the hexagonal system] 314 [Plate 5.—Gold in quartz from California (in color)] 280 [Plate 6.—Native silver in calcite. Argentite, the black masses throughout the white quartz (in color)] 281 [Plate 7.—Pyrargyrite as it appears after moderate exposure to the light; streak at left. Crystal form of pyrargyrite. Prousite as it appears after moderate exposure to the light; streak at left (in color)] 282 [Plate 8.—Native copper from Michigan. Chalcopyrite in tetrahedrons and an occasional octahedron; streak to the left (in color)] 283 [Plate 9.—Chalcocite crystals with the bluish tarnish. Tetrahedrite crystals; streak to left (in color)] 284 [Plate 10.—Tetrahedrons showing characteristic manner in which tetrahedrite occurs. A cube with the edges beveled and the corners cut in a form characteristic of cuprite] 315 [Plate 11.—Cuprite, the red crystals showing characteristic color, others showing the green tarnish of malachite. Malachite (green) and azurite (blue), the two minerals shown together as they very commonly occur (in color)] 285 [Plate 12.—Limonite. The crystal form in which goethite is found (in color)] 286 [Plate 13.—Hematite. Clinton iron ore, oolitic. Siderite crystals (in color)] 287 [Plate 14.—Crystal forms of hematite. A typical crystal of magnetite. The rhombohedron typical of siderite] 317 [Plate 15.—Pyrite crystals. Marcasite in concretionary form with radiate structure (in color)] 288 [Plate 16.—The pyritohedron. The pyritohedron with certain of its edges beveled by the cube faces, to show the relationship of these two forms] 318 [Plate 17.—Galena in crystals. Pyromorphite crystals (Green) (in color)] 289 [Plate 18.—Typical forms for cerrusite. Forms in which anglesite occurs] 319 [Plate 19.—Sphalerite, some the normal yellow and some crystals with the reddish tinge. (White is dolomite.) Zincite, streak to the left (in color)] 290 [Plate 20.—A characteristic form in which sphalerite may occur. Characteristic form for zincite crystals. Typical form of crystal of willemite] 320 [Plate 21.—Smithsonite in yellow crystals. Franklinite in octahedral crystals, streak to left (in color)] 291 [Plate 22.—Moss agates, showing the dendritic growth of manganitic minerals, like manganite or pyrolusite. Crystal form of manganite] 321 [Plate 23.—Crystals of green corundum in syenite, from Montana. Typical crystal forms of corundum] 322 [Plate 24.—Arsenopyrite, showing crystals massed so as to be incompletely developed. Realgar as it usually occurs in powdery incrustations (in color)] 292 [Plate 25.—Large crystals of stibnite; the light colored face is the one parallel to which cleavage occurs. Niccolite is a vein in slate (in color)] 293 [Plate 26.—Cobaltite, silver color, with pink tinge. Smaltite, pink is cobalt bloom (in color)] 294 [Plate 27.—Carnotite from Southwest Colorado. Cinnabar (in color)] 295 [Plate 28.—Cassiterite, twinned crystals. The crystal form in which both cassiterite and rutile occur when in simple crystals. Multiple twinning characteristic of rutile] 323 [Plate 29.—Crystal of spinel. Crystal forms in which dolomite occurs] 324 [Plate 30.—Two intergrowing or twinned quartz crystals. Diagram of the typical quartz crystal. A quartz crystal on which the left hand rhombohedron is represented by small faces, while the right hand rhombohedron has large faces] 316 [Plate 31.—Amethyst, not however deep enough colored for gems. Jasper, with botryoidal surface (in color)] 296 [Plate 32.—Banded agate from Brazil (in color)] 297 [Plate 33.—Common opal from Arizona. Siliceous sinter or geyserite from Yellowstone Park (in color)] 298 [Plate 34.—Orthoclase, a cleavage piece. Crystal forms of orthoclase. Diagram of a multiple twin of a plagioclase feldspar] 325 [Plate 35.—A group of microcline crystals from Pike’s Peak, Colo. Labradorite, showing multiple twinning (the striation) and the iridescent play of colors (in color)] 299 [Plate 36.—Crystal form of a pyroxene. Cross sections of a pyroxene crystal showing the lines of intersection of two cleavage planes. Cross sections of pyroxenes, showing typical forms taken by crystals. Augite crystals, in crystalline limestone (in color)] 300 [Plate 37.—Diagrams of amphibole crystals. Tremolite in silky fibrous crystals, asbestos. Hornblende crystals in quartzite] 326 [Plate 38.—The dodecahedron and the 24-sided figure characteristic of garnets. The garnet, grossularite. The garnet, alamandite (in color)] 301 [Plate 39.—Beryl of gem quality. Zircon in syenite (in color)] 302 [Plate 40.—Cyanite crystals in schist. A crystal of mica, showing basal cleavage (in color)] 303 [Plate 41.—Crystal form typical of topaz. A topaz crystal from Brazil. Crystal form typical of staurolite when simple. A typical twin of staurolite (in color)] 304 [Plate 42.—Epidote crystals. Typical forms of epidote crystals. Typical forms of tourmaline] 327 [Plate 43.—Serpentine. Chlorite (in color)] 305 [Plate 44.—The typical form of analcite. A typical natrolite crystal. The typical crystal form of stilbite. A sheaf-like bundle of fibrous crystals, typical of stilbite] 329 [Plate 45.—A group of calcite crystals. Typical forms of calcite] 330 [Plate 46.—Typical forms of aragonite. Typical form of the anhydrite crystal] 331 [Plate 47.—A piece of gypsum looking on the surface of the perfect cleavage, and showing the two other cleavages as lines, intersecting at 66°. Twinning is also shown. A simple crystal of gypsum. Twin crystals of gypsum.] 332 [Plate 48.—A group of barite crystals. Outline of the typical tabular barite crystal. The six-sided double pyramid, composed of three interpenetrating crystals, typical of witherite and strontianite] 328 [Plate 49.—Apatite crystals in crystalline calcite. The ends of apatite crystals showing common modes of termination (in color)] 306 [Plate 50.—A group of fluorite crystals. A group of halite crystals (in color)] 307 [Plate 51.—Sulphur crystals. Ice crystals, the top one, the end of a hexagonal prism; the two lower figures multiple twins as in snow flakes] 333 [Plate 52.—The Devil’s Tower, Wyoming, an example of igneous rock with columnar structure, and resting on sedimentary rocks] 334 Courtesy of the U. S. Geological Survey [Plate 53.—A coarse granite. Graphic granite] 335 [Plate 54.—Syenite. Gabbro] 336 [Plate 55.—Basalt-porphyry. The large white crystals are phenocrysts of plagioclase feldspar. Basalt-obsidian] 337 [Plate 56.—Amgydoloid] 338 [Plate 57.—The north face of Scott’s Bluff, Neb., showing sedimentary sandstones above and clays below. The type of erosion is characteristic of arid regions] 339 Courtesy of the U. S. Geological Survey [Plate 58.—Breccia. Conglomerate] 340 [Plate 59.—Calcareous shale. Coquina] 341 [Plate 60.—Foramenifera from chalk; enlarged about 25 diameters. Encrinal limestone; fragments of the stems, arms and body of crinoids] 342 [Plate 61.—Amber. Two bottles of petroleum, the left hand one with a paraffin base, the right hand one with an asphalt base (in color)] 308 [Plate 62.—Diatomaceous earth magnified 50 times. Two diatoms from the above enlarged 250 times] 343 After Gravelle, by the courtesy of Natural History [Plate 63.—A metamorphic rock, showing the contortion of layers due to expansion under heat] 344 [Plate 64.—A conglomerate partly metamorphosed to a gneiss. A typical gneiss] 345 [Plate 65.—Mica schist, with garnets. Chlorite schist (in color)] 309 [Plate 66.—Phyllite. A white marble, with black streaks due to graphite] 346 [Plate 67.—Serpentine composed of serpentite, hematite, and some calcite (in color)] 310 [Plate 68.—Claystones, simple and compound. A lime concretion, which on splitting disclosed a fern leaf of the age of the coal measures] 347 [Plate 69.—A septeria from Seneca Lake, N. Y. Pisolite from Nevada] 348 [Plate 70.—A geode filled with quartz crystals] 349 [Plate 71.—A quartz pebble from the bed of a New England brook. A pebble of schist and granite from the foot of Mt. Toby, Mass.] 350 [Plate 72.—An iron-nickel meteorite, of 23 lbs., which fell in Claiborne Co., Tenn. An etched slice of an iron meteorite which fell in Reed City, Osceola, Co., Mich.] 351 [Plate 73.—A stone meteor, about natural size, which fell in 1875 in Iowa Co., Iowa] 352

FIELD BOOK OF
COMMON ROCKS AND MINERALS

CHAPTER I
AN INTRODUCTION

Why

Why should one be interested in rocks and minerals? Because the whole world is made of rocks and minerals. They are the foundations on which we build. From them we draw all our metals, and the extent to which we utilize our minerals is a measure of the advance of our civilization. Fragments of rock are the soil from which, by way of the plants, we draw our food, and ultimately our life. The rocks make wild or gentle scenery, one at least of the sources of pleasure. Knowledge of rocks and minerals is then knowledge of fundamentals, of ultimate sources. Between finding the raw materials and their present uses there are usually many steps (so many that we forget that the beginning and end are united), as for instance in your watch. It is made of gold, brass, steel, agate, glass, and perhaps has luminous radium paint on the hands. It is a long way from finding and mining gold, chalcopyrite, hematite, carnotite, etc., through the raw materials, gold, copper, iron, etc., to the finished watch, but the minerals are the foundations of the watch; and it took centuries to find them and learn one by one how to use them, from the gold 10,000 years ago down to the radium within the last fifty years. Then too there is joy in going out into Nature’s wild and raw places, joy in being on the foundations of the earth, joy in the scenery, in the beauty of the minerals themselves.

But why collect the rocks and minerals? First because this is the way to know them. Both mineral and rocks require careful examination in order to see all those fine points by which they are distinguished. It is often necessary to compare one with another to get in mind the differences of form, color, streak, though with increasing familiarity these characteristics are recognized at first sight. It is the repeated examination which makes a rock tell the story of the country from which it came. Our first attempts to read the story give us only the most general facts. Nature’s book, written in the rocks, has to be read closely, often between the lines. Until we are used to the characters in which the words are written, we read slowly. When they look at Nature’s book, always open, most people do not read; for they do not know their letters. Every mineral is a letter, every rock a word, and we learn to read as we learn the minerals and rocks, and every time we go over them we get more facts coming out. The place where a rock or mineral occurs is of course the relation between them, and is involved in reading the story. No one today is a perfect reader. We are all learning to see more in the rocks day by day. So it is important to have the rocks and minerals where they can be handled and repeatedly examined, where we can turn to them in our leisure moments. Don’t stop when you have learned the name of a mineral or rock. You need more. See what it means. Secondly, minerals have beauties of form, color, and structure, and they do not fade. They will be as perfect in ten years as when found. We are all naturally crows, and love to gather the objects which interest us. It is not a bad habit, and only needs directing. Cultivate it. Have a hobby, and minerals and rocks are a good one; for they are like treasures in Heaven which “neither moth nor rust doth corrupt.” Not only will they give you pleasure, but they will be a constructive education, training the eye to see, and the mind to think straight. No one ever regretted the time and effort spent in collecting either minerals or rocks.

Collecting

In order to make a collection valuable two or three rules must be observed. In the case of rocks, collect large enough samples so that they will be characteristic, and clear in their make-up. The standard size for rocks is 3 × 4 inches on top and one to two inches thick according to the nature of the rock. Tiny fragments do not give the character of the rock as well, and they are all the time getting into confusion. Every specimen should be labeled, with at least its name and the exact locality from which it came. Composition, structural features, associations, and classification may be added, the more the better; for each item adds to the information and interest of the specimen. One may make his own labels or have printed blanks, and may put as much care and art into the labels as desired, the more the better. One thing is very important and that is to have a number on the label with a corresponding one on the specimen, so that in case they should get separated, they may be readily brought together, even by one who is not familiar with the individual specimens. Lastly, give your collection as good a place as possible, either in drawers, boxes or in a case. The specimens are worth being kept in order and where they can be readily seen and compared. Nature is systematic, and there is a reason for the order in which rocks and minerals are taken up. It is desirable either that this order, or some one of the orders of Nature appear in the collection. In this book the metals are the basis of classification, all those minerals primarily related to one of the metals being grouped together.

In collecting minerals, the size of the specimens can not be so regularly followed, but it should be followed when collecting non-crystalline minerals, and when possible. Crystals however are chosen from a variety of points of view, as perfection of form, color, examples of cleavage, twinning, etc.; so that in many cases smaller or larger examples must appear in the collection. It is always desirable that as many variations of a rock or mineral as possible should appear in the collection, and in many cases examples of the matrix from which the crystals came. When crystals are tiny, it is well to place them in vials, that they may not be lost.

Where

Where shall we start in making a collection? Near home. Get the local minerals and rocks first, and then range as widely as possible. The best places are bare and exposed rocks, especially where fresh and un-weathered surfaces are available. Quarries and where there has been blasting along roads offer fine opportunities. Fissures and cavities in the rocks are especially likely to have fine crystals, and in all localities continued search will reveal a surprising number of different minerals. The greatest variety occur in metamorphic rocks, or where igneous rocks come in contact with other rocks, but even the sedimentary rocks have a goodly range of minerals. All through the glaciated regions of the northern United States lie scattered boulders brought from afar, which will yield a surprising number of minerals and variety of rocks.

Equipment

One may start with a very simple equipment, a geologist’s or stone mason’s hammer which can be obtained at any hardware store, being sufficient for field work. Rocks should be broken, so as to show fresh surfaces and to get below the disintegrating effects of weathering. At home one should have a streak plate (a piece of unglazed porcelain), a set of hardness minerals (see [page 20]), and a small bottle each of hydrochloric and nitric acid. A pocket lens is useful in order to see more clearly the form of small minerals. These things can be purchased of any Naturalist’s Supply Co., like Ward’s Natural Science Establishment, P.O. 24, Beachwood Sta., Rochester, N. Y., or the Kny-Scheerer Corp., 483 First Ave., New York City. Success depends upon a quick eye, and persistent hunting. When traveling, opportunities are offered at frequent intervals to see and get new specimens.

Study Your Collection

Be sure and see the meaning in each rock and mineral. The history of the country is revealed in its rocks and minerals. Note whether the rocks are horizontal or folded, whether they change character from place to place, or vertically. In going over a piece of country you may locate an ancient mountain system now leveled, by noting a series of metamorphic rocks, with a central core of granite, the roots of former mountains. Don’t be afraid to draw conclusions from what you see. Later, when the opportunity offers, look up the region in the geological folio, bulletin, or map of that section, and check up your findings. These geological folios and bulletins, of which there is one for nearly every region, are a great help to collectors in suggesting where to look for various rocks and minerals. Write to the Director of the U. S. Geological Survey, Washington, D. C., for a catalogue of the publications of the United States Survey, or find out from him what are the maps or folios for the region in which you are interested. These U. S. publications cost but little. When opportunity presents itself, visit other collections. In them you will see some of the minerals or rocks which have puzzled you, and there is nothing quite so satisfactory as seeing the rocks or minerals themselves. No description can always be so convincing. Then too you will get suggestions as to localities that you can visit.

Literature

As your collection grows, if you find you have special interest in one or another branch of the field, you can get books giving more details in that line; and at the back of this book will be found a list of such books.

CHAPTER II
ON THE FORMS AND PROPERTIES OF MINERALS

Rocks

All we know of the earth by direct observation is confined to less than four miles depth; though by projecting downward the layers of rock that come to the surface, we may fairly assume a knowledge of the structure down to six or eight miles depth. This outer portion is often referred to as the “crust of the earth,” but the idea that the deeper portions are molten is no longer held. This outer portion is made of rocks, and a rock may be defined as, a mass of material, loose or solid, which makes up an integral part of the earth, as granite, limestone, or sand. The rocks (except glassy igneous ones) are aggregates of one or more minerals; either in their original form like the quartz, feldspar and mica of granite, or in a secondary grouping, resulting from the units having been dislodged from their primary position and regrouped a second time, as in sandstone or clay.

Minerals

Since the rocks are aggregates of minerals, it is best to take up the minerals first. A mineral may be defined as a natural inorganic substance of definite chemical composition. It is usually solid, generally has crystalline structure, and may or may not be bounded by crystal faces. A crystal is a mineral, bounded by symmetrically grouped faces, which have definite relationships to a set of imaginary lines called axes. There are between 1100 and 1200 minerals, of which 30 are so frequently present, and so dominant in making up the rocks, that they are termed rock-forming minerals. About 150 more occur frequently enough so that they can be termed common minerals, and one may expect to find a fairly large proportion of them. Some of these are abundant in one part of the country and rare in others, but this book is written to cover the United States, and so all those which have a fair abundance are included, though some will only be found in the west and others mostly in the east. Then there are some more minerals which are really rare, but which are cherished because of their beauty of color, and are used as gems. These are mentioned, and many of the gems are simply clear and beautiful examples of minerals, which in dark or cloudy forms are much more common. If one finds any of these rare minerals which are not mentioned in this book, he must turn to one of the larger mineralogies mentioned in the literature list to determine them.

Crystal Structure

A crystal is a mass of molecules, all of the same composition. A molecule in its turn is made up of atoms, and each atom is a unit mass of an element. Thus the calcite molecule is made up of one unit or atom of calcium, one of carbon, and three of oxygen (CaCO₃). These atoms are held together by an attraction, and make a molecule, and for the study of minerals the molecule is the unit. The mineral, calcite, is a mass of molecules all like the one above, and each molecule so small as to be invisible even with the aid of the most powerful microscope. When calcite is in crystal form, the molecules, like ranks of soldiers, are arranged each in its place, each at a definite distance from the other. While each molecule may vibrate or wiggle within certain limits it does not leave its place. (The comparison with soldiers is a good one for the molecules of one layer, but it must be remembered that in a crystal there are also like spacings and ranks up and down as well.) As long as the molecules remain in fixed ranks, up and down, forward and back, and sideways, the crystal is perfect. Calcite may be heated until it melts and becomes liquid. Then the molecules leave their definite arrangement and move about in all sorts of directions, like the soldiers after ranks have broken. So long as the molecules are thus free to move about but keep together, the substance is a liquid. There are cases when the molecules in this disorder take fixed positions without falling into ranks. Such minerals are non-crystalline and usually appear glassy. If still greater heat is applied to the mineral in liquid form, a point is reached (the vapor point), above which the molecules go flying away from each (like soldiers in a panic), each seeking to get as far from the other as possible, so only a container will prevent their dissipation. When in this condition a mineral is gaseous. When cooled, the reverse order obtains. The molecules of gas gather into a miscellaneous mob or liquid: and if this is further cooled (but not too suddenly), they fall into ranks and make a crystal. This may be illustrated with water. When above 212° F. it is steam (molecules wildly dissipated); when between 212° and 32° it is water (molecules close to each other, but milling like a herd of cattle); and when below 32° it is ice, the molecules ranged in perfect order, rank on rank.

Crystal Systems

With all the possible forms that crystals can and do take, there are six systems of arrangement. First there is the case where ranks, files, and vertical rows are all equal, and now to be scientific, instead of talking about ranks, files, etc., we use the term axes to express these ideas; the files or arrangements from front to back, being called the a axis, the ranks, or side to side arrangement the b axis, and the vertical arrangement the c axis. (See [Plate 1].) These axes are imaginary lines, but they represent real forces.

Isometric system

When the axes are all equal and at right angles to each other, a crystal is said to be in the isometric system. The cube is the basal form and each side is known as a face. The ends of the axes come to the middle of the cube faces. The essential feature of this system is that whatever happens to one axis must happen to all, which is another way of saying that all the axes are equal. If we think of the cube as having the corners cut off, we would have a new face on each of the eight corners, in addition to the six cube faces. Then if each of these new faces were enlarged until they met and obliterated the cube faces, an eight-sided figure, the octahedron, would result. In this the axes would ran to the corners. Another modification of the cube would be to bevel each of its twelve edges, making twelve new faces in addition to the six cube faces. If we think of these new faces being developed until they meet and obliterate the cube faces, there will result a twelve-sided figure, the dodecahedron. And the 24 edges of the dodecahedron could be beveled to make a 24-sided figure, and so on. Of course in Nature the corners are not cut, nor the edges beveled, but as a result of the interaction of the forces expressed by the axes and the distribution of the molecules, the molecules arrange themselves in a cube, octahedron, dodecahedron or combination of these basal forms.

Crystal formation

Crystals are formed in liquids as they cool or evaporate and can no longer hold the minerals in solution. Crystals start about a center or nucleus, and molecule by molecule, the orderly arrangement is increased and the crystal grows, there being no size which is characteristic. If free in the liquid the crystal grows perfectly on all sides, but if crystals are growing side by side, there comes a time when they interfere with each other. Then the free faces continue to grow and the orderly internal arrangement is maintained, though externally there is interference.

Tetragonal system

In the second or tetragonal system one axis (the c axis) is different from the other two, but all three are still at right angles with each other. This is saying scientifically that the lines of force are greater or less in one direction than in the other two, but they act at right angles to each other. The a and the b axes are equal and anything that happens to one of these two must happen to the other, but need not happen to the c axis. Thinking of the molecules that arrange themselves under this system of forces, it is clear that the simplest form will be a square prism, i.e., front to back, and from side to side the numbers of molecules will be equal, but up and down there will be a greater or lesser number. If the eight corners of this prism were cut, and these corner faces increased in size until they met, the resulting octahedron would be longer (or shorter) from top to bottom than from side to side or front to back, but the measurement from front to back would be equal to the one from side to side. In this system we may have the vertical edges of the prism beveled, and not have to bevel the horizontal ones, or we may bevel the horizontal edges and not the vertical ones. There is no dodecahedron in this system or in any other system than the isometric. The forms in this tetrahedral system are really a combination of the four sides of the square prism with such modifications as equally affect them all, with two ends which may be flat, or pyramidal, or modified pyramidal faces.

Orthorhombic system

The third system has all three axes unequal, but all three are still at right angles with each other. This is saying that the lines of force in the crystals are all at right angles to each other but of unequal value. The faces in this case are all in pairs. What happens at one end of an axis must happen at the opposite end, but does not need to happen at the ends of any of the other axes. We are dealing with pairs of faces (one at either end of an axis), and if three such pairs are combined in the simplest manner, the resulting figure will be a rectangular prism. If we cut the eight corners of this prism and enlarge the faces until they meet, the result is an octahedron, in which the distance from top to bottom, from side to side, or from front to back is not the same in any two cases. (See [Plate 2].) In this system if a face is made by beveling one edge of the prism there must be a corresponding face on the edge diagonally opposite, but there does not have to be one on any of the other edges. However if a corner is cut, that face affects all the axes and so all the corners must be cut. A great many crystals occur in this system, and some of them which are prismatic in shape may give trouble, for it is not uncommon for the vertical edges of the prism to be so beveled, that two of the original prism faces are obliterated, and the two remaining faces added to the four new faces make a six-sided prism, which at first glance seems to belong to the hexagonal system. (See [Plate 3], fig. 3.) Close examination however will show that, instead of all the prism faces being alike, as would be necessary for the hexagonal system, they are really in pairs, and one pair at least will be distinguished in some way, such as being striated, pitted, or duller.

Monoclinic system

The fourth system has all the axes unequal, the a axis and the b axis at right angles to each other, but the c axis is inclined to the a axis, meeting it at some other than a right angle. The monoclinic system is like the orthorhombic system except that it leans, or is askew, in one direction. The result is that the faces at the ends of the b axis are rhombohedral, while the others are rectangular. As in the foregoing system, the faces are in pairs at opposite ends of the axes; and as in the orthorhombic system, a face may occur on one edge and only have to be repeated on the edge diagonally opposite. The simplest form in this system will be made by combining the three pairs of faces at the opposite ends of the axes, which gives a prism, which is rectangular in cross section, but leans backward (or forward) if placed on end. As in all the systems, if a corner is cut, all must be cut; and if these corner faces are extended to meet each other, an octahedron results, in which, as in the prism, no two axes are equal. If this octahedron is properly orientated (i.e. with the a and b axes horizontal), it will lean forward or backward. Many minerals belong to this system; and, as in the orthorhombic system, it is not uncommon to have the vertical edges so beveled that two of the prism faces are obliterated, and the remaining two prism faces with the four new faces make a six-sided prism, which seems hexagonal. (See [plate 3], figure 3.) However, such a pseudo-hexagonal prism may be recognized by at least one pair of the faces having distinguishing marks (striæ, pits, or dullness), instead of all being just alike.

Triclinic system

The fifth or triclinic system has all the axes unequal, and no two of them intersect at right angles. As in the two preceding systems the faces occur in pairs at the opposite ends of the axes. This is the most difficult system in which to orientate a crystal, but fortunately only a few crystals occur in this system, such as the feldspars.

Hexagonal system

Lastly there is a group of crystals which have four axes, one vertical, and three in the horizontal plane which intersect each other at angles of 60°, all these three being equal to each other, but different from the vertical axis. The simplest form in this system is the six-sided prism. If one corner of this prism is cut all must be, and if these corner faces are extended to meet each other, a double-six-sided pyramid results. In this system if one of the vertical edges of the prism is beveled, all must be, but the horizontal edges need not be; or the horizontal edges may be beveled and the vertical ones not. The ends as they are related to the c axis may be developed independently of the prism, and so the prism may be simply truncated by a flat end, or have pyramids on either end.

Hemihedral forms

In this system it is quite common to have forms which result from the development of each alternate face of either the prism or the double pyramid. In the case of the prism, if every alternate face is developed (and the others omitted) a three-sided prism results, as in tourmaline. In the case of the double pyramid if the three alternate faces above are united with the three alternate faces below, a six-sided figure is formed, which is known as the rhombohedron, as all the faces are rhombohedral in out-line and all equal. These forms in which only half the faces are developed are known as hemihedral forms. The same sort of thing may happen in the isometric system in the case of the octahedron, and also in the case of the octahedron of other systems. When half the faces of the octahedron are developed, two above unite with two below and make a four-sided figure, known as a tetrahedron. (See [plate 10].) While tetrahedrons may occur in any of the first five systems they are not common outside the isometric system.

Twinning

Another modification of the simple forms which will be met occasionally is twinning. By this is meant two crystals growing together as though placed side by side on some one of the faces, and then revolved until the two axes which would normally be parallel are at some definite angle with each other, 60°, or 180° which is commoner. The surface of contact between the two crystals is called the composition face, and as no more material can be added on that face the crystals continue to grow developing the other faces, and we find faces in contact with each other which should be at the opposite end or other side of the crystals. This contact of faces which should not come in contact, and the presence of reentrant angles are indications of twinning. In some minerals the twinning may be repeated time and again, and if the twinning is on one of the end faces a branching structure results, as in frost and snow crystals, or the multiple twinning may be of crystals growing side by side when the final form will approximate a series of thin sheets placed side by side as in some feldspars. The peculiar forms characteristic of individual minerals are taken up under the respective minerals.

Other important properties of minerals are hardness, cleavage, specific gravity, streak, luster, and color.

Hardness

Hardness may be defined as the mineral’s resistance to abrasion or scratching. It is measured by comparing a mineral with Moh’s scale, a set of ten minerals arranged in the order of increasing hardness, as follows:

1 [talc] 2 [gypsum] 3 [calcite] 4 [fluorite] 5 [apatite] 6 [feldspar] 7 [quartz] 8 [topaz] 9 [corundum] 10 [diamond]

A set for measuring hardness may be purchased from any dealer in mineral supplies. For rough determination, as in the field, the following objects have the hardness indicated; the finger nail 2¼, a penny 3, a knife blade about 5.5, and glass not over 6. In testing, a mineral is harder than the one it will scratch, and softer than the one by which it is scratched. For instance, if a mineral will scratch calcite and is scratched by fluorite, it is between 3 and 4 in hardness, say 3.5. When two samples mutually scratch each other they are of equal hardness. Care must be used in determining hardness, especially with the harder minerals; for often, when testing a mineral, the softer one will leave a streak of powder on the harder one, which is not a scratch. One should always rub the mark to make sure it is really a groove made by scratching.

Cleavage

Cleavage is the tendency, characteristic of most minerals, and due to the arrangement of their molecules, to cleave or break along definite planes. The cleavage of any mineral is not irregular or indefinite, but characteristic for each mineral, and always parallel to possible or actual faces on the crystal, and always so described. For instance galena has three cleavages, all equally good, and parallel to the cube faces; so it is said to have cubic cleavage. In the same way fluorite has octahedral cleavage, and calcite rhombic cleavage. In some minerals cleavage is well developed in one plane, and less developed in other planes, or it may be lacking altogether. The varying degrees of perfection by which a mineral cleaves are expressed as, perfect or imperfect, distinct or indistinct, good or poor, etc.

Specific gravity

The specific gravity of a mineral is its weight compared with the weight of an equal volume of water, and is therefore the expression of how many times as heavy as water the mineral is. For instance the specific gravity of pyrite is 5.1, which is saying it is 5.1 times as heavy as water. In a pure mineral the specific gravity is constant, and an important factor in making final determinations. As ordinarily obtained, a piece of pure mineral is weighed in air, which value may be called x. It is then immersed in water and again weighed, and this value is called y. The difference between the weight in air and that in water is the weight of an equal volume of water. Then we have the following formula:

specific gravity =

Various balances have been devised for making these measurements, but any balance which will weigh small objects accurately, may be adapted to specific gravity work, by hanging a small pan under the regular weighing pan. When using this balance, care is taken to see that the lower pan is always submerged in water, even while the mineral is being weighed in air, so that when weighed in water in the lower pan, the weight of this lower pan has already been considered.

Streak

By streak is meant the color of the mineral when powdered. For some minerals, especially metallic ores, it is of great importance, for it remains constant, though the color of the surface of the mineral changes materially. It is most readily determined by rubbing a corner of the mineral on a piece of unglazed porcelain. Small plates, known as “streak plates” are made for this purpose.

Luster

The luster of a mineral is the appearance of its surface by reflected light, and it is an important aid in determining many minerals. Two types of luster are recognized; metallic, the luster of metals, most sulphides and some oxides, all of which are opaque on their thin edges; and non-metallic, the luster of minerals which are more or less transparent on their thin edges, and most of which are light colored. The common non-metallic lusters are; vitreous, the luster of glass; resinous, the appearance of resin; greasy, oily appearance; pearly, the appearance of mother-of-pearl; silky, like silk due to the fibrous structure; adamantine, brilliant like a diamond; and dull, as is chalk.

Color

When used with caution color is of the utmost importance in determining minerals, especially in making rapid determinations. In metallic minerals it is constant and dependable; but in the non-metallic minerals it may vary, due to the presence of small amounts of impurities which act as pigments. Color depends on chemical composition, and when not influenced by impurities is termed natural; but when the color is due to some inclosed impurity it is termed exotic. In this latter case caution must be used in making determinations. Many minerals are primarily colorless, but take on exotic colors as a result of the presence of small quantities of impurities; for instance, pure corundum is colorless, but with a trace of iron oxide present becomes red, and is called the ruby, or with a trace of cobalt becomes blue and is called sapphire.

CHAPTER III
THE MINERALS

KEY TO THE MINERALS, BASED ON HARDNESS, COLOR, ETC.

The Gold Group

Gold was undoubtedly the first metal to be used by primitive man; for, occurring as it did in the stream beds, its bright color quickly attracted the eye, and it was so soft, that it was easily worked into various shapes, which, because they did not tarnish, became permanent ornaments. The metal is associated with the very earliest civilizations, being found in such ancient tombs as those at Kertsch in Crimea and in northern Africa and Asia Minor. It was used in the cloisonné work of Egypt 3000 years B.C. In America the Indians, especially to the south, were using it long before the continent was discovered.

Of all the metals gold is the most malleable, and its ductility is remarkable, for a piece of a grain’s weight (less than the size of a pin head) can be drawn out into a wire 500 feet long; and it can be beaten into a thin leaf as thin as ¹/₂₅₀₀₀₀ of an inch in thickness, and thus a bit, weighing only a grain, can thus be spread over 56 square inches.

It forms very few compounds, but has a considerable tendency to make alloys (i.e., mixtures with other metals without the resulting compound losing its metallic character). In Nature gold is never entirely pure, but is an alloy, usually with silver, there being from a fraction of 1% up to 30% of the silver with the gold, the more silver in the alloy, the paler the color of the gold. Australian gold is the purest, having but about .3% of silver in it, while Californian gold has around 10% and Hungarian gold runs as high as 30% of silver. Another alloy fairly abundant in Nature is that with tellurium, such as calaverite (AuTe₂) which is a pale brassy yellow, similar to pyrite, but with the hardness of but 2.5. Another combination includes gold, silver and tellurium, sylvanite, (AuAgTe₄) a silvery white mineral with a hardness of but 2. Such combinations are known as tellurides and the calaverite is mined as a source of gold at Cripple Creek, Colo., while the sylvanite is one of the important ores of gold in South Africa. Occasionally gold is also found alloyed with platinum, copper, iron, etc. Jewelers make several alloys, “red gold” being 3 parts gold and 1 of copper, “green gold” being the same proportions of gold and silver, and “blue gold” being the combination of gold and iron. Our gold coins are alloys, nine parts gold and one of copper, to give them greater durability. Most of the gold recovered from nature is found native, i.e., the pure metal, or with some alloy.

[Gold]
Au
[Pl. 5]

Usually non-crystalline, but occasionally showing cube or octahedral faces of the isometric system; hardness 2.5; specific gravity 19.3; color golden yellow; luster metallic; opaque.

Gold is mostly found as the metal and is readily recognized by its color, considerable weight, hardness, malleability, and the fact that it does not tarnish. It usually occurs in quartz veins in fine to thick threads, scales or grains, and occasionally in larger masses termed “nuggets.” It is insoluble in most liquids so that when weathered from its original sites, it was often washed down into stream beds, to be found later in the sands or gravels, or even in the sea beaches. When thus found it is termed “placer gold,” and its recovery is placer mining. Most of the original discoveries of gold have been in these placer deposits; and from them it has been traced back to the ledges from which it originally weathered. In the placer deposits the size of the particles varies from fine “dust” up to large nuggets, the largest found in California weighing 161 pounds; but the largest one found in the world was the “Welcome Nugget,” found in Australia, and weighing 248 pounds. When gold was discovered in California in 1848, this became the chief source for the world, but later this distinction went to Australia, and now belongs to South Africa, which today yields over half the annual supply.

The ultimate source of gold is from the lighter colored igneous rocks, like granites, syenites, and diorites, throughout which it is diffused in quantities too small to be either visible or worth while to extract. It becomes available only when it has been dissolved out by percolating waters and segregated in fissures or veins, either in or leading from these igneous rocks. Generally this transfer of gold has taken place when the rocks were at high temperatures, and by the aid of water (and perhaps other solvents) which was also at high temperatures. The presence of gold in sandstones, limestones, etc., is secondary, as is also its presence in sea water, in which there is reported to be nearly a grain (about five cents worth) in every ton of water. Beside the direct recovery of gold from gold mining, a great deal is obtained from its association with iron, copper, silver, lead and zinc sulphides, in which it is included in particles too fine to be visible, but in quantities large enough to be separated from the other metals after they are smelted.

In the United States gold is found in the Cordilleran region from California to Alaska, in Colorado, Nevada, Arizona, Utah, the Black Hills of South Dakota, and in small quantities in the metamorphosed slates of North and South Carolina, Georgia, and in Nova Scotia.

The Silver Group

Though much commoner than gold, silver did not attract the eye of man as early, probably because it tarnishes when exposed to air or any other agent having sulphur compounds in it, and a black film of silver sulphide covers the surface. Its first use was for ornaments, and some of these found in the ruins of ancient Troy indicate its use as early as 2500 B.C. A thousand years later it was being used to make basins, vases and other vessels.

Silver is next to gold in malleability and ductility, so that a grain of silver can be drawn out into a wire 400 feet long, or beaten into leaves ¹/₁₀₀₀₀₀ of an inch in thickness. As a conductor of electricity it is unsurpassed, being rated at 100% while copper rates 93%. Silver is also like gold in the freedom with which it alloys with other metals, such as gold, copper, iron, platinum, etc. All our silver coins, tableware, etc., have some copper alloyed with the silver to give it greater hardness and durability.

Unlike gold, silver freely enters into compounds with the non-metals, which is the reason that it is not found primarily in its native state, but usually as a sulphide. Its ultimate source is in the igneous rocks, few granites or lavas, on analysis, failing to show at least traces of silver. Before it is available as an ore, or mineral, it has been dissolved from the original magma, and segregated in fissures or veins, along with such minerals, as quartz, fluorite, calcite, etc. This seems to have taken place while the igneous rocks were still hot, and by the agency of vapors and liquids which were also hot. The presence of silver in sedimentary and metamorphic rocks, or even in sea water, is secondary.

The primary deposition of silver is usually in the form of sulphides, the commoner of which are, argentite or silver sulphide, pyrargyrite or silver and antimony sulphide, and prousite, or silver and arsenic sulphide. Its occurrence as native silver, or the chloride, cerargyrite, is secondary and due to the reactions which have taken place when sulphide deposits have been subjected to weathering agents.

The United States produces about 25% of the world’s supply, Mexico some 35%. It is especially found along the Cordilleran ranges of both North and South America.

[Silver]
Ag
[Pl. 6]

Usually non-crystalline, but occasionally showing cube or octahedron faces of the isometric system; hardness 2.5; specific gravity 10.5; color silvery white; luster metallic; opaque.

When found in its native state silver is usually in wirey, flakey, or mossy masses; but sometimes masses of considerable size occur, the most famous being an 800 pound nugget found in Peru, and another of 500 pounds weight found at Konsberg, Norway, and now preserved in Copenhagen. When exposed to the air the surface soon tarnishes and takes on a black color which must be scraped off to see the real color.

Like gold, silver is usually found associated with other metals, like iron, copper, lead and zinc; and much of the silver recovered is obtained in connection with the mining, especially of copper and lead. Some lead ores have so much silver in them that they are better worth mining for the silver; galena, for instance, under such circumstances being termed argentiferous galena. Native silver is a secondary mineral, having been formed by the reduction of some one of its sulphides by water, carrying various elements which had a greater affinity for the sulphur.

Silver is found along with copper in the Lake Superior region, and in Idaho, Nevada, and California.

[Argentite]
AgS
[Pl. 6]
silver glance

Usually in irregular masses, but sometimes in cubes; hardness 2.5; specific gravity 7.3; color and streak lead gray; luster metallic; opaque on thin edges.

Argentite, the simple sulphide of silver, is the chief source from which silver is obtained. It looks like galena, and has the same hardness, streak and specific gravity, but can be distinguished by the galena having a very perfect cubic cleavage while the argentite has no cleavage. Argentite is easily cut with a knife (sectile). It is usually found in irregular masses, but sometimes in cubes which make very choice cabinet specimens; and is associated with such other minerals as galena, sphalerite, chalcopyrite, pyrite, fluorite, quartz, and calcite.

It occurs in fissures and veins all through the Cordilleran regions, especially in California, Colorado, Nevada (Comstock Lode), Arizona (Silver King Mine) and about the shores of Lake Superior.

[Pyrargyrite]
Ag₃SbS₃
[Pl. 7]
ruby silver or dark red silver

Usually occurs in irregular masses; hardness 2.5; specific gravity 5.8; color dark red to black; streak purplish red; luster metallic to adamantine; translucent on thin edges.

Pyrargyrite, the sulphide of silver and antimony, is distinguished by its dark red color and the purplish streak. It may look like prousite, but is easily distinguished from the latter which has a scarlet streak. It also at times looks like hematite and cinnabar, but the hematite has a hardness of 6, and the latter has the bright red color throughout, while pyrargyrite turns black when exposed to the light, so that the characteristic red color will be seen only on fresh surfaces. The characteristic red color can only be kept on the mineral if it is constantly protected from the light.

Sometimes pyrargyrite occurs in crystals and these belong to the hexagonal system, and are prisms with low faces on the ends, as on [plate 7], and the mineral is peculiar in that the faces on the opposite ends are unlike.

Pyrargyrite is found mostly in fissures and veins of quartz, fluorite, calcite, etc., and associated with pyrite, chalcopyrite, galena, etc. It is fairly common in Colorado in Gunnison and Ouray counties, in Nevada, New Mexico, Arizona, etc.

[Prousite]
Ag₃ AsS₃
[Pl. 7]
light red
silver

Usually occurs in irregular masses; hardness 2.5; specific gravity 5.6; color scarlet to vermilion; streak the same; luster adamantine; transparent on thin edges.

In general this mineral is very like pyrargyrite, but has the scarlet color and streak which are entirely characteristic. It is likely to have the surface tarnished black, which happens on exposure to light, so that it is essential to be sure that fresh surfaces are being examined. Occasionally it is found in crystals, of the same type as the preceding mineral. It is generally found associated with pyrargyrite.

[Cerargyrite]
AgCl
horn silver

Usually found in irregular masses or incrustations; hardness 1 to 1½; specific gravity 5.5; color pearly gray, grayish green to colorless, but turning violet brown on exposure to light; luster resinous; transparent on thin edges.

This mineral is usually found in thin seams or waxy incrustations, but it may occur in crystals in which case they are cubes. It is very soft and easily cut with a knife, which with its tendency to turn violet-brown on exposure to light, makes it easy to identify. Cerargyrite is a secondary mineral, resulting from the action of chlorine-bearing water on some one of the sulphides of silver. It is found in the upper portions of mines, especially those in arid regions.

The Copper Group

After gold the next metal to be utilized was copper. About 4000 B.C. our early forefathers found that by heating certain rocks, they obtained a metal which could be pounded, ground and carved into useful shapes. Curiously enough the rocks which had the copper also had some tin in them, so that this first-found copper was not pure, but had from five to ten per cent of tin in it, making the resulting metal harder, and what we call bronze. It was some thousands of years later before they distinguished the copper as a pure metal, but it worked and made good tools. The newly found metal was not as ornamental as gold; but, because it could be made into tools, it had a tremendous influence on man’s development. As the bronze tools began to take the place of the stone implements, the “Age of Bronze” was ushered in. In America the Indians in the Lake Superior region found native copper weathered out of the rocks and later mined it, and they too pounded it into knives, axes, needles, and ornaments, but probably never learned to melt it and mold their tools. At any rate they were not as far advanced in using this metal when Columbus landed as were the southern Europeans 6500 years earlier. Since the use of iron became general, copper has not held such a dominant place, but it still is “the red metal” which holds the second most important place.

It is malleable and ductile, though not equal to gold or silver in these respects. It is a good conductor of electricity and a very large amount of copper is used in electrical manufacture, roofing, wire, etc. It alloys with other metals; ten parts copper and one of tin being bronze, ten of copper and one of zinc is brass, and copper with aluminum is aluminum bronze.

Like silver and gold, copper is widely diffused through the igneous rocks, but before it is available, it must be leached out by solvents and concentrated in veins, fissures, or definite parts of the lavas or granites. The primary ores are those which, while the igneous rock was still hot, were carried by hot vapors and liquids into the fissures and there deposited, mostly as sulphides. There is a long list of these, but in this country, the following are the commoner ones; chalcocite the sulphide of copper, chalcopyrite the sulphide of copper and iron, bornite another combination of copper, iron and sulphur, and tetrahedrite copper and antimony sulphide. When these primary ores are near enough to the surface to come in contact with waters carrying oxygen, carbon dioxide or silica in solution, they may give up their sulphur and take some one of these new elements and we have such forms as cuprite, the oxide of copper, malachite and azurite, carbonates of copper, or chrysocolla, the silicate of copper. Native copper is also a secondary deposit laid down in its present state by a combination of circumstances which deprived it of its original sulphur. In general copper mining can not be profitably carried on for ores with anything less than a half of one percent in them; and the use of such low grade ores has only been possible for a few years, as the result of inventing most delicate processes in the smelting.

The United States produces about a quarter of the world’s supply of copper, with Chile ranking second with about 17%.

[Copper]
Cu
[Pl. 8]

Usually in irregular masses; hardness 2.5; specific gravity 8.9; color copper red; luster metallic; opaque. Native copper, easily determined by its color and hardness, is generally found in irregular grains, sheets, or masses, on which may sometimes be detected traces of a cube or an octahedral face, showing that it belongs to the isometric system. The most famous locality is the Upper Peninsula of Michigan which may be taken as typical. Here, long before it was known historically, the Indians found and dug out copper to make knives, awls, and ornaments.

In this region, beds of lava alternate with sandstones and conglomerates. The copper was originally in the lavas, but has been dissolved out, and now fills cracks and gas cavities in the lavas, and also the spaces between the pebbles of the conglomerate. This locality has been very famous both because of the quantity mined, and also because of the strikingly large masses sometimes found. Today but little of the ore runs above 2 percent copper, and it is mined if it has as little as ½ of one percent.

While nowhere near as abundant, native copper occurs in the same way in cavities and cracks in the trap rocks of New Jersey, and along the south shore of the Bay of Fundy. It is also known from Oregon, the White River region of Alaska, and in Arctic Canada.

[Chalcopyrite]
CuFeS₂
[Pl. 8]
copper pyrites or yellow copper ore

Occurs in crystals of irregular masses; hardness 4; specific gravity 4.2; color bronze yellow; streak greenish black; luster metallic; opaque on thin edges.

Chalcopyrite resembles pyrite, but its color is a more golden yellow, and its surface tarnishes with iridescent colors. Then too the hardness of chalcopyrite is but 4 as compared with 6 for pyrite. When in crystals this mineral belongs to the tetrahedral system as the c axis is but .985 in length as compared with I for the two other axes. This difference is so little that, to the eye, the octahedron appears to belong to the isometric system. Chalcopyrite occurs in octahedrons and tetrahedrons (as on [plate 8]), the latter being the form where but half of the octahedral faces are developed. However by far the most frequent mode of occurrence is in irregular masses.

This is the most important primary ore of copper, and is widely distributed, being found either in lavas, or in veins, or in fissures connected with igneous rocks. Apparently the deposits were made, either at the time of eruptive disturbances or shortly afterward, from vapors or hot solutions carrying the copper sulphides (and other sulphides) from the molten igneous rocks. Chalcopyrite is usually associated with pyrite, galena, sphalerite and chalcocite, as well as quartz, fluorite and calcite. It is found in all the New England States, in New York, New Jersey, Pennsylvania, Maryland, Virginia, North Carolina, Tennessee, Missouri, and all the Rocky Mountain and Pacific Coast States.

[Bornite]
Cu₃FeS₃
purple copper ore

Occurs in granular or compact masses; hardness 3; specific gravity, 5; color bronze-brown with a bluish tarnish; streak gray-black; luster metallic; opaque on thin edges.

Bornite is also known as erubescite, blushing ore, variegated copper, peacock copper, etc., all of which names refer to the highly iridescent tarnish which fresh faces soon take on when exposed to the air. Though usually in masses, it is sometimes found in rough cubes of the isometric system. In this country it is not abundant enough to be used as an ore, but is likely to be found with other ores like chalcopyrite or chalcocite. In the east it has been found at Bristol, Conn., and near Wilkesbarre, Penn., while in the west it may be expected to occur wherever other sulphide minerals of copper are found.

[Chalcocite]
Cu₂S
[Pl. 9]
copper glance

Occurs in fine grained compact masses; hardness 2.5; specific gravity 5.7; color dark leaden gray; streak black; luster metallic; opaque on thin edges.

Chalcocite is one of the important ores of copper, especially in Arizona and the Butte District of Montana. It resembles argentite in color and general appearance, but is readily distinguished by being brittle and having a tendency to tarnish to bluish or greenish colors on fresh surfaces. Occasionally it occurs in crystals which are in the orthorhombic system; but the edges of the prism are so beveled that there are six sides and the prism resembles a hexagonal prism (see [page 16]).

In the Butte, Mont., district, the most important copper region in the United States, fully 50% of the ore is chalcocite, which is a derivative of the originally deposited chalcopyrite, the latter having lost its iron. In the veins of this district chalcopyrite, bournite, tetrahedrite, and several other copper minerals not described in this book, occur all together, and with them also gold, silver and arsenic minerals. The gold amounts to about 2¼ cents per pound of copper, and the silver is in somewhat less quantity. These veins were first opened to get the silver ores, which were the more important ones down to a depth of 200 to 400 feet. Below these depths the copper became much more important. It was the weathering which had removed a large part of the copper minerals in the upper levels of the veins, but had left a large part of the silver. Chalcocite is also important in most of the Utah and Arizona mines.

In the east it has been found at Bristol, Simsbury and Cheshire, Conn., and in the west it is found in all the Cordilleran States.

[Tetrahedrite]
Cu₃SbS₃
Pl. [9] & [10]
gray copper ore

Occurs in irregular masses and in tetrahedrons of the isometric system; hardness 3.5; specific gravity 4.7; streak dark brown; luster metallic; opaque on thin edges.

In its crystalline form the tetrahedrite occurs in tetrahedrons, which generally have faces formed by beveling the edges and by cutting the corners, as in the two figures of [plate 10]. Chalcopyrite may also occur in tetrahedrons, but its golden yellow color is entirely different from the gray-black of the tetrahedrite. When in masses the hardness and the streak which is dark brown, are very characteristic.

In England and Bolivia tetrahedrite is an important ore of copper, but in this country it is simply a copper mineral which is widely distributed, and associated with most of the mining enterprises, but is in no case the important ore. It has been found sparingly through the New England States, at the Kellogg Mines in Arkansas, and abundantly in Colorado, Montana, Utah, Arizona, Nevada and New Mexico.

[Cuprite]
Cu₂O
Pl. [9] & [10]
red copper ore

Occurs in isometric cubes, octahedrons, and dodecahedrons, or in masses; hardness 3.5; specific gravity 6; color dark brownish-red; streak brownish-red; luster metallic; translucent on thin edges.

When in crystals cuprite is easily determined, but when in masses its fresh surfaces may suggest prousite, but the streak and hardness are quite different in the two cases. Sometimes its color suggests hematite, but the latter has the hardness of 6. When found it is often coated with a thin film of green, which is malachite.

Except when found as native copper, the ore which contains the greatest percentage of copper is cuprite with 88.8% of copper. It is likely to occur in any of the deposits of copper ore, where they are in arid climates and above the level of the underground water, and is very frequently associated with malachite and azurite. In the Bisbee, Arizona, district cuprite is one of the important ores.

Besides the normal occurrence described above, cuprite may be found in two other varieties; one where the crystals have grown side by side and so only the ends have been free for continuous additions of the mineral, which has resulted in a fibrous mass known as “plush copper ore” or chalcotrichite; the other an earthy mixture of limonite and cuprite, which is brick red in color, and termed “tile ore.”

Cuprite is found sparingly in New England, more abundantly at such places as Summerville and Flemington, N. J., Cornwall, Penn., in the Lake Superior region, and fairly abundantly in the Cordilleran States.

[Malachite]
CuCO₃·Cu(OH)₂
[Pl. 11]

Usually occurs in nodular or incrusting masses; hardness 3.5; specific gravity 4; color green; streak a lighter green; luster adamantine, silky or dull; translucent on thin edges.

The vivid green of malachite is usually enough to determine it at once, but one may be sure by trying a drop of acid on it, in which case it effervesces as is characteristic of so many carbonates, but this is the only carbonate which is vivid green. Generally the malachite is in irregular masses, but crystals are occasionally found. These are extremely small and needle-like, and belong to the monoclinic system. In the Ural Mountains there is a locality where these crystals grow in fibrous masses, usually radiating from the center. Malachite in such nodules has a silky luster. These rare nodules have furnished the rulers of Russia with a unique and much prized material for making royal gifts. In European museums and palaces one finds many objects carved from this form of malachite, and marked as gifts of the czars of Russia.

In the United States malachite is widely distributed, appearing as green streaks and stains where copper minerals have been exposed to the air. It is the green tarnish which appears on bronze and copper when exposed to the weather. It is found in large quantities in New Jersey, Pennsylvania, Wisconsin, Nevada, Arizona, Utah, New Mexico, etc. The Bisbee mine in Arizona is the place that has furnished museums with so many of the handsome specimens of malachite associated with azurite. These are the most striking specimens for the vividness of their colors that appear in any collections.

Malachite has been known since about 4000 B.C., the Egyptians having mines where they obtained it between the Suez and Mt. Sinai. In those early days it was particularly a child’s charm, protecting the wearer from evil spirits. It is still used as a stone of lesser value in making some sorts of jewelry.

[Azurite]
2CuCO₃·Cu(OH)₂
[Pl. 11]

Occurs as short prismatic or tabular crystals of the monoclinic system; hardness 4; specific gravity 3.8; color azure blue; streak lighter blue; luster vitreous; translucent on thin edges.

Azurite is another very striking mineral fully characterized by its color and streak. Like malachite it effervesces in acid. It is very near to malachite in composition, and by increasing its water content, can and freely does change to the green mineral; so that few specimens of azurite are without traces of malachite. It is found in the same places as malachite, but is not as abundant in the east.

Azurite with the accompanying malachite is cut and polished to make semi-precious stones for some forms of jewelry.

[Chrysocolla]
CuSiO₃·2H₂O

Never occurs in crystals, but in seams and incrustations; hardness 2-4; specific gravity 2.1; color bluish-green; streak white; luster vitreous; translucent on thin edges.

This rather rare mineral often appears in opal- or enamel-like incrustations, and its color is variable ranging from the typical bluish-green to sky-blue or even turquoise blue. This is a mineral resulting from the action of silica bearing waters, coming in contact with most any of the copper minerals, and is found accompanying cuprite, malachite, azurite, etc. It is never in large enough quantities to be used as an ore, but its striking color attracts attention and it can be found fairly frequently, especially in the west.

The Iron Group

Pure iron is a chemical curiosity which looks very much like silver. As obtained from its ores, or as it occurs in Nature, iron always has some impurities with it, such as carbon, silicon, sulphur and phosphorus, and these are highest in the crudest iron such as “pig-iron.” Its malleability and ductility are only a little less than for gold and silver, and so it has a wide range of qualities for use by man. It is only rarely found native in minute grains in some of the dark lavas. There is however one remarkable exception to this statement, in that on Disco Island, Greenland, there is a basaltic rock, from which are weathered great boulders of native iron up to 20 tons in weight. This iron is very like that occurring in meteorites, and probably came from great depths in the earth’s interior. The specific gravity of iron is 7.8. It makes up around 5% of the crust of the earth, and probably occurs in much larger percentages in the interior of the earth.

Iron was discovered by man later than gold or silver or copper, about 1000 B.C.; but once found it was so much more abundant than any of these that it soon dominated over copper, and from Roman times to the present has been the basis of progress in civilization, and these times are well called “the iron age.”

Iron unites freely with the non-metals, and occurs as sulphides, oxides, carbonates, etc., and is also present as a secondary metal in that great group of minerals known as the silicates (see [page 97]). It alloys with a wide range of other metals, every combination altering the properties of the iron, and thus making it useful in a still greater range of manufacture. The introduction of ¼ to 2½% of carbon into iron makes steel, which is harder (in proportion to the amount of carbon) and stronger than the pure iron.

Iron compounds are among the most numerous and important of the colors in Nature’s paint box, limonite furnishing the browns which color the soil and so many of the rocks, hematite giving the red color to other abundant rocks, and magnetite often coloring igneous rocks black, while the chlorophyll which gives the green color to plants is an iron compound, as is also the hemoglobin which gives the red to our blood.

Iron is present in all igneous rocks, and secondarily in the sedimentary and metamorphic rocks. It is soluble in water, and so is being constantly transferred from place to place, and changes from one compound to another, according to the circumstances in which it is placed.

The primary forms are pyrite, magnetite and the silicates. When in weathered rocks the iron is changed to limonite, siderite or hydrated silicates. Hematite is an intermediate oxide from which the water contained in limonite has been driven off by moderate heat or bacterial action.

[Limonite]
2Fe₂O₃·3H₂O
[Pl. 12]

Never crystalline, occurs in mammillary, botryoidal and stalactitic forms, or in fibrous, compact, oolitic, nodular or earthly masses; hardness 5.5; specific gravity 3.8; color yellow-brown to black; streak yellow-brown; luster metallic to dull; opaque.

Limonite is a very common mineral, the color, streak and hardness identifying it readily. Iron rust is its most familiar form. When powdered it is the ochre yellow used in paints. Being so universally distributed, it is to be expected it will occur in a variety of ways. First, there is the fibrous type found lining cavities, in geodes, or hanging in stalactites in caves. This has a silky luster, an opalescent, glazed or black surface, and is in mammillated or botryoidal masses. Second, it may occur in compact masses in veins, where it was deposited by waters; which, circulating through the adjacent rocks, gathered it from the rocks, and, on reaching the open seams, gave it up again. Third, it may occur in beds on the bottom of ponds, where it was deposited by waters which gathered it as they flowed over the surface of the country rocks. Measurements in Sweden show that it may accumulate in such places as much as six inches in the course of twenty years. In ponds and swamps, the decaying vegetation forms organic compounds, which cause the precipitation of the iron from the water, as it is brought in by the streams. This sort of iron in the bottom of ponds or swamps is also known as “bog iron.” Another form in which limonite may occur in ponds, lakes, or even the sea, is in oolitic masses. In this case the iron forms in tiny balls, with perhaps a grain of sand at the center, and one coat of iron after another formed around it, like the layers of an onion. If the resulting balls are tiny this is called oolitic (like fish eggs), but if the balls are larger it is pisolitic (like peas). Bacteria probably have a good deal to do with the precipitation of limonite in this manner. Fourth, limonite occurs in earthy masses, usually mixed with impurities like clay and sand, which are the residue left behind, where limestones have been dissolved by weathering. The fifth mode of occurrence is known as gossan, or “the iron hat,” which is a mass of limonite capping a vein of some sulphide mineral, like pyrite, chalcopyrite or pyrrhotite, which has been exposed to weathering; and in these minerals the sulphur has been removed, leaving a mass of limonite over the vein. This is particularly common in the west. Limonite is quite easily fusible and so was probably the first ore from which early man extracted iron.

Limonite is iron oxide, with 3 molecules of water of crystallization (or constitution) associated with every 2 molecules of the oxide. If limonite is moderately heated the water is driven out and the resulting compound is hematite, the same oxide, but without the water. In this case and many other similar cases, as gypsum, opal, etc., we have two or more minerals resulting from the presence or absence of water in the mineral. The water molecules have a definite place in the arrangement of molecules which determines the structure of the mineral. Sometimes the water is driven out at a temperature around 212 F., in which case it is called, water of crystallization, but in other cases as gypsum, a considerably higher temperature is required to drive out the water, and then it is called, water of constitution. In all cases the removal of the water changes the arrangement of molecules and a new mineral results, with characteristics of its own.

In this case limonite is only one of a series of minerals which have the Fe₂O₃ molecule as a basis, and that incorporate more or less water into their molecular construction as follows:

Of these goethite is crystalline, the others non-crystalline. They may occur pure or in all sorts of mixtures, the mixtures usually being lumped under limonite. The limonite is far the commonest of the series, goethite is fairly common, but the others are rare as pure minerals.

Limonite is found in all parts of all states and in every country. Though so common, it is by no means an important source of iron today, only about one percent of the iron mined in this country coming from this source, though in Germany, Sweden and Scotland it is relatively much more important.

[Goethite]
Fe₂O₃·H₂O
[Pl. 12]

Occurs in lustrous brown to black orthorhombic prisms, usually terminated by low pyramids; hardness 5; specific gravity 4; color brown to black; streak brownish-yellow; luster imperfect adamantine; opaque.

Goethite, named for the poet Goethe, who was interested in mineralogy, is much less abundant than limonite or hematite, but occurs with them, when they are in veins. Its usual form is an orthorhombic prism with the edges beveled, and a low pyramid on either end. The crystals usually grow in clusters, making a fibrous mass, often radiated, in which case it is known as “needle iron stone”; or the prisms may be so short as to be almost scales; when, because of the yellowish-red color, it is called “ruby mica”. It is found in many states, including Connecticut, Michigan, Colorado, etc.

[Hematite]
Fe₂O₃
Pl. [13] & [14]
specular iron

Occurs in compact, mammillary, botryoidal, or stalactitic masses of dark red to black color, or in earthy masses of bright to dark red; hardness 6; specific gravity 5.2; color ochre red to black; streak cherry red to dark red; luster metallic, vitreous, or dull; opaque on thin edges.

Hematite is readily distinguished from other red minerals by its hardness and streak. It may occur in crystals, which belong to the hexagonal system, and are usually hemihedral forms of the double pyramid, or rhombohedrons. These rhombohedrons usually have the edges beveled, as in [Pl. 13], A; or are tabular in form as a result of the beveling of two of the opposite edges to such an extent that a form like [Pl. 13] B results. However the usual occurrence is in non-crystalline masses, which represent transformations from limonite by the loss of water of crystallization on the part of the limonite. In such cases we have fibrous, oolitic or compact masses, according to the form in which the limonite occurred. The transformation from limonite into hematite involves some heat to drive out the water of crystallization, but nothing like what is involved in metamorphism.

Hematite is the source of 90% of the iron mined in this country. Part of it comes from the famous Clinton iron ore, a layer a foot or more in thickness; starting in New York State, and extending all down the Appalachian Mountains to Alabama, where it is ten or more feet thick and the basis of the Birmingham iron industries. Then there are tremendous deposits of earthy to compact hematite, probably derived from limonite, around the west end of Lake Superior. This latter region yields today around 75% of the iron for this country.

Loose earthy masses of hematite are often known as “ochre red,” and were used by the Indians for war paint. Today the same sort of material is obtained by powdering hematite and using it for red paint. The red color in great stretches of rock is due to the presence of small amounts of hematite, acting as cementing material. The red of the ruby, garnet, spinel, and the pink of feldspars and calcite are due to traces of hematite.

This mineral is very common and found in every state.

[Magnetite]
Fe₃O₄
[Pl. 14]
Magnetic iron ore

Occurs in masses or in isometric octahedrons or dodecahedrons; hardness 6; specific gravity 5.8; color black; streak black; luster metallic; opaque on thin edges.

Magnetite is another important ore of iron, and is peculiar in being strongly magnetic; its name being derived, according to Pliny, from that of the shepherd Magnes, who found his iron pointed staff attracted by the mineral when he was wandering on Mount Ida. This magnetic property has been repeatedly used to locate beds of magnetite, and is very helpful in separating magnetite from the “black sands,” of which it so often forms a part. These sands however generally have magnetite with so much titanium in it that they are unfit for smelting.

Magnetite is found in association with igneous or metamorphic rocks, and often represents limonite or hematite which has been altered as the result of high temperatures. Some of it, in the igneous rocks especially, was undoubtedly in the molten magma and has crystallized out from the magma while it was still hot. It is the form of iron always indicative of former high temperatures. It is an ore mineral for about 3% of the iron in this country, but in Scandinavia and some other countries, it plays a leading role as the source of iron.

It is found in the Adirondack Mountains, in New Jersey, Pennsylvania, Arkansas, North Carolina, New Mexico, and California.

[Siderite]
FeCO₃
Pl. [13] & [14]
Spathic iron

Occurs in fibrous botryoidal masses or rhombohedral crystals, sometimes with curved faces; hardness 3.5; specific gravity 3.8; color gray-brown; streak white; luster vitreous; translucent on thin edges.

Like hematite this mineral belongs to the hexagonal system, and crystallizes in hemihedral form, making the rhombohedron. Its faces are often curved, which is rare in minerals, only a few forms like this and dolomite having other than plane faces. When siderite crystals grow in clusters, the crowding often results in growth on one face only, making a mass of fibrous character, and in such cases the surface of the mass is botryoidal in contour. The mineral is likely to oxidize, losing its gray-brown color, and becoming limonite. In the United States it is scarcely ever used as an ore for iron, but in Germany and England a great deal of iron is smelted from this mineral.

It occurs in Massachusetts, Connecticut, New York, throughout the Appalachian Mountains, and also in Ohio.

[Pyrite]
FeS₂
Pl. [15] & [16]
iron pyrites

Occurs as cubes, octahedrons and pyritohedrons, or in compact masses, scales or grains; hardness 6; specific gravity 5.1; color brassy yellow; streak greenish-black; luster metallic; opaque on thin edges.

This is one of the commonest of all minerals. It is found in all kinds of rocks, with all kinds of associations, in all parts of the world. Its crystals are isometric, and cubes and octahedrons are abundant. The pyritohedron is also a common form, and characteristic of this mineral. It is a hemihedral form derived from a 24-sided form, i.e. the cube with four faces on each side. On this 24-sided form each alternate face has developed and the others have disappeared, resulting in a 12-sided form, known as the pyritohedron, which differs from the dodecahedron in that each of its faces is five-sided instead of rhomboidal. When in crystals pyrite can not be easily confused with any other mineral; but when in masses it is often mistaken for gold, chalcopyrite, pyrrhotite or marcasite. From the first two, the color should be sufficient to distinguish it, for they are golden yellow. Pyrrhotite is bronze yellow, and marcasite is paler yellow. Then too in hardness pyrite is much harder than any of these minerals except marcasite. This last is the one which is most likely to cause real difficulty. Its lighter color, and the fact that it usually comes in fibrous masses are the best distinctions.

In spite of being so abundant pyrite is scarcely ever used as an ore for iron, because the sulphur makes the metal “short,” or brittle, and the sulphur is not easily gotten entirely out of the iron; but pyrite is used largely in the manufacture of sulphuric acid, so important to many of our industries.

Other sulphides are commonly mixed with pyrite, such as chalcopyrite, arsenopyrite, argentite, etc.; but the most important impurity is gold, which is often scattered through the pyrite in invisible particles, and sometimes in quantities enough to make it worth while to smelt it for the gold.

Pyrite is particularly the form in which the sulphur compounds of iron appear in rocks which have been highly heated, and is to be expected in metamorphic rocks and also igneous rocks, especially in fissures and veins leading from the igneous rocks. It may occur in sedimentary rocks, but in these last it is usually marcasite.

[Marcasite]
FeS₂
[Pl. 15]
white pyrite

Occurs in orthorhombic crystals, usually grouped to make fibrous or radiating masses, or non-crystalline in masses; hardness 6; specific gravity 4.8; color pale brassy-yellow; streak greenish-gray; luster metallic; opaque on thin edges.

Marcasite has the same chemical composition, as pyrite, and looks like it, but is lighter colored and usually occurs in fibrous masses. It is the commoner form in limestones and shales, while pyrite is more likely to occur in igneous and metamorphic rocks. It seems probable that marcasite is due to a more hasty precipitation from cold solutions, while pyrite is deposited more slowly from hot solutions.

Isolated crystals of marcasite are rare; but, if formed, they belong to the orthorhombic system. Usually some form of twinning is present, and because of the multiple character of the twinning, marcasite crystals usually show a ragged outline, with reentrant angles. It is most abundant in radiated masses, which appear fibrous on the broken surfaces. It decomposes easily, taking oxygen from the air and forming, even in museum cases, a white efflorescence or “flower,” which is iron sulphate or melanterite. In moist air it takes water and decomposes to sulphuric acid which may change the surrounding limestone to gypsum. Marcasite is found wherever limestones and shales are the country rock.

[Pyrrhotite]
Fe₁₁S₁₂
Magnetic pyrites

Occurs in masses; hardness 4; specific gravity 4.6; color bronze; streak grayish-black; luster metallic; opaque on thin edges.

Tabular crystals are known, but are very rare. They belong to the hexagonal system. This form is easily distinguished from the other yellow minerals by being magnetic. It is by no means as abundant as the two preceding sulphides of iron, but does occur fairly frequently in veins in igneous rocks, and less frequently in limestones, large quantities of sulphuric acid being made from a deposit in limestone at Ducktown, Tenn. It will be found in most states. When associated with nickel it is an important source for the latter mineral, as at Sudbury, Canada. Pyrrhotite is very like a substance found in meteorites, known as troilite.

The Lead Group

After learning how to get iron from the rocks by rude smelting methods, the early peoples tried heating various rocks, and some time around 500 B.C. stumbled upon lead, which is rather easily separated from its ores. This metal was used through Roman times to make pipes, gutters, etc.

Lead is a soft metal, fairly malleable, but with little ductility, and still less tensile strength. Though one of the commoner metals, it does not occur as pure metal in Nature. It is diffused in minute quantities through the igneous rocks, and also is found in the sedimentary rocks and in the sea water. Its minerals are few, galena, the sulphide of lead, being the commonest, and at the same time the form in which lead is primarily deposited. Galena may also represent a secondary deposition. The other minerals, cerrusite, anglesite, and pyromorphite are results of modification of the galena when it lies near enough to the surface to be acted on by weathering agents, like water and air. Lead minerals are usually associated with zinc minerals, there being but few places where the minerals of the one group occur without the other. Most lead when first smelted from its ore, contains a greater or less amount of silver in it, sometimes enough so that the lead ore is better worth working for the silver than for the lead.

Lead is used in making pipes, gutters, bullets, etc., and in its oxide forms in the manufacture of paints and glass. Eighty-three parts of lead with 17 parts of antimony make type metal. Lead and tin alloy to make solder. Lead and tin with small amounts of copper, zinc and antimony make pewter. The United States produce about 20% of the world’s supply of this metal.

[Galena]
PbS
[Pl. 17]
lead glance

Occurs in cubes or cleavable masses; hardness 2.5; specific gravity 7.5; color lead-gray; streak lead-gray; luster metallic; opaque.

While there is quite a group of lead-gray minerals, galena is easily identified by its cleavage, which is perfect in three directions parallel to the cube faces. Even a moderate blow of the hammer will shatter a mass of galena into small cubic pieces. The crystals often have the corners cut by octahedral faces, and occasionally the edges are beveled by dodecahedral faces. It is not uncommon to find crystals of large size, several inches across. If galena has 1 to 2% of bismuth as an impurity, curiously enough, the cleavage changes to octahedral, but this is a rare occurrence.

Galena may occur as a primary mineral in veins associated with igneous intrusions, or in irregular masses in metamorphic rocks; but it is more often found in irregular masses in limestones, where the limestone has been dissolved, and the cavities thus formed, filled with secondary deposits of galena. It also occurs at the contact between igneous rocks and the adjacent rock, whatever this may be. Sometimes it is found in residual clays.

Among the most important lead deposits are the Cœur d’Alene district in Idaho, where galena with a high percentage of silver is mined; the Leadville, Colo., district where lead, silver and gold occur together in veins; the Joplin, Mo., district, where lead and zinc ores occur together in irregular masses in limestones; and the Wisconsin district of similar character.

When found galena is usually associated with sphalerite, argentite chalcopyrite, pyrite and calcite. It will be found in every state.

[Cerrusite]
PbCO₃
[Pl. 18]
White lead ore

Occurs in fibrous or compact masses, or in orthorhombic crystals, usually on galena; hardness 3.5; specific gravity 6.5; colorless; streak white; luster adamantine; transparent on thin edges.

While the crystals of this mineral simulate hexagonal, they are actually orthorhombic, the simple form being an octahedron with two of its edges beveled, making double six-sided pyramids (see [Pl. 18] A.) Usually prism faces are present. Twinning is common, both the simple contact sort, as shown on [Plate 18] B, and also the sort in which three crystals have grown through each other, so as to make a six-rayed crystal. The considerable weight, and the fact that it effervesces in acid serve to identify cerrusite. When pure it is colorless, but impurities cause it to appear white, gray or grayish-black, and sometimes it has a tinge of blue or green.

It is likely to occur wherever galena is found, as a secondary mineral derived from the galena. In this country it is not used as an ore, for, as in the Leadville district, veins which have cerrusite near the surface change at moderate depths, and galena takes the place of the cerrusite. It is found all down the Appalachian Mountains, and in all the Cordilleran States. Especially fine specimens have come from the Cœur d’Alene district in Idaho.

[Anglesite]
PbSO₄
[Pl. 18]

Occurs in grains and masses, or in tabular and prismatic orthorhombic crystals; hardness 3; specific gravity 6.3; colorless; luster adamantine; transparent on thin edges.

Two modes of occurrence are characteristic, one in cavities in galena, the other in concentric layers around a nucleus of galena. In the former case fine crystals are developed, in the latter the mineral is in masses. The crystals look like those of barite, but are soluble in nitric acid while the barite is insoluble. Sometimes the crystals are prismatic with pyramidal faces instead of the tabular form.

It is found in the lead mines associated with galena, and in this country is not used as an ore for lead, but in Mexico and Australia it is abundant enough to be mined as an ore. Exposed to water which has carbon dioxide in it, and most surface waters have some, it readily changes to cerrusite. It is found in Missouri, Wisconsin, Kansas, Colorado, and Mexico.

[Pyromorphite]
Pb₅Cl(PO₄)₃
[Pl. 17]
Green lead ore

Occurs in small barrel-shaped hexagonal crystals, and in fibrous or earthly masses; hardness 3.5; specific gravity 7; color green to brown; luster resinous; translucent on thin edges.

Pyromorphite is found in the upper levels of lead mines, and is formed by the decomposition of galena. Its green color (sometimes shading off toward brown), considerable weight and resinous luster, serve to distinguish this mineral. The crystal form is that of a simple hexagonal prism, with the ends truncated. It is found in Phœnixville, Penn., Missouri, Wisconsin, Colorado, New Mexico, etc.

The Zinc Group

Zinc and copper made the brass of early Roman times; but even then, zinc was not known as a separate metal, the brass being made by smelting rocks in which both zinc and copper occurred, the zinc never being isolated until much later. Some time in the later Roman times it seems to have been obtained separately, but then and all down through the Middle Ages zinc and bismuth were confused. Our earliest record of zinc being smelted, as we know it today, was about 1730 in England. In those earlier days, the product, zinc, or bismuth, or both together, were known as “spelter,” and this name has clung to zinc in mining and commercial circles; so that today, if one looks for quotations in the newspaper, he often finds zinc under the head of spelter.

Zinc, like lead, is diffused in small quantities through all the igneous rocks. In places it is segregated in fissures or veins leading from the igneous rocks, along the contact between igneous rocks and either sedimentary or metamorphic rocks, in limestones where solution cavities have been formed and later filled with zinc minerals, and as a residue where limestones have been weathered away. In all these places it is closely associated with lead.

The sulphide, sphalerite, is the primary mineral, and the other minerals, like zincite, smithsonite, calamine, willemite, franklinite, etc., are secondary, resulting from modifications of the original sphalerite. In connection with zinc minerals the region of Franklin Furnace, N. J., is especially interesting, for at that place are found two large metamorphosed deposits containing a wide range of zinc minerals, several of which are not found anywhere else.

Zinc is soft and malleable, but is only slightly ductile, and has little tensile strength. It alloys with several metals, and in this form is most useful today; three parts of copper to one of zinc making brass; four or more parts of copper and one of zinc, making “gold foil”; copper and zinc (a little more zinc than copper) making “white metal”; three parts of copper to one of zinc and one of nickel making German silver; etc. Zinc is also used in large quantities in galvanizing iron, sheets of iron being dipped into melted zinc and thus thinly coated. It is also used in batteries and a wide range of chemical industries.

[Sphalerite]
ZnS
Pl. [19] & [20]
zinc blende, black jack

Occurs in grains, in fibrous or layered masses, or in isometric crystals; hardness 3.5; specific gravity 4; color yellow-brown to almost black; streak light yellow to brownish; luster resinous to adamantine; translucent on thin edges.

When in crystals sphalerite occurs most commonly either in dodecahedrons or in tetrahedrons (hemihedral forms of the isometric octahedron). The cleavage is fairly good and parallel to the faces of the dodecahedron. The difficulty usually is to get large enough crystalline masses to see this cleavage clearly, but by examining the angles between the faces of cleavage pieces they will be found to be the same as those on a dodecahedron. When the mineral is pure, it has the color of resin, but sometimes it is reddish to red-brown, and then it is called “ruby zinc,” more often it is dark brown due to the presence of iron as an impurity. This is what the miners call “black-jack.” The presence of iron also tends to make the streak darker. The hardness, streak and cleavage will usually determine this mineral readily.

Sphalerite is the primary ore of zinc and is usually found in fissures and veins leading from masses of igneous rocks, or along the surface of contact where igneous rocks like granite or lavas come against such metamorphic rocks as gneisses, schists, or crystalline limestones. In the region of Joplin, Mo., however, the sphalerite is of secondary character, having been gathered by waters circulating through the limestones, and deposited in them in irregular pockets. This Joplin district has produced more zinc than any other in the world. The United States annually produces about 25% of the world’s supply of this metal.

Sphalerite is always associated with galena, and such other minerals as argentite, pyrite, chalcopyrite, fluorite, quartz, calcite and barite, are very apt to be present. It will be found in almost every state, especially in fissures and veins, and less frequently in cavities in limestones.

[Zincite]
ZnO
Pl. [19] & [20]
red zinc ore

Usually occurs massive, but may be found in crystals; hardness 4; specific gravity 5.6; color deep red; streak orange; luster subadamantine; translucent on thin edges.

When in crystals zincite forms in hexagonal prisms with hexagonal pyramids on the ends. This is rather rare, most of the zincite being found in massive form. The cleavage is parallel to the prism faces and perfect. The deep red color and orange streak are wholly characteristic.

This mineral is so common at Franklin Furnace, N. J., as to be an important ore, but it is very seldom found elsewhere. This district, as mentioned before, is a peculiar one for zinc minerals. The zinc beds are in a metamorphosed limestone, and into this are intruded numerous dikes of granite. Probably the zinc was originally present in the bed of limestone as smithsonite, calamine and other secondary minerals of zinc. When intruded by the hot granite the smithsonite (carbonate) may well have been altered to the oxide, zincite; while the calamine (hydrous silicate) became the simple silicate, willemite.

[Willemite]
ZnSiO₄
[Pl. 20]

Occurs in masses or in crystals; hardness 5.5; specific gravity 4.1; color pale yellow when pure; luster resinous; translucent on thin edges.

Willemite is another of the minerals which are distinctively characteristic of Franklin Furnace, and found elsewhere very rarely. It is so common there as to be one of the principal ores, and mostly occurs in irregular masses, but is also found in crystals. These are hexagonal prisms, with a three-sided (rhombohedral) pyramid on the ends. The color when pure is whitish or greenish-yellow, but with small amounts of impurities it may be flesh-red, grayish-white or yellowish-brown. When in crystals it is easily determined; but when massive it looks like calamine, and can only be distinguished by placing a bit of the mineral in a closed tube and heating it, in which case calamine will give off water vapor, while willemite will not.

This mineral is one of those resulting from metamorphic alteration and is derived from calamine, when the latter loses its water of crystallization. It is common at Franklin Furnace, N. J., and also found occasionally elsewhere, as at Salida, Colo., and in Socorro Co., New Mexico.

[Calamine]
Zn₂(OH)₂·SiO₃

Occurs as crystalline linings in cavities, or as botryoidal or stalactitic masses; hardness 5; specific gravity 3.4; colorless to white; luster vitreous.

Calamine resembles both smithsonite and willemite when in non-crystalline masses. From the smithsonite it is easily separated by the fact that in nitric acid the smithsonite effervesces and the calamine does not. From willemite it is harder to distinguish, but a piece may be placed in a closed tube and heated. If it is calamine water vapor will be given off, if willemite nothing happens. When calamine occurs in crystals these are orthorhombic and mostly tabular, and the crystals are peculiar in that the two ends are terminated differently.

Both this and smithsonite are secondary minerals and usually occur together when zinc is found in limestones. It is abundant at Franklin Furnace and Sterling Hill, N. J., and also found at Phœnixville, Penn., in Wythe Co., Va., and Granby, Mo.

[Smithsonite]
ZnCO₃
[Pl. 21]
Dry bone

Usually occurs as incrustations, grains, earthy or compact masses, and as crystals; hardness 5; specific gravity 4.4; color white, yellow, greenish or bluish; streak white; luster vitreous; transparent on thin edges.

When pure this mineral is colorless, but, as it occurs, it is usually white, or tinged with some shade of yellow, green, or blue, but in all cases its streak is white. The crystals are rhombohedrons often with edges beveled or corners cut by other faces. It resembles calamine and willemite, but is readily separated from either of these by the acid test, for smithsonite effervesces when acid is placed on it.

Next to sphalerite, smithsonite is the commonest of the zinc minerals. It is a secondary mineral, resulting from the action of lime-charged water acting on sphalerite, and so is likely to be found wherever zinc minerals occur in a limestone region. In the Wisconsin-Illinois-Iowa district it serves as a minor ore of zinc, and is termed here “dry bone.” It is also found in the Missouri and Arkansas districts, and in Europe is an important ore for zinc.

[Franklinite]
(ZnMn)Fe₂O₄
[Pl. 21]

Occurs in compact grains or masses, and in isometric octahedrons; hardness 6; specific gravity 5; color black; streak reddish-brown; luster metallic; opaque on thin edges.

This is a mineral peculiar to the Franklin Furnace region, from which it gets its name. It looks like magnetite, but its reddish-brown streak and lack of magnetism distinguish it. When it occurs in octahedrons, the edges are rounded, while those of magnetite are sharp. It is a complex and variable oxide of zinc, iron and manganese, which has resulted from the metamorphism of the beds in which it occurred probably being originally something quite different.

The Manganese Group

Though manganese was known in the mineral pyrolusite in early times, it was then thought to be magnetite or magnetic iron ore. It was not until 1774 that it was isolated and recognized as a distinct element.

Manganese is one of the lesser elements in the crust of the earth, making less than .07 of one percent, but as an alloy with other metals, especially iron, it has attained a considerable importance to man. It is used chiefly with iron, 20% of manganese making the alloy, spiegeleisen, a combination which occurs in Nature in Germany, and from 20% to 80% making ferromanganese. These alloys are in great demand because they make an especially tough steel essential in the manufacture of munitions. The sources for manganese are the oxide ores, manganite, pyrolusite and psilomelane, which have been formed as secondary minerals, as a result of the weathering of silicates which carry manganese. They occur widely enough, but throughout the United States the deposits are small, and this is one of the elements in which this country is not self-sufficient. The largest producer of manganese is Russia; however she consumes almost all of her output at home, and our supply comes from the next largest producers, India, the Union of South Africa, and the Gold Coast. A shift in trade may be expected when Brazil’s recently discovered ore body in Matto Grosso is brought into full production. Besides being used as an alloy, manganese is employed in making paints and dyes, for clearing glass, and for some types of electric batteries.

[Pyrolusite]
MnO₂

Occurs in earthy or fibrous masses; hardness 1-2; specific gravity 4.8; color black; streak black; luster dull; opaque.

Pyrolusite occurs in soft masses and incrustations, usually leaving a sooty mark on the fingers. Sometimes it seems to be in crystals, but these are pseudomorphs which have the form of manganite, from which the pyrolusite has formed as a result of the water having been driven from the manganite. Frequently pyromorphite and manganite will be found together, and in some cases the outer part of a mass or crystal will be pyrolusite, while the center is still manganite. Psilomelane is another oxide of manganese with water and may appear very like pyrolusite, but both manganite and psilomelane have much greater hardness than does pyrolusite. If there is difficulty in deciding about pyrolusite, it may be placed in a closed tube and heated. It will not be affected by the heat, while, under the same circumstances, both manganite and psilomelane will give off water vapor.

Pyrolusite usually occurs in black streaks or pockets in residual clays which have formed as a result of the decomposition of limestones. It may also occur in dendritic forms in seams and crevices (see manganite). It is found in Vermont, Massachusetts, Virginia, Arkansas, Colorado, California, etc.

[Psilomelane]
MnO₂·H₂O

Occurs in compact botryoidal or stalactitic masses; hardness 5-6; specific gravity 4.2; color black; streak brownish-black; luster metallic; opaque on thin edges.

Psilomelane is very like pyrolusite, and often occurs with it. It is distinguished by its greater hardness, and the fact, that when heated in a closed tube, it gives off water vapor. From manganite it is more easily distinguished, for it never occurs in crystals, while the manganite is usually crystalline. This and pyrolusite are the principal ores of manganese.

Wad is an impure form of psilomelane, having some iron oxide mixed with the manganese oxide, usually limonite; or the impurity may take the form of a copper, cobalt, lithium or barium oxide.

Psilomelane is found at Brandon, Vt., in Arkansas, Colorado, California, etc.

[Manganite]
Mn₂O₃·H₂O
[Pl. 22]

Occurs in prismatic crystals, or in columnar or fibrous masses; hardness 4; specific gravity 4.4; color steel gray; streak reddish-black; luster submetallic; opaque on thin edges.

This is the form taken by manganese oxide when it crystallizes in the presence of moisture, and pyrolusite frequently changes to manganite when exposed to moisture. The crystals are orthorhombic prisms, with striated sides and the ends truncated. These prisms usually occur in bundles and give the mineral a fibrous appearance. Manganite is not hard to identify, the striations on the crystals and the streak being very characteristic.

In seams and tiny crevices this mineral, and often pyrolusite, grows in a branching manner, resembling tree-like or “mossy” masses. This is termed dendritic, and the growths of manganese minerals are called dendrites. One of the most curious of these is when the “mossy” growth is inclosed in chalcedony, making the so-called moss agate. These moss agates are abundant through the Rocky Mountains and are frequently cut for semi-precious stones. The finest ones however come from India and China.

Manganite is found in the Lake Superior region, Colorado, etc.

[Rhodochrosite]
MnCO₃

Occurs in compact cleavable masses; hardness 4; specific gravity 3.5; color rose to dark red; streak white; luster vitreous; translucent on thin edges.

This usually occurs in pink to red masses which cleave readily parallel to the faces of the rhombohedron. When it is found in crystals, which are rare, these too are rhombohedrons. It is usually found in veins as a gangue mineral with copper, silver or zinc ores. Its beautiful color and the fact that it effervesces in acid serve to distinguish this mineral. It is found at Branchville, Conn., at Franklin Furnace, N. J., and in veins with silver in Colorado, Nevada, and Montana.

The Aluminum Group

Though aluminum is one of the most abundant of all the metals, making some 8% of the crust of the earth, its union with other elements is so firm, that only recently have methods been found for getting the metal free. It was first isolated in 1846, but up to 1890 the extraction of aluminum was so expensive, that it could not be widely used. About that time electrical processes were applied to its extraction, and since then the price has steadily dropped, until now it is under $.20 per pound. It is very malleable, and ductile, and has high tensile strength. Exposed to the air, water or ordinary gases, it does not tarnish; and it is very light, an equal bulk weighing about a third as much as iron. The combination of lightness and strength, and the fact that it is a good conductor of electricity, have made it available for a wide range of uses, such as electrical apparatus, delicate instruments, boats, aeroplanes, and domestic utensils.

It is an essential component of all the important rocks, except sandstone and limestone, and combines to a greater or less degree in a host of minerals. Though present in clays, shales, argillites, feldspars, and micas, it is only from bauxite that it has been successfully extracted. Aside from the small number of simple compounds of aluminum grouped here, it also takes a part in the make-up of a large series of minerals termed silicates, treated a little further on in this book.

It alloys with other metals, especially copper. The union of copper and a small amount of aluminum makes aluminum-bronze, which looks like gold and is used for watch chains, pencil-cases, etc., and also for the antifriction bearings of heavy machinery. A small amount added to steel prevents air holes and cracks in casting.

[Corundum]
Al₂O₃
[Pl. 23]

Occurs in cleavable masses or in hexagonal crystals; hardness 9; specific gravity 4; colorless, red, yellow, blue, or gray; luster vitreous to adamantine; translucent to transparent on thin edges.

Corundum is readily recognized by its hardness, second only to that of the diamond. The crystals may be simple six-sided prisms, hexagonal pyramids or combinations of the two. The cleavage is usually described as parting, for it is by no means perfect, but when it is recognizable it is parallel to the faces of a rhombohedron, and cleavage pieces may appear almost cubic.

When in clear and perfect crystals this mineral is one of the most highly prized of all the gems. Clear and colorless it is known as the “Oriental white sapphire”; when tinged with blue it is the sapphire; when colored yellow, the “Oriental topaz”; when green, the “Oriental emerald”; when purple, the “Oriental amethyst” and when red, the ruby. Sapphires range from colorless to deep blue, the value depending on the shade of the blue, and increasing as the color deepens. The Oriental topaz can easily be confused with the true topaz, which is a much commoner and less valuable gem, but can be distinguished by the hardness, topaz having a hardness of but 8. The name emerald is applied to several green gems, mostly to beryl, which is not so hard and is the true emerald. The Oriental emeralds have a value about the same as diamonds. Rubies of clear and deep color are the rarest of all gems, ranging in value about three times as high as diamonds of equal size. The most sought-for shade is the so-called “pigeon-blood red,” and the value of a stone of this sort is almost dependent on the whim of the buyer. The best of the rubies come from granites or metamorphosed limestones in Burma; the best sapphires from Ceylon, though both of these, and some of the other corundums of gem quality, have been found in North Carolina and Montana.

Around these stones, which have been used so long among the Hindus, Persians, Jews, Egyptians, and Christians, a wealth of lore has been woven. The sapphire was Saturn’s stone, and a talisman to attract Divine favor. Where tradition makes the stone on which the ten commandments were written the sapphire, it is probable that, what was really meant, is lapis lazuli, as is also the case when sapphires are mentioned as building stones for the celestial gates. The ruby in ancient lore is termed “lord of stones,” “gem of gems” etc., and so protected its wearer that he was safe from injury in peace or war.

When corundum is colored brown by impurities of iron, it is termed corundum, when black by greater quantities of iron, it is emery. These varieties are far the commonest form in which corundum occurs, and when ground to finer or coarser powder make the commercial emery. Emery is likely to be found in sands, making so-called “black sands,” where it has accumulated as a result of the weathering to bits corundum-bearing rocks. In some one of its forms, corundum is found in Massachusetts, Connecticut, New York, New Jersey, and all down the Appalachian Mountains, also in Colorado, Montana, California, etc.

[Bauxite]
Al₂O₃·2H₂O

Occurs in grains, or oolitic or clay-like masses; hardness 1-3; specific gravity 2.5; color white to yellowish-white or reddish-brown.

Bauxite never comes in crystals, but is usually in earthy masses, which have resulted from the decomposition of granitic or volcanic rocks, in circumstances where hot alkaline waters were present. This explanation seems to apply especially to the deposits in France, which were first the chief source of the bauxite, and may be applicable to those in Georgia and Alabama. Some of the other deposits, however, do not seem to have had any hot water available, and the deposit appears more like simple decomposition of the underlying rocks by alkaline waters.