Fluorine
At ordinary temperatures the element fluorine is a colorless gas, which was not obtained pure until 1888, because it could not be contained in vessels of glass, gold, platinum, etc. At that time it was made and kept in a vessel composed of an alloy of platinum and iridium. Its most important compound is hydrofluoric acid, a fuming liquid, which is mostly used to etch or dissolve glass. It occurs in several minerals, like tourmaline, turquois, etc., but the only one used to obtain the hydrofluoric acid is fluorite.
[Fluorite]
CaF₂
[Pl. 50]
Fluor spar
Occurs in crystals and cleavable masses; hardness, 4; specific gravity, 3.2; colorless or some shade of violet, green, yellow, or rose; luster vitreous; transparent on thin edges.
Fluorite usually occurs in beautiful cubic crystals, often with the edges and corners beveled by smaller faces, and occasionally in twins, which seem to have grown through each other. There is perfect cleavage parallel to each of the octahedral faces, which often, as in the illustration on [Plate 50], show as cracks cutting off the corners.
Since fluorite loses weight and color on heating, it is concluded that the colors are due to the presence of hydrocarbon compounds. The red and the green fluorite when heated to above 212° F. become phosphorescent, as may be seen if they are thus heated and exposed to the light, then taken into the dark.
Fluorite is quite commonly the gangue mineral associated with metallic ores, and is also likely to occur with topaz, apatite, etc. It is generally in such places that it seems to have been deposited from hot vapors, rising from igneous magmas.
It is the only mineral at all common from which fluorine can be obtained, and is used for making hydrofluoric acid, and other chemical compounds of this element. It is, however, of much greater importance as a flux in reducing iron, silver, lead and copper ores. In the industries it finds a place, being used to make apochromatic lenses, cheap jewelry, and for the electrodes in flaming arc lamps.
Fluorite is widely distributed, some of the better known localities being Trumbull and Plymouth, Conn., Rossie and Muscalonge Lake, N. Y., Gallatin Co., Ill., Thunder Bay, Lake Superior, Missouri, etc.
[Halite]
NaCl
[Pl. 50]
Salt
Occurs in crystals, and in cleavable and granular masses; hardness, 2.5; specific gravity, 2.1; colorless to white; luster vitreous; transparent on thin edges.
Halite is common salt, occurring in cubic crystals, with perfect cubic cleavage. Its form, hardness, taste, and solubility in water make it easy to determine.
Halite is the most abundant salt in sea water, making about 2.5% out of the total of 3.5% of solids in solution. It is also a prominent, when not the leading, salt in solution in the waters of inland lakes, like Great Salt Lake, or the Dead Sea, there being 20% of halite in the former and 8% in the latter, though the total of solid in solution in the water of the Dead Sea is greater than that in Great Salt Lake.
The great salt deposits are mostly the result of the evaporation of the water of arms or isolated portions of former oceans; the salt, gypsum, etc., left by the drying sea, having been buried beneath later sediments. Other bodies of salt represent the disappearance of ancient lakes. There are also the curious “salt domes” of Louisiana and Texas, which are immense, roughly circular, subterranean masses of salt extending to as yet unknown depths which are thought to have been formed by masses of salt from some deep source bed pushing their way upward through the overlying formations by plastic flowage. As the upthrust took place the sediments were arched into domes. Some of these domes are today important sources of rock salt.
There are extensive beds of salt under parts of New York, Michigan, Ohio, Oklahoma, Kansas, etc., which are mostly worked by drilling wells into the salt layer, then introducing hot water to dissolve the salt. The brine thus formed is pumped to the surface, and the salt recovered by evaporation in pans. During the process, skeleton crystals of salt with concave faces may form, but in Nature the crystals are uniformly solid cubes.
[Boracite]
Mg₇Cl₂B₁₆O₃₀
Occurs in small crystals or granular masses; hardness of crystals, 7; of the masses, 4.5; specific gravity 3; colorless to white; luster vitreous; transparent to translucent on thin edges.
Small crystals, associated with salt and gypsum, occur in the beds and incrustations, which result from the drying up of alkaline lakes, especially in Nevada and southern California. The crystals are orthorhombic, but appear like perfect cubes, with the edges beveled and part of the corners cut. They are not easily dissolved in water, but quickly go into solution in hydrochloric acid.
[Colemanite]
Ca₂B₆O₁₁ + 5H₂O
Occurs in crystals or compact masses; hardness, 4.5; specific gravity, 2.4; colorless to white; luster vitreous; translucent on thin edges.
The crystals when they occur, are monoclinic; but usually colemanite is a bedded deposit, which has resulted from the drying up of a saline lake. It was first found in Death Valley, Cal., in 1882, then near Daggett, Cal., and since then in several similar locations in Nevada and Oregon. The deposits are of all grades of purity, the colemanite being mixed with varying quantities of mud. Today this mineral is the chief source of borax, which is used in medicines, cosmetics, colored glazes, enamel, and as a preservative.
[Borax]
NaB₄O₇ + 10H₂O
Occurs in crystals or in powdery incrustations; hardness, 2; specific gravity, 1.7; colorless to white; luster vitreous; translucent on thin edges.
The crystals are tiny and monoclinic, this mineral being usually obtained by the evaporation of the saline waters of such lakes as Clear and Borax Lakes in southern California, or from the muds of salt marshes, like Searles Borax Marsh in California. Originally most of our borax came from a large saline lake in Tibet, but now most of it is obtained from colemanite. Borax is soluble in water, giving it a sweetish taste.
[Sulphur]
S
[Pl. 51]
Occurs in crystals, incrustations or compact masses; hardness, 2; specific gravity, 2; color yellow; streak yellow; luster resinous; translucent on thin edges.
Aside from the numerous compounds, such as the sulphides of the metals like pyrite, galena, sphalerite, etc., and the sulphates, like gypsum, barite, anglesite, etc., sulphur occurs in its elemental form in Nature. In this case it may be in crystals, which are orthorhombic and usually occur as octahedrons, with the upper and lower ends truncated, either by a basal plane, or by a lower octahedron, or by both. Incrustations and compact masses are, however, much the commoner mode of occurrence. The incrustations are found mostly about volcanic regions, where the sulphur has risen from the molten lavas as a sublimate, and on cooling has been deposited in crevices or on the adjacent surfaces. Irregular masses of sulphur are often found where sulphide minerals, like pyrite or galena have been decomposed in such a way as to leave the sulphur behind. The extensive beds of sulphur are usually associated with gypsum, and are thought to be the result of water, containing bituminous matter, so acting on gypsum as to remove the calcium and oxygen as lime, and leave the sulphur. Finally many waters carry sulphates in solution, from which the sulphur may be precipitated by certain sulphur bacteria, making thus incrustations on the bottom of ponds or lakes.
Sulphur is used for making matches, gunpowder, fireworks, insecticides, in medicine, vulcanizing rubber, etc. It is widely distributed, however, most of the present world’s production is from deposits associated with the “salt domes” of Texas and Louisiana. A “caprock” of gypsum and anhydrite overlies many of these which often contains elemental sulphur. Wells are drilled into this, and the sulphur is melted by the introduction of hot steam. This melted sulphur is then pumped to the surface and run into molds.
Some of the best known localities are Sulphurdale, Utah, Cody and Thermopolis, Wyo., Santa Barbara Co., Cal., Humboldt Co., Nev., and about the hot springs of the Yellowstone Park.
[Ice]
H₂O
[Pl. 51]
water
Occurs solid as ice, snow and frost, or liquid as water; hardness, 2; specific gravity, .92; colorless to white; luster adamantine; transparent on thin edges.
Though we seldom think of ice, and its liquid form, water, as a mineral, still it is one, and perhaps the most important of all minerals, as well as the most common. Ice melts at 32° F. and vaporizes at 212° F., being then termed steam. Because it is so common and liquid at ordinary temperatures it acts as a solvent for a host of other minerals, and is therefore the agent by which they are transported from place to place and redeposited in veins and beds.
Not only does water act as a transportation agent for minerals in solution, but is also the agent of erosion and weathering. Water vaporizes slowly when exposed to the air at all temperatures above freezing, and so it is slowly rising from the surface of the sea or lakes or moist ground into the air, where it would accumulate until the air was saturated, if the air would only keep still and at a uniform temperature. The air will hold a given amount of water vapor, which is, for example, 17 grams per cubic meter when the temperature is 68° F., but at 59° F. it will hold only 12½ grams, or at 50° F. only 9 grams. Thus the air is more or less completely saturated at higher temperatures, and when the temperature is lowered the air can not hold all it has taken up, and it is precipitated in dew, rain or snow, most often as rain. When the rain falls it mechanically carries away, and more or less slowly transports to other places particles of rock, being thus the agent of erosion; and when it is slowed down, as on entering the quiet water of a lake or the sea, it drops the mechanically carried sediment and makes sedimentary deposits.
Another very important and unique feature of water is that on freezing it expands about ¹/₁₁th of its former bulk, so that, as a result, ice floats, and also wherever water in crevices is frozen, the crevices are enlarged. In locations where this freezing and melting take place repeatedly throughout a year, there the breaking up of rocks is rapid.
This is hardly the place to take up a complete discussion of water, but its action as a solvent, mechanically, and in freezing, melting, and vaporizing is the basis of a large part of the study of geology.
When water crystallizes, as in forming ice, it is in the hexagonal system. It tends to twinning and a snow-flake is made up of a large number of twinned crystals, each diverging from the other at 60°. When ice is formed in the air or on the surface of water it forms these complex and beautiful multiple twins, of which but a couple are suggested here. Beneath the surface the hexagonal crystals grow downward into the water, parallel to each other, making a fibrous structure, which is very apparent when ice is “rotten,” which is the time at which the surfaces of the prisms are separating, because the molecules leave the crystal in the reverse order to which they united with it. Frost in marshy or spongy ground will often show this fibrous growth beautifully.
CHAPTER IV
THE ROCKS
Broadly speaking a rock is an essential part of the crust of the earth, and includes loose material, like sand, mud, or volcanic ashes, as well as compact and solid masses, like sandstone and granite. Rocks are aggregates of minerals, either several minerals grouped together, as are mica, quartz and feldspar to make granite, or large quantities of a single mineral, like quartz grains to make sandstone.
The rocks are most conveniently classified according to their mode of origin, into three main groups, igneous, sedimentary, and metamorphic. The igneous rocks are those which have solidified from a molten magma, like lavas, granites, etc. The sedimentary rocks are those which represent accumulations of fragments or grains, derived from various sources, usually the weathering of other rocks, and deposited by such agents as water, wind and organisms. Metamorphic rocks are those which were originally either igneous or sedimentary, but have been altered by the actions of heat, pressure and water, so that the primary character has been changed, often to such an extent as to be obscured.
Rocks once formed in any of the above ways are being constantly altered in character by the various processes of nature. Those exposed on the surface are weathered to pieces, and the fragments are transported by wind or water to accumulate elsewhere as sedimentary rocks. Those buried deep beneath the surface are affected by the high temperature and pressure of the depths of the earth and thus metamorphosed. For instance a granite exposed on the surface is slowly weathered, some parts being carried away in solution by the rain water, others less soluble remaining as grains of quartz, mica or kaolin. These are transported by water and sorted, the finer kaolin being carried to still and deep water, the quartz and mica accumulating in some lowland as sand. This sand will in time be cemented to a sandstone, later slowly buried beneath the surface. If buried deep it will feel the effect of the interior temperature, which increases as one goes down at the rate of one degree F. for every 50 feet. If this should be in a region where folding and mountain-making takes place, the material under the folds would be melted (because of the relief from pressure which would permit the high temperature to act freely) and become igneous rock, either coming to the surface as lava, or remaining below the surface and making a granite or similar rock; while the sedimentary material not melted but near enough to the molten material to be affected, would be metamorphosed, in this case to a quartzite. Much of the interest and profit in studying rocks, will come from the understanding which they will give as to the history of that particular part of the earth’s crust where they are found.