THE MINERALS
The vein minerals, which are deposited in conduits for hot spring water, commonly possess attractive crystal forms; they include barite, quartz and amethyst, fluorite, calcite, datolite, galena, sphalerite, pyrite and others almost too numerous to list. Almost equally attractive crystals may be obtained from some metamorphic rocks, in which they have formed as heat and pressure abetted the growth of certain minerals at the expense of their less favored fellows; this group contains garnet, kyanite, chlorite, amphibole, epidote and many others. Less spectacular are the minerals resulting from the decay of rocks by percolating surface water, such as kaolin, limonite, some calcite, and the bright-colored copper carbonates. Two additional types of minerals are formed as the result of normal sedimentary and igneous processes, which will be described at length in connection with these two kinds of rocks. So, after the rock specimens are sorted from the minerals, the latter may profitably be arranged into five groups:
1. The Vein Minerals. 2. The Minerals of Pegmatites and Igneous Rocks. 3. The Minerals of Metamorphic Rocks. 4. The Minerals of Soils and Rock Decay. 5. The Minerals of Sedimentary Rocks.
The Vein Minerals
The mineral list which follows is far from complete; it contains only those minerals which are most commonly found in casual visits to the localities discussed in connection with the local tours of the Connecticut Valley. Additional species are listed and described in any textbook on mineralogy.
QUARTZ is a hard, white or colorless mineral which will scratch glass easily. In the technical language of the crystallographer, crystals are hexagonal or six-sided prisms, terminated by hexagonal pyramids; and the six flat faces which make the sides, together with the six triangular faces which form the apex, are readily recognized. Massive forms break with a curved or conchoidal fracture and were used by the Indians to make arrow-heads. The mineral is very abundant in all the lead veins and trap quarries; and in some of the latter, specimens of the purple variety of quartz, amethyst, are common. A black, smoky variety has been discovered in the pegmatite dikes of the highlands. Chemically quartz is the dioxide of silicon (SiO₂).
CALCITE breaks along three smooth surfaces or cleavage planes. Each surface is rhomb-shaped, and the six rhombic faces fit together into a characteristic rhombohedral form. A knife will scratch the mineral easily. Calcite is abundant in the white veins of the trap quarries and is the principal constituent of the crystalline limestones in the Hoosac Valley between North Adams and Pittsfield. Calcite is a carbonate of lime (CaCO₃).
BARITE resembles calcite because it can be scratched with a knife and has three smooth cleavage planes. It differs in having one cleavage perpendicular to the other two, which intersect at angles of 78°. The mineral is more than four times the weight of an equal volume of water, and it feels heavy. It is found in the lead veins at West Farms, Hatfield and Leverett. In large quantities it has commercial value as a source of the element barium, for it is the sulphate of barium (BaSO₄).
GALENA is the chief metallic mineral in the veins at Leverett, Hatfield and Loudville. It is very heavy and has a metallic gray color; it breaks into perfect cubes. A knife scratches it easily and crumbles it to a black powder. The mineral is a lead sulphide (PbS).
SPHALERITE is a lustrous, resinous brown mineral in these same veins. It cleaves into multi-faced fragments and is softer than a knife. Chemically it is the sulphide of zinc (ZnS).
PYRITE is the deceptive golden-colored, metal-like mineral which has earned the name of “fool’s gold.” It will scratch glass, and it crushes to a black powder. The materials in it are iron and sulphur (FeS₂).
CHALCOPYRITE resembles pyrite but will not scratch glass and has a greenish yellow color. It is a compound of copper, iron and sulphur (CuFeS₂).
The veins in the Connecticut Valley region contain many other minerals, among which must be mentioned datolite, natrolite, apophyllite, thomsonite, fluorite and babbingtonite in the lavas; and siderite, rhodochrosite, rhodonite, wulfenite and pyromorphite in the older veins of the highlands.
Minerals of Pegmatites and Igneous Rocks
The minerals found in pegmatites are legion. More than thirty can be collected on any trip to Collins Hill near Portland, Connecticut, or to the Ruggles Mine near Grafton Center, New Hampshire. Only the minerals appearing most commonly in pegmatites are described, but a list of others is appended as an aid in consulting a textbook. Igneous rocks contain practically the same suite of minerals as pegmatites, but in smaller grains.
MICROCLINE is a white to flesh-colored feldspar with two almost perpendicular cleavages. It will scratch glass or a knife. One cleavage face shows a grid of translucent and transparent lines intersecting at 90°.
ALBITE is the second most abundant feldspar. It is white and may generally be recognized by its two cleavage surfaces at 86°. Its growth may be likened to piling a series of plates with their surfaces parallel to one of the cleavages; during growth the plates are laid alternately face up and face down, so that the 86° cleavage edges zigzag in and out, forming a surface which, on the average, is perpendicular to the growth cleavage surface. The separate plates can usually be detected as fine bands or striations. The mineral scratches either glass or a knife.
MUSCOVITE is the white mica found in tabular crystals that can be cleaved into flexible and elastic sheets. It can be scratched and cut easily with a knife or shears.
BIOTITE is an amber-colored to black mica. Like muscovite it is flexible and elastic, but it is slightly more brittle.
TOURMALINE crystals occur in triangular prisms with the corners bevelled so as to give them a rounded appearance. They lack cleavage, are very brittle, and will scratch glass. Black is their usual color, but red and green varieties are present in many pegmatites.
SPODUMENE crystals are white to pale rose in color, and they occur as flattened prisms with bevelled corners. They cleave parallel to the surfaces bevelling the corners. The mineral is much harder than a knife, and the cleavage surfaces have a lustrous, slightly satiny appearance.
RADIOACTIVE MINERALS occur in many pegmatites and metamorphic rocks of this region. The species which have been formed as a result of recent alteration are brilliant golden or green encrustations in cracks or on a pitchy-black nucleus. The most abundant ones are uranite, autunite and torbernite. Older primary or source minerals are pitchy-black and are surrounded by a narrow rusty red zone or “halo,” ¹/₁₆ to ⅛ inch wide; an elongate species resembling a rusty hand-made nail is allanite; the more pitchy, irregular-shaped mineral is usually uraninite or pitchblende.
Other minerals found in pegmatites in the Connecticut Valley region include beryl, apatite, zircon, garnet, fluorite and lepidolite.
Most of the minerals in normal igneous rocks are too minute to be recognized easily, but a few have distinctive characteristics which serve to identify them. QUARTZ is a hard, dark, glassy-looking mineral without cleavage. ORTHOCLASE and MICROCLINE feldspar are hard, flesh-colored (occasionally white) minerals with flat cleavage surfaces. The minerals making the white lathlike mosaic on the weathered surface of the Range at the Mount Holyoke House are LABRADORITE feldspar. They are about ¼ inch long and ¹/₅₀ inch thick—too small to permit testing by ordinary physical methods, although unweathered pieces have essentially the same physical properties as orthoclase and microcline. The MICAS are flaky and reflect light like minute pieces of tinfoil; muscovite is white, and biotite is amber-colored to black. CHLORITE resembles mica but is less lustrous and is dark green.
Some minerals of igneous rocks do not appear in pegmatites. Among them is OLIVINE, which has almost the same color as chlorite but is harder than a knife and is massive or granular. It is commonly associated with massive green SERPENTINE, which is softer than a knife. These three minerals are especially abundant in rocks found in the vicinity of Blandford, Massachusetts, and Dover and Chester, Vermont.
AUGITE is a dark brown to black pyroxene which occurs between the mosaic of whitish labradorite feldspar prisms in the weathered diabase near the Mount Holyoke House.
AMPHIBOLE crystals are dark green to black, “match-shaped” crystals. They have almost the same hardness as a knife and are characterized by two cleavages parallel to their length and intersecting at 56°. The mineral is also abundant in metamorphic rocks and is frequently reported as a “fossil fern” from ledges at Charlemont and Shelburne Falls.
Minerals of Metamorphic Rocks
The principal minerals of metamorphic rocks include many which are likewise present in pegmatites and igneous rocks, such as microcline, albite, quartz, muscovite, biotite, amphibole, serpentine and tourmaline. But there are others which are more exclusively metamorphic:
GARNET occurs in twelve- or twenty-four-sided red crystals. It is much harder than a knife. The geometric form is diagnostic, and crystals up to an inch thick are obtainable in Plainfield, Massachusetts, and at Grafton, Chester, and Gassetts in Vermont. They occur in a muscovite schist, in which the muscovite flakes are wrapped around the individual crystals.
TALC is a white to pale-green mineral found around the margins of intrusive rocks that are rich in olivine and serpentine. It is foliated and is so soft that even solid masses will rub off on cloth. It is present in the green marble quarry near Westfield.
KYANITE is a sky-blue, bladed mineral, with two excellent cleavages at nearly 90°, and a good smooth fracture at almost 90° to both. One face is harder than a knife and the other two are softer. It is very abundant in the country rock southeast of the Westfield marble quarry.
The Minerals of Soils and Rock Decay
Aluminous minerals decay to KAOLINITE, and those with a high iron content alter to LIMONITE. Both these products of decomposition form a sticky paste in their original forms. Kaolinite is white to yellow, and limonite or ochre is yellow to orange. Limonite also appears in orange-colored or brown balls, in icicle-like masses, and in thin beds. Specimens have approximately the hardness of a knife. Quartz does not decay easily and remains behind in solid granules.
The Minerals of Sedimentary Rocks
Most sedimentary rocks are formed by the cementation of deposits of transported waste, derived from older materials. They may contain anything. The minerals which undergo rapid decay break down to limonite, kaolinite and quartz, leaving only the more resistant varieties, which include, in order of decreasing resistance, quartz, microcline, orthoclase, albite and muscovite. Less abundant constituents are garnet, tourmaline, zircon and magnetite.
Certain kinds of sedimentary rocks may be formed through other agencies—for example, limestone, which is composed of calcite, initially precipitated by lime-secreting organisms or by the evaporation of lime-charged waters. The effects of organic activity may be seen in the limestone near Bernardston, but most of the calcite now present in the rocks of western Massachusetts is of vein or metamorphic derivation. Salt (halite) and gypsum are formed by the evaporation of saline waters, but only the vacated casts of salt crystals have been detected in the Triassic sediments of the valley.