Mineral and Rock Collections

Travelers are inveterate collectors of mementos, and those who travel up and down and across the Connecticut Valley and who delve into its geologic history may well be interested in gathering records of its past. The best records are not in notes or printed pamphlets—not even in this volume on the subject; they are to be found imprinted in the rocks and minerals themselves. But the value of records is measured solely by their utility, and utility is achieved by systematic arrangement. So the authors will venture a few suggestions on collecting and arranging the minerals and rocks which are present in the valley and in the bordering uplands.

One mineral may come from a vein, which is the record of a fissure beneath a hot spring; another comes from a dike, which was a molten igneous rock. This specimen is a conglomerate or consolidated gravel washed into place by an ancient stream; that is a slate which was transformed from clay by intense squeezing and shearing. And if these four specimens were to constitute the nucleus of a collection, the need for classification is apparent. The first two are minerals, which are substances of limited chemical composition and well defined physical properties. The last two are rocks, which are aggregates of minerals or of mineral grains. And the minerals may be further classified according to their separate modes of origin. So, too, with the rocks. Their mineral composition indicates some of the conditions which existed where the minerals originated; the shapes of the mineral grains reveal the process which moved them to their present site; and the arrangement of grains discloses the conditions existing during aggregation at this new locality. Mineral make-up, size, shape, and arrangement of the grains provide means of recognizing major rock varieties—namely, sedimentary, igneous and metamorphic types,—and also of reading each rock’s history.

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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 108 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, 109 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.

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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.

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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 112 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.

THE ROCKS

Rocks record three distinct methods which nature employs in the aggregation of minerals. The sedimentary rocks register the work of wind, water and ice. Deposits left by wind and water are generally stratified or bedded, and they, together with glacial deposits, are composed of fragments which touch one another and are cemented at the points of contact. Igneous rocks record the solidification of hot liquids which injected themselves into older rocks or filled crevices, and which, upon cooling, formed masses of closely fitting crystals. The third group includes types which are crystalline like the igneous rocks, and which may be laminated somewhat like the sediments; they show effects of heating and squeezing until their original forms and even their minerals were changed. These are the metamorphic rocks.

Anyone who wants an orderly record of geologic history will arrange his rocks into these three groups—the sedimentary, the igneous, and the metamorphic. In the Connecticut Valley the metamorphic rocks reveal the ancient phases of earth history, and the 113 sediments contain the details of younger or later geological episodes. The igneous rocks have a wider historical range; and, like the other types, they record a long period of violence and upheaval which seems out of harmony with the placid countryside for which they now provide a solid foundation.

The Sedimentary Rocks

The sedimentary rocks are built from the disintegrated wreckage of older ones. The products of rock decay are picked up and dragged, or carried in suspension or solution, by wind, running water, or moving ice. They are deposited when and where the transporting agent can no longer function. Such rocks are usually layered because the transporting power of the carrying agent fluctuates. Bands of one kind of material, separated by dissimilar materials above and below, are called beds.

The bedded or stratified rocks of the Connecticut Valley vary greatly, from the coarse bouldery deposits in Mount Toby to the fine-textured, red and black laminated beds at Whittemore’s Ferry. Conglomerate, arkose, graywacke, shale and even limestone are represented, but there is little true sandstone. Sandstone is an even-textured, granular rock, most commonly composed of cemented quartz grains. Its uniformity of grain-size and composition reflects prolonged weathering of the original rock and good sorting of the fragments as they were transported to their new resting place. The sequence of exposure, transportation and deposition was too rapid in the ancient Connecticut Valley to permit appreciable decay and sorting; hence sandstones are absent. Limestones and salt beds are likewise rare, but the metamorphosed limestones which are found in the western highlands and in the Berkshire valley demonstrate that limestone-forming processes played a significant, if intermittent, part in the history of the region.

CONGLOMERATE is consolidated gravel. Pebbles and boulders of all sizes are packed together by the stream which was moving them, and 114 the spaces between the larger fragments are filled with the sand that settled in from the stream bed. The entire mass is cemented by silica, limonite, carbonates or some other substance deposited by percolating ground-water. The Devil’s football near the Mount Holyoke House is a famous piece which was dislodged from the hillside above; and excellent specimens may be collected on Mount Toby, on Mount Sugarloaf, and in the cut at Mount Tom Junction.

ARKOSE resembles conglomerate, but the individual grains consist of mineral fragments, among which reddish feldspar is prominent. Quartz and mica may be present, too; and all the pieces are characteristically angular, commonly ranging from ¹/₁₆ to ⅛ inch in size. The rock is red and crumbles easily. Beds of arkose alternate with conglomerate on the steep sides of Mount Sugarloaf.

GRAYWACKE is light to dark gray in color, and the fragments composing it are sand size pieces of older rocks. A few mineral grains, such as quartz, may be present, but mica is rare. Graywacke occurs interbedded with arkose in some parts of the valley.

SHALE is a thinly laminated sediment composed of microscopic quartz, feldspar, mica and kaolinite grains. Most shales in the Connecticut valley were deposited as muds in old lake beds. Some are red and record ephemeral pools, but others show from their black color, their coal layers, and their fish skeletons, that the water bodies in which they accumulated remained in existence for a comparatively long time.

LIMESTONE is a rock composed of calcium carbonate, and it consists essentially of an aggregate of calcite crystals or calcite fragments. It will give off gas bubbles in a very dilute solution of hydrochloric acid, and it exhibits other properties peculiar to the mineral calcite. A thin, sandy limestone bed has been identified in several sections of Holyoke.

The Igneous Rocks

Igneous rocks were once molten, and in this hot fluid state some were extruded at the surface as lava flows. Congealed flows reveal the motion, which brought them to their present resting places, in the banding and streaks that are so evident in the patterns of steam holes and minerals; but their massive structure bears witness to stagnation as they hardened. Other molten masses insinuated themselves into underground openings, where they solidified as intrusives, 115 varying in size from small dikes less than an inch wide, to huge masses that can be measured in miles in any direction. Most of the igneous rocks in the highlands of western and central Massachusetts are massive intrusive types; light-colored varieties predominate, but some dark-colored dikes cut the older rocks both east and west of the valley. Dark-colored, massive and banded lavas are conspicuous in the ranges within the valley.

Igneous rocks may be divided into three general groups for practical classification, and each major group may be further subdivided. Rather conveniently each of the major groups may be recognized by the prevalent color of its rocks—whether dark, medium-colored, or light. And within each major classification there may be flows, characterized by banded structures and fine textures; small intrusives composed of well formed crystals in a fine-grained groundmass; and large intrusives consisting of goodsized, equi-granular crystals. Not all of these types can be found in central Massachusetts, but the variety of igneous rocks is surprising and offers some excellent possibilities for the collector.

The Dark Rocks

The dark rocks owe their color to iron-bearing minerals like olivine, pyroxene (augite), amphibole and biotite. All of these minerals weather to a rusty red surface, which is typical of their outcrops at many places.

BASALT is a black rock, dense in some places but perforated with bubble holes or vesicles, at others. It occurs throughout the length of the Holyoke, Tom and Pocumtuck Ranges; and fragments of basalt are abundant in the Granby tuff and agglomerate.

DIABASE resembles basalt but is distinguished by the thin, short crystals embedded in it. These crystals of labradorite feldspar resemble pieces of clipped thread, and they sparkle in reflected light. Almost all dark-colored dikes and the slowly cooled central portions of thick lava flows consist of diabase.

PERIDOTITE is a dark green, coarse, granular rock composed of 116 olivine and subordinate amounts of pyroxene. It occurs near Westfield and Blandford, and at many places in Vermont.

The Medium-Colored Rocks

The medium-colored rocks contain approximately the same proportions of light- and dark-colored minerals. The dark iron-bearing minerals are relatively stable, but the light-gray feldspars decompose to kaolin and give the weathered rock a chalky white surface. Surface flows of this group are unknown in central Massachusetts, but the coarsely granular intrusives are well represented.

GRANODIORITE PORPHYRY is a greenish-gray rock occurring in many dikes in the western highlands. It has rectangular crystals of andesine feldspar up to ⅛ inch across, and these have a dull porcellaneous luster. A few dark-green amphibole crystals are only slightly smaller. Both feldspars and amphiboles are embedded in a very fine-textured, pale greenish groundmass.

GRANODIORITE is a gray equigranular rock containing flesh-colored microcline feldspar, white andesine feldspar, greenish flakes of chlorite, needles of amphibole and sparse grains of brown biotite. All crystals are about ¹/₃₂ inch thick and commonly display a parallel arrangement. This rock forms huge irregular masses at Williamsburg, Whately and Belchertown.

The Light-Colored Rocks

The light-colored rocks are well represented by dikes and large masses but not by recognizable surface flows in central Massachusetts. Their exposures have rarely weathered much, because the predominant minerals are quartz, microcline, orthoclase and albite, which resist decay.

QUARTZ PORPHYRY is a light gray rock that is found in dikes. It has porcelain-white cleavable feldspars up to ⅛ inch thick, and dark glassy quartz of equal size in a granular mass of very fine-grained crystals. Intrusives of this type are numerous in the vicinity of Whately.

GRAY ALBITE GRANITE occurs in many dikes and small irregular masses throughout the highlands. All crystals have approximately 117 the same size and rarely exceed ¹/₃₂ inch in thickness. They consist of white orthoclase and albite, dark sugary quartz, and brown to black biotite.

RED MICROCLINE GRANITE is found in very large, irregular intrusives in the highlands. The crystals are ¹/₁₆ inch or more in thickness. The red color is due to the flesh-colored microcline. Quartz is dark and glassy, and muscovite is the typical mica.

The Metamorphic Rocks

Metamorphic rocks were once sedimentary or igneous rocks which have been changed by intense pressure, by heat, or by solutions moving through them. Pressure usually produces a sheeted or foliated structure along which the rock exhibits a tendency to part—somewhat like the pages in a book that was bound before the ink was dry. Percolating solutions may produce chemical alterations in the original materials and even crystallize new substances along the foliated surfaces within the rock, much as water circulating through cooled soil may solidify to ice and cause heaving. Many of the rocks in the highlands bordering the Connecticut Valley are highly foliated or banded in consequence of the mechanical deformation they suffered when the ancient upland mountain system was created. They include the slates, schists and gneisses. A few massive types, like marble, serpentine and soapstone, owe their origins chiefly to the effects of heat or of the hot, chemically charged solutions which permeated them.

SLATES are fine-grained rocks characterized by flat, parallel cleavage surfaces which usually cross the original sedimentary structure. They were formed from shales, by shearing and compression during ancient mountain-making movements. Slates crop out beside the station platform at Brattleboro, Vermont, and at many places southward along Federal Highway 5 to Greenfield.

SCHIST is foliated, too, but it is composed largely of cleavable minerals, such as chlorite, muscovite, biotite and amphibole, which are distributed along the cleavage surfaces. These minerals result from the chemical activity of hot solutions circulating along a slaty cleavage, re-crystallizing 118 old materials, and bringing in new to make these coarse mineral flakes. The schist receives its specific name (biotite schist, chlorite schist, etc.) from the mineral which accentuates its cleavage structure.

A few schists contain large crystals which bulge the schistose surfaces outward around them. Garnet is characteristic in this role, and a muscovite schist with garnets in it is called a GARNETIFEROUS (or garnet-bearing) MUSCOVITE SCHIST. Other minerals with occurrences similar to the garnet are microcline, albite, staurolite, amphibole, tourmaline, pyrite and magnetite.

GNEISS is a banded rock containing cleavable minerals, but it lacks the cleavage structure of schist. The cleavable minerals (biotite, muscovite, amphibole, etc.) may give the gneiss its specific name, but as often as not, the name is derived from the whole mineral assemblage, or from an assumed origin, as in the case of granite gneiss. As in the igneous rocks, the mineral ensemble is held together by interlocked quartz and feldspar grains. Black-banded biotite gneiss and hornblende gneiss are the most abundant varieties in the neighborhood of the metropolitan reservoir east of Pelham.

MARBLE is a granular rock composed of calcite crystals. It is formed when heat volatilizes the bituminous coloring agents of ordinary limestone and simultaneously causes enlargement of the calcite grains. It is the principal rock in the Berkshire Valley in which North Adams, Adams and Pittsfield are located.

OPHICALCITE is a lime-silicate rock. It is formed by the chemical reactions of hot solutions on limestone or marble at considerable depth within the earth. The original calcite is converted into diopside, garnet, vesuvianite and tremolite, forming a rock that may be massive, or which may preserve some of the original bedded structure. It is found in association with the crystalline limestone and magnetite at the old iron mine, located one mile north of Bernardston.

SERPENTINE is a dark-green rock made almost exclusively of the mineral serpentine. It results from the reaction of hot solutions on olivine and pyroxene rocks (peridotites). Serpentinite is present in the Westfield marble quarry and at Zoar on the south side of the Deerfield River.

SOAPSTONE is composed principally of talc. It, too, results from the chemical activity of hot solutions ascending through serpentine and causing the mineral transformation. Bodies of this material are associated with the serpentinite at Westfield and Zoar, and northward in 119 sections of Vermont. It is mined for talc, but in colonial days it found many uses. The colonists used cross-cut saws to make blocks for foot warmers in their sleighs, to control the heat in the old wood-fired ovens and to make water pipes before iron and lead were available in adequate quantities. Many soapstone articles may be seen—and purchased—in Wiggins Country Store and in other good antique shops through the valley. One of the most primitive Indian cultures in this region utilized soapstone pots, and exhibits are on display at both the Springfield Museum of Natural History and the Amherst College Museum.

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