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 (SiO_2).
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 (CaCO_3).
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 (BaSO_4).
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
(FeS_2).
CHALCOPYRITE resembles pyrite but will not scratch glass and has a
greenish yellow color. It is a compound of copper, iron and sulphur
(CuFeS_2).
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/_1_6 to 1/8
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/_5_0 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.