96 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
crystals,
1 to 2 feet long, are very common. Some masses weighing half a ton are
almost purely mica. All of the mica shows one or more of the defects
known as twinning, wedge structure, and ruling. None of it will yield
any plate mica. Several of the mica books observed were 1 foot thick
(at right angles to the cleavage). Near the walls of the pegmatite mass
the mica books tend to orient themselves with their long axes
perpendicular to the contact, though only within 6 inches or so of the
wall is there any noticeable decrease in the coarseness of the
pegmatite. The quartz of this pegmatite is mostly opaque but is pure
white. Spodumene is unusually abundant in long flat crystals, some of
them 2-1/2 feet long and 3 to 4 inches thick. The color is light gray to white. Some of the spodumene is intimately intergrown with quartz.
A
remarkable feature of this deposit is the presence in the pegmatite of
irregular masses of medium-grained granite, which in some parts
consists of muscovite, quartz, and plagioclase, and along certain
bands or irregular bunches is one-third to one-half bright pink
tourmaline, producing a stone of considerable beauty. Under the
microscope the principal minerals are seen to be quartz, muscovite,
pink tourmaline, and basic oligoclase (extinction angles up to 17°:
refractive index near balsam). In the thin section only very faint
pleochroism is seen in the tourmaline. Tourmaline constitutes the
largest crystals in the rock and shows a tendency toward the
development of radiate bundles, one-eighth to one-fourth inch across,
made up of small prisms. The average size of grain, exclusive of the
tourmaline crystals, is from 0.3 to 0.6 millimeter. This granite is
plainly a crystallization from the pegmatite magma and, like the
pegmatite, numbers quartz, muscovite, and pink tourmaline among its
chief constituents. Many large spodumene crystals are embedded in this
tourmaline granite. Its quantity and uniformity are not sufficient to
give it any commercial importance.
In
the pegmatite, greenish-black tourmaline occurs in crystals averaging
one-half inch to 1-1/2 inches in diameter and 4 to S inches in length.
They are commonly associated with quartz or clevelandite and only
rarely are in contact with muscovite, being rare in the more micaceous
parts of the pegmatite. Pink to gray opaque tourmaline also occurs,
generally surrounded by quartz. One aggregate exposed in a loose quartz
fragment is 7 inches long. It is a brush-shaped aggregate of tourmaline
crystals and enlarges from a diameter of about 2-1/2 inches at the base to about 4 inches at the top, the cross section being nearly circular.
Most
of the schist exposed near this mine is somewhat weathered. Noticeable
contact metamorphism, though confined to the immediate vicinity of the
pegmatite, has been more severe than along most of
