22 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
Muscovite phase.—A
few deposits showing local increases in the proportion of muscovite
have been worked for this mineral in the past, but have not been
commercially successful.
TEXTURE.
The
pegmatites show remarkable differences in coarseness, some, especially
the narrower dikes and sills, being but little coarser than
medium-grained granites, though differing strikingly from the latter in
texture, and others containing single crystals of nearly pure feldspar
20 feet across and single bend crystals the diameter of a hogshead. The
majority of the deposits are nearer the lower limit than the higher.
Only the coarsest deposits are commercially valuable for their
feldspar, quartz, mica, or gem minerals, and these constitute a
relatively small percentage of the total mass of pegmatitic material
present in any district. In most of the pegmatites worked commercially
the feldspar and quartz crystals will not average more than 4 or 5 feet
in diameter.
Irregularity of grain.—The
most striking characteristic of the texture of the pegmatites, with
the exception of the graphic intergrowths described below, is their
irregularity. Typical granites show considerable uniformity in the
size of grains of the same mineral species, but the pegmatites show no
such regularity, a feldspar crystal, for example, being as likely to be
two or three or even ten times as large as an adjacent crystal as to be
of equivalent size.' This feature is shown on a microscopic scale in
Plate II.
Graphic granite.—Most
of the pegmatites contain much graphic granite, formed by an intimate
intergrowth or interpenetration of large single crystals of quartz and
feldspar. In certain directions through these intergrowths the quartz
forms an angular pattern somewhat resembling the cuneiform inscriptions
of the ancients. (See PL XVIII.) Fine-grained phases pass in the most
gradual manner into coarser graphic granite (PI. VII); and the latter,
by decrease in the percentage of quartz, may pass into masses of pure
feldspar, or by decrease in the percentage of feldspar into masses of
pure quartz. Much of the material mined as "spar" is coarse-grained
graphic granite containing from 10 to 20 per cent of free quartz.
On
casual inspection the coarser types of graphic granite appear to
contain a somewhat larger proportion of feldspar than the finer-grained
types. Chemical analyses of graphic granites of different coarseness
from Maine and from other districts indicate, however, that the
proportion of feldspar to quartz bears no marked relation to the
coarseness. Such variations as do occur are within relatively narrow
limits and appear to be dependent on the composition of the feldspar
and on other factors not yet understood. Analyses of graphic granites
of widely different coarseness from the Fisher
