24 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
The
long straight blades of biotite are idiomorphic with respect to the
quartz and feldspar grains and their intergrowths. The biotite blades
are paralleled in many specimens by an abundance of microscopic blades
of muscovite alternating with thin layers of quartz. Other blades of
muscovite traverse the rock in the same manner that biotite does,
though they are much smaller.
The
coarseness of the graphic intergrowth of quartz and feldspar described
above varies notably, even where only one quartz and one feldspar
individual are involved. There does not appear, however, to be any
important difference in the relative proportions of the minerals
involved. On the contrary, it is notable that the areas of graphic
intergrowth, whether coarse or fine, terminate very abruptly against
areas of pure feldspar that are crystallographically continuous with
the feldspar of the intergrowth (PL VIII) and that show no transition
through intergrowths characterized by progressively smaller proportions
of quartz. The quartz and feldspar thus appear to be intergrown in
rather definite proportions or not at all-*1
Very
little is known of the physical-chemical conditions that produce the
peculiar types of crystallization described above. The mode of
occurrence in the Alaskan and New Mexican examples suggests, however,
that the brush-shaped crystals developed rapidly in a border portion of
the magma where the temperature gradient was
a Adolph
Knopf, of the U. S. Geological Survey, in a personal communication
states that he has observed radiating graphic intergrowths identical
with those of the Maine specimens in rocks from the Cape Mountain
region near Cape Prince of Wales in Alaska. The specimen brought from
(he field was indistinguishable in appearance from the Maine specimens
except that the graphic intergrowth was slightly finer grained. Its
mode of occurrence, however, was wholly different. It occurs at the
border of sills of microcline-orthoclase-biotite granite which radiate
from a central granite massif at (heir contact with limestones. The
latter have been metamorphosed by the granite with the development,
within 3 feet of the contact, of numerous contact-metamorphic minerals.
The contact zones of micrographic granite range up to 8 inches or so in
width, though the ind ividua! brush-shaped crystals are not over 4
inches in length. In a microscopic section parallel to the long axis of
one of the "brushes" and about parallel to the c (001) pinacoid of one
of the feldspar crystals, the cross sections of the quartz bands are
for the most part elongate rod-shaped. In some of these the long axis
is the direction of fastest light transmission; in others it is (lie
direction of slowest. Single microcline crystals may be intergrown with
several quartz crystals, each with slightly different orientation and
slightly different trend of their blades, which repeat in miniature the
brushlike forms assumed by the microcline crystals. A feature observed
in both the Maine and Alaskan specimens is the frequent abrupt
termination of the quartzes along planes transverse to the axes of the
brushes, other sets of intergrown quartzes beginning with equal
abruptness farther on. The microcline is crystallographically
continuous across these hiatuses. In other places the fine graphic
intergrowths are succeeded abruptly along planes at right angles to the
length of the brushes by coarse ones. As in the Maine specimen, grains
of albite, usually graphically intergrown with quartz, occur between
some of the quartz-microcline " brushes," and between others occurs a
granular aggregate of quartz, microcline, and albite. There are
occasional short blades of biotite.
Bands
of brush-shaped intergrowths of feldspar and quartz, similar in
appearance to the Maine and Alaska occurrences but of microscopic
dimensions, were also studied in a microscope slide in the collection
of the United States Geological Survey. This shows the contact between
a coarse quartz diorite and an intrusive aplite dike from New Mexico.
For 0.15 millimeter from the diorite contact occurs a granular
aggregate of quartz and feldspar so fine that it may represent a
devitrified glass; the next zone, 0.G0 millimeter in average width, is
made up of brush-shaped intergrowths of feldspar and quartz radiating
from the finely crystalline zone mentioned above into the main mass of
aplite.
Frank
C. Calkins, of the United States Geological Survey, reports in a
personal communication the occurrence near Anaconda, Mont., of borders
1| inches or so in width similar to the Alaskan and Maine occurrences
in general appearance, at the contact between diorite and an intrusive
mass of biotite granite. No specimens of these were collected for study
but presumably they are similar in structure to those here described.
