Practically
this whole mass is a graphic intergrowth of quartz, with white to pale
pink orthoclase and microcline. Some of the feldspar crystals of this
intergrowth are shown by reflections from their cleavage faces to be 2-1/2 feet
across. The coarseness varies rapidly from point to point even within
the range of a single feldspar individual. At the south end of the
outcrop the graphic granite grades into pegmatite of irregular
texture, showing some masses of pure feldspar 2 to 3 inches across.
Both the graphic granite and this irregular-textured pegmatite inclose
scattered biotite laths.
At
the south end of this exposure also there is some associated gray
gneiss. In one place the pegmatite cuts directly across the folia of
the gneiss. In other places graphic granite forms knots or short lenses
up to 6 inches in width between the gneiss folia. The mass of grapliic
granite exposed in this ledge is the largest continuous mass observed
by the writer in the State.
ECONOMICALLY IMPORTANT PEGMATITE MINERALS.
FELDSPAR.
The
feldspars are compounds of alumina and silica with one or more of the
bases potash, soda, and lime; rarely barium is present. They fall into
two principal groups, the potash-soda feldspars and the lime-soda
feldspars, both of which may be present in the same deposit or even
intergrown in the same crystal.
POTASH-SODA FELDSPARS.
The
principal representatives of the potash-soda feldspar group are
orthoclase .and microcline, both of which have the composition KAlSi308 or K2O.Al203.6Si02.
These two varieties have also the same crystal form and are similar in
most of their physical properties. For commercial purposes they may be
regarded as identical, for they can not be distinguished from each
other with the unaided eye and are often associated in the same
crystal. The theoretical percentage composition of pure orthoclase or
microcline is silica (Si02), 64.7 per cent; alumina (A1203), 18.4 per cent; and potash (K20),
16.9 per cent. Soda may partly or completely replace potash in these
feldspars. If it is more abundant than the potash, the feldspar is
called anorthoclasc.
The
feldspar of the potash-soda group mined in the United States is mostly
pale flesh colored to nearly white, though that from Bedford, X. Y.,
is reddish and that from near Batchellerville, N. Y., is pearl gray.
The potash spars from Norway and from Bedford, Ontario, are reddish in
color. The cause of the reddish color is not definitely known, but in
some feldspars it seems to be due to the presence of small quantities
of finely divided iron oxide. The per-
