120 PEGMATITES AND ASSOCIATED ROCKS OF MAINE.
centage of iron oxide is smaller, however, in many pink feldspars than in those of lighter color. All the1
pink spars burn perfectly white, and the iron content is too small to
be in the least detrimental in pottery manufacture. Fresh feldspar is
so hard that only with difficulty can it be scratched with a knife
blade.
As
found in the quarries, the potash-soda feldspars seldom show true
crystal faces, but when undecomposed break readily into angular
pieces, bounded in part by smooth cleavage faces. There are three
directions of cleavage, intersecting at definite angles, which are
practically identical in orthoclase and microcline and are only
slightly different in the soda-bearing feldspars of this group. Only
two of the cleavages are well denned, and these invariably intersect
approximately at right angles. Both of these principal cleavage
surfaces show a high luster, comparable tc that exhibited by a plate of
glass, though one cleavage face is a trifle less brilliant than the
other. The hardness and the two lustrous cleavage planes intersecting
at right angles are usually sufficient to identify a mineral as
belonging to the group of potash-soda feldspars.
Recent
experiments have shown that the potash-rich feldspars have no definite
melting point, as metals have, for example. Fusion tests made on finely
powdered microcline in the geophysical laboratory of the Carnegie
Institution a showed that at 1,000° 0. traces of
sintering were evident; at 1,075° the powder had formed a solid cake;
at 1,150° this cake had softened somewhat; and at 1,300° it had become
a viscous liquid which could be drawn out into glassy threads. In most
of the determinations complete fusion has taken place in the dry state
at temperatures below Seger cone No. 9, which fuses at 1,310° C, or
2,390° F.
The
great bulk of the feldspar quarried in the eastern United States and in
Canada belongs to the class described above, being orthoclase or
microcline or an intergrowth of the two. In most quarries this is
associated with minor quantities of soda feldspar—albite or oligoclase—
occurring either in separate crystals or delicately intergrown with the
potash feldspar, as shown in Plate XVII. The presence of the soda spar
renders the ground product slightly more fusible. The specific gravity
of orthoclase and microcline varies from 2.54 to 2.56.
LIME-SODA FELDSPARS OR PLAGIOCLASES.
The
lime-soda group of feldspars, the plagioclases, as they are called,
form a continuous series ranging from pure soda feldspar, albite, at
one end to pure lime feldspar, anorthite, at the other end. The
chemical composition of albite is represented by the formula NaAlSi308 (designated Ab) or Na2O.Al203.6Si02, being similar to that
oDay,
A. L., and Allen, E. T., The isomorphism and thermal properties of the
feldspars: Pub. 31 Carnegie Inst, of Washington, 1905, pp. 13-75; also
Am. Jour. Sci., 4th ser., vol. 10, 1905, pp. 93-142.
