Anorthite Composition, Crystallization & Structure
Composition. Calcium-aluminum silicate, CaAl2Si2O8 = Silica 43.2, alumina 36.7, lime 20.1. Soda is usually present, in small amount, in the albite molecule, NaAiSi3O8.
Crystallization. Triclinic. Crystals usually prismatic parallel to vertical axis. Twinning common according to albite and pericline laws (see Above)
Structure. Massive cleavable. Crystals rare.
Physical Properties. Cleavage in two directions at 85° 50’. One cleavage (parallel to base) better than the other. H. =6. G. = 2.75. Vitreous to pearly luster. Color white, grayish, reddish. Transparent to opaque.
Tests. Fusible at 4.5. Dissolves slowly in hydrochloric acid and yield a silica jilly upon evaporation. Gives a strong test for calcium (see under oligoclase) and only a slight yellow flame (sodium).
Occurrence. A rock- making mineral, particularly in the dark-colored basic igneous rocks. Associated with various calcium and magnesium silicates. Occurs in andesite, basalt, diorite, gabbro, morite, etc, in chrysolite-bearing rocks, in amphibolites. Also in druses of ejected volcanic blocks and in the granular limestone of contact deposits. Found in the lavas of Mount Vesuvius; of Japan, etc.
Name. Derived from the Greek Word meaning oblique, because of its triclinic crystallization.
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