Sanidine KAlSi3O8

Optical Properties:

Biaxial negative, small 2V, first order gray interference color, simple twins, 2 cleavages. Both photos above are under crossed polarizers; on the left is a sanidine grain with Carlsbad twin; on the right, the large grain at the right is sanidine; the phenocrysts at the left edge of the photo are quartz. The rocks in both cases are rhyolites.

Occurrence:

Restricted to volcanic rocks or very high temperature contact metamorphic rocks (sanidinite facies) that are rich in potassium.