Perthite KAlSi3O8 and Plagioclase (Na-Ca Al Silicate)

Optical Properties:

Perthite is actually two minerals: an intergrowth of sodic plagioclase in K-feldspar (orthoclase or microcline). The intergrowths are commonly stringy (as in the photo above), but they may be globular, lensoid, or other shapes. First order gray interference colors. When the optic sign is determined, it may be either + or -, depending on whether the lens is focused on K-feldspar or intergrown plagioclase.

Occurrence:

Common in siliceous intrusive rockssuch as granite and quartz monzonite; also in syenite, trachyte, and some metamorphic rocks.