Microcline KAlSi3O8

Optical Properties:

Biaxial negative; first order gray interference color; "basketweave" twinning (multiple twins crossing at almost right angles), as shown in the pictures above, is almost always present; note how twins pinch and swell, unlike most plagioclase twins. In the photo on the right microcline occurs with biotite and quartz.

Occurrence:

Common in quartz-rich intrusive rocks such as granite, and in some moderate to high temperature metamorphic rocks; it does not occur in volcanic rocks. It may be intergrown with plagioclase to form perthite. At higher temperature, orthoclase forms instead of microcline.