Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. It is common in granite and pegmatites. Microcline forms during slow cooling of orthoclase. Sanidine is a polymorph of alkali feldspar stable at yet higher temperature. Microcline has cross-hatch twinning that forms as a result of the transformation of monoclinic orthoclase into triclinic microcline.