A monoclinic mineral, KAlSi₃O₈; feldspar group; prismatic cleavage; partly ordered, monoclinic potassium feldspar dimorphous with microcline, being stable at higher temperatures; also a general term applied to any potassium feldspar that is or appears to be monoclinic; e.g., sanidine, submicroscopically twinned microcline, adularia, and twinned analbite. It is a common rock-forming mineral and occurs esp. in granites, granite pegmatites, felsic igneous rocks, and crystalline schists, and is commonly perthitic.