Sanidine's the high temperature form of potassium feldspar (K,Na)(Si,Al)4O8. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase's a monoclinic polymorph stable at lower temperatures. At yet lower temperatures, microcline, a triclinic polymorph of potassium feldspar,'s stable. Due to the high temperature and rapid quenching, sanidine can contain more sodium in its structure than the tw… (More on Sanidine)