Orthoclase is a common mineral in igneous and metamorphic rocks, consisting of potassium aluminum silicate (KAlSi2O8). It occurs in the form of vitreous crystals in the monoclinic system, of various colors, notably white and pink. Orthoclase is one of the end-members (pure compounds) of the feldspar group. Hardness 6–6.5, relative density 2.5–2.6.
See also microcline.