Orthoclase is a feldspar mineral containing K[AlSi3O8]. It is an important rock-forming mineral. It has a monoclinic crystal structure, and the crystals are columnar or blocky. Crystal twinning is common, and the aggregates are blocky, flaky or granular. The colour is white, colourless, light red, yellow, grey or green with white streaks. The diaphaneity ranges from transparent to translucent with a vitreous or pearly lustre. Its specific gravity is 2.55–2.63, and its Mohs hardness is 6–6.5. It has complete cleavage, uneven fractures and mainly forms in igneous and metamorphic rocks such as granite, pegmatite, rhyolite, trachyte, syenite, gneiss and schist.