ortoklaasi

Ortокlaasi, known in English as orthoclase, is a potassium feldspar mineral with the chemical formula KAlSi3O8. It is one of the alkali feldspars and the monoclinic polymorph of KAlSi3O8; the other K-feldspars are sanidine (high-temperature) and microcline (low-temperature). Orthoclase forms during crystallization of silica-rich magmas at intermediate temperatures and is a common component of granitic and rhyolitic rocks, as well as pegmatites. It also occurs in some metamorphic rocks and in weathered sediments.

Physically, orthoclase is typically colorless, white, pink, or salmon, and ranges from transparent to translucent. It has a vitreous luster, a hardness of about 6, and a specific gravity around 2.54. It cleaves in two directions at roughly 90 degrees, a diagnostic feature of feldspars, and can exhibit Carlsbad twinning in thin section.

Orthoclase is a major mineral in continental crustal rocks and serves as a key potassium reservoir in felsic rocks. It weatheres to clay minerals such as kaolinite and contributes potassium to soils. In geology and geochronology, potassium-argon dating often uses orthoclase-bearing rocks, though argon retention and post-crystallization alteration can complicate results.

Its name derives from Greek orthos ("straight") and klasis ("cleavage"), referring to its cleavage direction. Related K-feldspars include microcline and sanidine, which differ in crystal symmetry and temperature of stability.