Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Monoclinic
    Chemistry : KAlSi₃O₈
    Rarity : Ubiquitous

Orthoclase is a potassium feldspar, dimorphic from microcline and which forms a series with celsian. It is one of the essential minerals of the earth's crust as an important constituent of many acidic plutonic rocks (granite, granodiorite) and their pegmatites. Orthoclase also appears in highly metamorphic rocks. The adularia is a low temperature variety. Its name comes from the Greek orthos (right) in allusion to the almost right angle formed by its perfect cleavages along {001} and {010}. Orthoclase occurs in cleavable to granular masses, or in crystals that are most often short prismatic with an orthorhombic or sometimes tetragonal appearance, but also tabular, flattened according to {010}. It is translucent to transparent, its luster is glassy to pearly, it is colorless to white, gray, yellow or reddish (fine inclusions of hematite) and easily alters to kaolinite or phyllites in outcrops. It is a feldspar widely used in the porcelain and ceramics industry. it is used as an adjuvant in the glass industry. Mixed with clay and potash, crushed orthoclase is the basis of the glaze of porcelain and enamels. A minor part goes into the manufacture of "soft" abrasives. Some transparent yellow varieties are cut as gemstone.

Main photo : Carlsbad twin orthoclase from Pitkin County, Colorado, USA © Michael C. Roarke

Orthoclase crystals are frequently twinned according to the twin laws of Carlsbad (the most common), Baveno and Manebach.

Quartz crystals can grow epitaxially on orthoclase, giving geometric structures or unique ornamental materials such as zebradorite.

No fakes listed for this mineral species.