feldspaatit

Feldspaatit, or feldspar, is a group of rock-forming tectosilicate minerals that constitutes a large portion of the Earth's continental crust. Feldspars crystallize from magma as alkali feldspars (such as orthoklaas, mikroklin, and sanidiin) and through a continuous solid-solution series with calcium and sodium in the plagioclase group (from albite to anorthite). The two main subgroups are alkali feldspars (KAlSi3O8) and plagioclases with a Na-Ca end-member system (NaAlSi3O8–CaAl2Si2O8).

Feldspar minerals typically exhibit two directions of cleavage at about 90 degrees and have a vitreous to pearly luster. They range in color from white and colorless to pink, gray, or reddish. Hardness on the Mohs scale is around 6. Microcline can show exsolution lamellae, which are visible in some crystals.

Geologically, feldspar is a major constituent of felsic igneous rocks such as granite, granodiorite, and rhyolite, and it also occurs in metamorphic and sedimentary rocks. Weathering of feldspar contributes to clay mineral formation and soil development.

Industrially, feldspars are important raw materials for glassmaking and ceramic production, acting as fluxes that lower melting temperatures. They are also used as fillers and, in the case of some minerals, as ornamental stones and in geochronology (potassium-argon dating using alkali feldspars).