feldspat

Feldspar is a group of rock‑forming tectosilicate minerals that make up a large portion of the Earth's crust, typically described by the general formula (K, Na, Ca)AlSi3O8. The two main subgroups are alkali feldspar, which is rich in potassium or sodium (examples include orthoclase, microcline, and sanidine), and plagioclase feldspar, which forms a continuous solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8).

Plagioclase feldspar comprises a wide range of compositions from Na-rich albite to Ca-rich anorthite, and often exhibits characteristic striations on cleavage surfaces. Alkali feldspar grains tend to be more uniform in composition and include potassium-rich varieties. Feldspars crystallize in igneous rocks such as granites, granodiorites, and rhyolites and are also common in metamorphic rocks; they occur as primary crystals in felsic magmas and as detrital grains in sedimentary rocks.

Physically, feldspar minerals are typically white to pink, gray, or colorless, with a Mohs hardness of about 6. They exhibit two directions of cleavage at or near 90 degrees (plagioclase differs in some details) and generally have good cleavage visibility under magnification. In many rocks, feldspar can be altered to sericite or clay minerals during weathering.

Industrially, feldspar is an important industrial mineral. Alkali feldspars are widely used as fluxes in glassmaking and ceramics, helping to lower melting temperatures, while plagioclase varieties are used in glazes and as fillers. Feldspar-rich rocks also serve as a source for radiometric dating in potassium-bearing minerals.

The name feldspar derives from Feldspat, the German term for this mineral group.