feldspar

Feldspar is a group of rock-forming minerals that accounts for about 60% of the Earth's crust. They are aluminosilicates with a framework of linked tetrahedra, in which silicon is largely replaced by aluminum; the available cations are potassium (K), sodium (Na), and calcium (Ca).

Feldspars are divided into alkali feldspars (potassium and sodium) and plagioclase feldspars (a solid solution series between albite, NaAlSi3O8, and anorthite, CaAl2Si2O8). The alkali feldspars include orthoclase, microcline, and sanidine. Plagioclase includes albite, oligoclase, andesine, labradorite, and anorthite.

Key physical properties: two cleavages at near 90 degrees; hardness about 6; vitreous luster; colors from white through pink, gray, to brown; typical crystal habit is prismatic; density around 2.55–2.76 g/cm3.

Occurrence: feldspar crystallizes from magma and is a dominant mineral in felsic igneous rocks such as granite, rhyolite, and their volcanic equivalents; it also occurs in metamorphic rocks and in sediments where it weathered from older rocks.

Uses: feldspar is a primary source material for the glass and ceramics industries, acting as a flux and providing alumina; it is also used as filler in paints, plastics, and paper and contributes to the composition of some concretes and construction materials.

Weathering: feldspar alters to clay minerals such as kaolinite and illite, contributing to soil development and sedimentary processes.

Occurrence and extraction: large-scale deposits exist worldwide, notably in pegmatites and granitic rocks; processing involves crushing and screening to produce feldspar concentrates used by industrial sectors.