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One of a group of rock-forming minerals, the most abundant group in the Earth's crust. They are the chief constituents of igneous rock and are present in most metamorphic and sedimentary rocks. All feldspars contain silicon, aluminum, oxygen, linked together to form a framework; spaces within this structure are occupied by sodium, potassium, calcium, or occasionally barium, in various proportions.
Feldspars form white, gray, or pink crystals and rank 6 on the Mohs' scale of hardness.
The four extreme compositions of are orthoclase, KAlSi3O8; albite, NaAlSi3O8; anorthite, CaAl2Si2O8; and celsian, BaAl2Si2O8. These are grouped into plagioclase feldspars, which range from pure sodium feldspar (albite) through pure calcium feldspar (anorthite) with a negligible potassium content; and alkali feldspars (includingorthoclase and microcline), which have a high potassium content, less sodium, and little calcium.
The type known as moonstone has a pearl-like effect and is used in jewelry. Approximately 4,000 metric tons of feldspar are used in the ceramics industry annually.