ETYM German feldspath; feld field + spath spar.
Several minerals forming part of all crystalline rocks and decomposing into clay or china clay.
Any of a group of hard crystalline minerals that consist of aluminum silicates of potassium or sodium or calcium or barium; SYN. felspar.
One of a group of rock-forming minerals; the chief constituents of igneous rock. Feldspars all contain silicon, aluminum, and 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 feldspar 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, (including orthoclase), 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.