Feldspar is the name of a group of rock-forming minerals which make up as much as 60% of the Earth's crust.Feldspar. What is Feldspar? Industrial Minerals Association. Retrieved on July 18 2007.

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, and they can also occur as compact minerals, as veins, and are also present in many types of metamorphic rock."Metamorphic Rocks." Metamorphic Rocks Information. Retrieved on July 18 2007. Rock formed entirely of plagioclase feldspar (see below) is known as anorthosite."Gem, Rock, and Mineral Postage Stamps Featuring Anorthosite." Anorthosite Retrieved on February 8 2008. Feldspars are also found in many types of sedimentary rock."Weathering and Sedimentary Rocks." Geology. Retrieved on July 18 2007.

Etymology

Feldspar is derived from the German Feld, field, and Spat, a rock that does not contain ore. "Feldspathic" refers to materials that contain feldspar. The alternative spelling, felspar, has now largely fallen out of use.Harper, Douglas. "Feldspar." Feldspar Online Etymology Dictionary. Retrieved on February 8 2008.

Compositions

K-feldspar endmember KAlSi₃O₈

Albite endmember NaAlSi₃O₈

Anorthite endmember CaAl₂Si₂O₈

Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or more properly plagioclase feldspar. Only limited solid solution occurs between K-feldspar and anorthite, and in the two other solid solutions, immiscibility occurs at temperatures common in the crust of the earth. Albite is considered both a plagioclase and alkali feldspar. In addition to albite, barium feldspars are also considered both alkali and plagioclase feldspars. Barium feldspars form as the result of the replacement of potassium feldspar.