Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals that make up about 41% of the Earth's continental crust by weight.^[2][3]

Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.^[4] Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite.^[5] Feldspars are also found in many types of sedimentary rocks.^[6]

The name feldspar derives from the German Feldspat, a compound of the words Feld, "field", and Spat meaning "a rock that does not contain ore".^[7] The change from Spat to -spar was influenced by the English word spar,^[8] meaning a non-opaque mineral with good cleavage.^[9] Feldspathic refers to materials that contain feldspar. The alternate spelling, felspar, has fallen out of use.

This group of minerals consists of tectosilicates. Compositions of major elements in common feldspars can be expressed in terms of three endmembers:

Solid solutions between K-feldspar and albite are called "alkali feldspar".^[10] Solid solutions between albite and anorthite are called "plagioclase",^[10] 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.

Alkali feldspars are grouped into two types: those containing potassium in combination with sodium, aluminum, or silicon; and those where potassium is replaced by barium. The first of these include:

Potassium and sodium feldspars are not perfectly miscible in the melt at low temperatures, therefore intermediate compositions of the alkali feldspars occur only in higher temperature environments.^[14] Sanidine is stable at the highest temperatures, and microcline at the lowest.^[11][12] Perthite is a typical texture in alkali feldspar, due to exsolution of contrasting alkali feldspar compositions during cooling of an intermediate composition. The perthitic textures in the alkali feldspars of many granites can be seen with the naked eye.^[15] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Barium feldspars are also considered alkali feldspars. Barium feldspars form as the result of the substitution of barium for potassium in the mineral structure. The barium feldspars are monoclinic and include the following:

The plagioclase feldspars are triclinic. The plagioclase series follows (with percent anorthite in parentheses):

Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes. The immiscibility gaps in the plagioclase solid solutions are complex compared to the gap in the alkali feldspars. The play of colours visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae. The specific gravity in the plagioclase series increases from albite (2.62 g/cm3) to anorthite (2.72–2.75 g/cm3).

Chemical weathering of feldspars results in the formation of clay minerals^[18] such as illite and kaolinite.

About 20 million tonnes of feldspar were produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt).^[19]

Feldspar is a common raw material used in glassmaking, ceramics, and to some extent as a filler and extender in paint, plastics, and rubber. In glassmaking, alumina from feldspar improves product hardness, durability, and resistance to chemical corrosion. In ceramics, the alkalis in feldspar (calcium oxide, potassium oxide, and sodium oxide) act as a flux, lowering the melting temperature of a mixture. Fluxes melt at an early stage in the firing process, forming a glassy matrix that bonds the other components of the system together. In the US, about 66% of feldspar is consumed in glassmaking, including glass containers and glass fiber. Ceramics (including electrical insulators, sanitaryware, pottery, tableware, and tile) and other uses, such as fillers, accounted for the remainder.^[20]

Bon Ami, which had a mine near Little Switzerland, North Carolina, used feldspar as an abrasive in its cleaners. The Little Switzerland Business Association says the McKinney Mine was the largest feldspar mine in the world, and North Carolina was the largest producer. Feldspar had been discarded in the process of mining mica until William Dibbell sent a premium quality product to the Ohio company Golding and Sons around 1910.^[21]

In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, and luminescence dating.

In October 2012, the Mars Curiosity rover analyzed a rock that turned out to have a high feldspar content.^[22]