Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.

Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock. Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite. Feldspars are also found in many types of sedimentary rock. 

Compositions

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

Potassium-Feldspar (K-spar) 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.

Alkali feldspars

The alkali feldspars are as follows:

    orthoclase (monoclinic), — KAlSi3O8
    sanidine (monoclinic) —(K,Na)AlSi3O8
    microcline (triclinic) — KAlSi3O8
    anorthoclase (triclinic) — (Na,K)AlSi3O8

Sanidine is stable at the highest temperatures, and microcline at the lowest. 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.[9] Microperthitic textures in crystals are visible using a light microscope, whereas cryptoperthitic textures can be seen only with an electron microscope.

Plagioclase feldspars

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

    albite (0 to 10) — NaAlSi3O8
    oligoclase (10 to 30) — (Na,Ca)(Al,Si)AlSi2O8
    andesine (30 to 50) — NaAlSi3O8 — CaAl2Si2O8
    labradorite (50 to 70) — (Ca,Na)Al(Al,Si)Si2O8
    bytownite (70 to 90) — (NaSi,CaAl)AlSi2O8
    anorthite (90 to 100) — CaAl2Si2O8

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 solution 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.

Barium Feldspars

The barium feldspars are monoclinic and comprise the following:

    celsian — BaAl2Si2O8
    hyalophane — (K,Na,Ba)(Al,Si)4O8

Feldspars can form clay minerals through chemical weathering.

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