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Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust.[1]
Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock.[2] Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite.[3] Feldspars are also found in many types of sedimentary rock.[4]
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Feldspar is derived from the German Feld, "field", and Spath, "a rock that does not contain ore". "Feldspathic" refers to materials that contain feldspar. The alternate spelling, felspar, has now largely fallen out of use.[5]
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[1]
Anorthite endmember CaAl2Si2O8[1]
Solid solutions between K-feldspar and albite are called alkali feldspar.[1] Solid solutions between albite and anorthite are called plagioclase,[1] 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.
The alkali feldspars are as follows:
Sanidine is stable at the highest temperatures, and microcline at the lowest.[6][7] 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.
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 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.
The barium feldspars are monoclinic and comprise the following:
Feldspars can form clay minerals through chemical weathering.[10]
About 20 million tonnes of feldspar was produced in 2010, mostly by three countries: Italy (4.7 Mt), Turkey (4.5 Mt), and China (2 Mt).[11]
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. Pottery (including electrical insulators, sanitaryware, tableware, and tile) and other uses, such as fillers, accounted for the remainder.[12]
In earth sciences and archaeology, feldspars are used for K-Ar dating, argon-argon dating, thermoluminescence dating and optical dating.
This article incorporates public domain material from the United States Geological Survey document "Feldspar and nepheline syenite".