Solid solutions between K-feldspar and albite are called alkali feldspar. potassium feldspar (K-spar) endmember K Al Si 3 O 8.Compositions of major elements in common feldspars can be expressed in terms of three endmembers: The feldspar group of minerals consists of tectosilicates, silicate minerals in which silicon ions are linked by shared oxygen ions to form a three-dimensional network. Feldspars are also found in many types of sedimentary rocks. Rock formed almost entirely of calcic plagioclase feldspar is known as anorthosite. įeldspars crystallize from magma as both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock. Feldspars make up about 60% of the Earth's crust, and 41% of the Earth's continental crust by weight. The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. Feldspar crystal 18 cm × 21 cm × 8.5 cm (7.1 in × 8.3 in × 3.3 in) from Jequitinhonha valley, Minas Gerais, southeastern BrazilĬompositional phase diagram of the different minerals that constitute the feldspar solid solution.įeldspar (sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium.
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