Feldspars comprise a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust. They are divided into two groups, K-feldspars, which are rich in potassium, and plagioclase feldspars that contain no potassium.
The major K-feldspars are orthoclase, sanidine, microcline and anorthoclase.
The major plagioclase feldspars are albite and its varieties oligoclase, and albite variety andesine, anorthite and its variety labradorite.
Perthite is a variety of the feldspar group that occurs as an intergrowth of albite or its variety oligoclase with a microcline host, occasionally also with an orthoclase host (Mindat).
Feldspars are primary minerals; they are essential constituents of granite, syenite, diorite, gabbro, rhyolite, trachyte, andesite, basalt and , sandstone.
Feldspars are common constituents of phyllite and gneiss, and medium constituents of quartzolite.
K-feldspars are minerals of the greenschist and amphibolite facies.
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