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-feldspar, which
are rich in potassium, and
feldspars that contain no potassium.
The major K-feldspar are orthoclase, sanidine, microcline and anorthoclase.
The major plagioclase feldspars are albite, oligoclase, andesine, labradorite (an intermediate member of the albite - anorthite series) and anorthite.
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-feldspar are minerals of the greenschist and amphibolite facies.
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