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).
Mesoperthite is a perthite with approximately equal amounts of alkali feldspar and plagioclase (Mindat).
Myrmekite is an intergrowth of plagioclase feldspar (commonly oligoclase) and vermicular quartz, generally replacing K-feldspar; formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity. The quartz occurs as blobs (Mindat).
Sunstone from Oregon is a labradorite with inclusions of copper crystals that cause patches of red colour (https://www.youtube.com/watch?v=ejucgGmeJMA).
Moonstone a trade name for any white feldspar with a blue schiller. Commonly microcline rarely orthoclase, and the plagioclase series (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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