Feldspars are a group of rock-forming tectosilicateminerals that make up about 41% of the Earth‘s continental crust by weight.[2]
Feldspars crystallize from magma as veins in both intrusive and extrusive igneous rocks and are also present in many types of metamorphic rock.[3] Rock formed almost entirely of calcic plagioclase feldspar (see below) is known as anorthosite.[4] Feldspars are also found in many types of sedimentary rocks.[5]

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Internal Mold of a Turretellid Gastropod: Fossil
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