From Academic Kids
Feldspar is the name of an important group of rock-forming minerals which make up perhaps as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive rocks; they occur as compact minerals, as veins, and are also present in many types of metamorphic rock. They are also be found in many types of sedimentary rock.
This group of minerals consists of framework or tectosilicates:
- orthoclase a potassium-aluminium silicate,
- microcline also a potassium-aluminium silicate, and
- plagioclase a sodium-aluminium silicate to a calcium-aluminium silicate isomorphous series: