Stable below 700°C with an ordered Si-Al arrangement, in contrast to 'high albite'. Oligoclase: A plagioclase feldspar with an albite : anorthite molar ratio ranging from 90 : 10 to 70 : 30. Originally described from Danvikstull, Stockholm, Södermanland, Sweden. Oligoclase-Albite: A variety of Oligoclase with Anorthite:Albite between 80:20 and 90:10.
Feldspars (KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8) are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust. Feldspars crystallize from magma in both intrusive and extrusive igneous rocks, as veins, and are also present in many types of metamorphic rock.
anorthite (90 to 100) CaAl 2 Si 2 O 8. Intermediate compositions of plagioclase feldspar also may exsolve to two feldspars of contrasting composition during cooling, but diffusion is much slower than in alkali feldspar, and the resulting two-feldspar intergrowths typically are too fine-grained to be visible with optical microscopes.
Plagioclase is a major constituent mineral in the Earth's crust, and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks. Plagioclase is also a major constituent of rock in the highlands of the Earth's moon.