Types of Feldspar

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.

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Potassium-dominant feldspars with unknown crystal symmetry and Al-Si ordering state.

An 50 Ab 50 to An 30 Ab 70 (Na,Ca)Al(Si ... Crystal of plagioclase var. labradorite (Ca,Na)[Al(Al,Si)Si2O8] ... 3 thoughts on “ Feldspars I: Plagioclase ...

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.