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Anorthite

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Anorthite is one of the plagioclase feldspars, an important group of minerals abundant in the Earth's crust. Anorthite's chemical formula is CaAl2Si2O8.

Mineralogy

Anorthite is the calcium-rich endmember of the plagioclase system, the other endmember being albite (sodium-rich). The range of intermediate compositions have varying ratios of calcium and sodium ions.

Occurrence

Anorthite occurs in igneous and metamorphic rocks. Its type localities are Monte Somma and Valle di Fassa, Italy,

Details

Crystallographic systemTriclinic
ColoursNormally colourless or white, colourless in thin section.
LustreVitreous
Hardness6-6.5
Refractive index 1.51
Cleavage3: 1 perfect, 1 good, 1 poor
TwinningMultiple lamellar and Carlsbad twins
PleochroismYellow to dark green to transparent to opaque


See also

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