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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.

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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,


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


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