Anorthiet

Anorthiet, also known as anorthite in English, is the calcium-rich end-member of the plagioclase feldspar solid solution series. Its chemical formula is CaAl2Si2O8, and it forms a compositional series with albite (NaAlSi3O8) and other plagioclase minerals. As a tectosilicate, it is a framework mineral in igneous and metamorphic rocks.

In terms of structure and appearance, anorthiet crystallizes in the triclinic system. It typically occurs as colorless to white crystals or grains, often tinted by impurities to gray, pink, or brown. It has two good cleavages at or near 90 degrees and a vitreous to pearly luster. On the Mohs scale, its hardness is about 6, and its specific gravity is around 2.7 to 2.8. Exsolution lamellae and striations on cleavage surfaces are common features in plagioclase minerals, including anorthite.

Geological occurrence and formation are linked to calcium-rich magmas. Anorthiet forms in certain igneous rocks such as anorthosites, where it can be the dominant mineral, and it also appears in various metamorphic rocks and in the crustal portions of mafic to felsic plutonic rocks. The mineral is notably abundant in the Moon’s highland crust, where large expanses of anorthite-rich plagioclase provide major evidence for the early crustal differentiation of the Moon.

Significance and uses: the calcium-to-sodium ratio in plagioclase minerals, including anorthite, helps geologists interpret magma evolution and crust formation in planetary bodies. As a feldspar mineral, plagioclase materials are important industrially for glass and ceramic applications, though pure anorthite itself is not widely mined.