Anorthite

Anorthite is the calcium-rich end-member of the plagioclase feldspar series, a calcium aluminium silicate. It sits at the opposite end of the series from sodium-rich albite, with intermediate compositions such as labradorite grading between them.

Properties

Anorthite is triclinic with two good cleavages meeting near 90 degrees, a hardness of 6–6.5 and frequent fine polysynthetic twinning seen as parallel striations on cleavage faces. It is usually white, grey or colourless with a vitreous lustre.

Occurrence

It is a high-temperature feldspar, characteristic of calcium-rich igneous rocks such as anorthosites and many basalts, of contact-metamorphosed limestones, and of meteorites and lunar highland rocks. It is a rock-forming member of the feldspar group.