Anorthite

Anorthite is the calcium-rich end-member of the plagioclase feldspar series, the counterpart to sodium-rich albite. Its name, from Greek for "not straight," refers to its oblique (triclinic) crystal symmetry.

It is a high-temperature feldspar typically found in calcium-rich, silica-poor igneous rocks such as gabbro, basalt, and especially anorthosite, a rock composed largely of calcic plagioclase. Anorthite is usually white to gray and translucent.

Beyond Earth, anorthite is geologically famous as a major constituent of the lunar highlands and a component of many meteorites, making it important in planetary science. Pure anorthite is relatively uncommon compared with intermediate plagioclase compositions.

Anorthite crystallizes at high temperatures from calcium-rich, sodium-poor magmas, making it characteristic of mafic igneous rocks such as gabbro, norite, and basalt, and the dominant mineral in anorthosite. It is among the first plagioclase compositions to crystallize as a basaltic magma cools.

It also forms in high-grade metamorphic rocks, contact metamorphic skarns, and calcium-rich aureoles. Terrestrial localities include volcanic rocks in Japan, Italy (Monte Somma, Vesuvius), and India (the variety indianite). Beyond Earth, anorthite-rich anorthosite forms the bright lunar highlands and occurs in calcium-aluminum inclusions in primitive meteorites, the oldest solids in the solar system.

Anorthite is a hard (Mohs 6-6.5) feldspar with two cleavages near 90 degrees, a vitreous luster, and a white streak. It is usually white, gray, or colorless, sometimes with reddish tints.

As a plagioclase it may display fine albite twinning striations on cleavage faces, distinguishing it from potassium feldspars. Its association with dark mafic rocks like gabbro and basalt is a useful field clue.

Distinguishing anorthite from intermediate plagioclase (labradorite, bytownite) by eye is difficult; precise identification relies on optical properties or chemical analysis of the calcium content. It can be confused with other pale silicates but is set apart by feldspar cleavage and hardness.

Anorthite has limited direct commercial use compared with sodium and potassium feldspars, since its high calcium content makes it less ideal as a ceramic flux, though calcic feldspar rocks are used in some glass and ceramic and aggregate applications.

Its greatest significance is scientific. Anorthite is central to understanding basaltic and anorthositic rocks, igneous differentiation, and the geology of the Moon and meteorites, where it records the earliest history of the solar system.

Collectors value well-formed crystals from classic volcanic localities. Anorthite has little gemstone or metaphysical tradition, and its main importance is as a rock-forming and planetary-science mineral.