Anorthosis

Part of speech: noun

Definitions

1. A geological process involving the alteration of plagioclase feldspar minerals in igneous rocks, where calcium-rich varieties are transformed into sodium-rich forms through metamorphism or hydrothermal activity
2. The chemical and mineralogical modification occurring in volcanic or plutonic rocks, characterized by the conversion of calcium-rich plagioclase into a sodium-rich mineral assemblage under changing temperature and fluid conditions
3. A metamorphic phenomenon in which original calcium-rich plagioclase crystals in igneous formations are replaced by sodium-rich feldspar, resulting from metasomatic reactions or thermal alteration in the rock body

Etymology: The root is Greek: "an-" meaning "not" and "orthos" meaning "straight" or "upright." It names the mineral anorthite, a feldspar whose crystals lack right angles. That mineral in turn gave its name to anorthosite, the calcium-rich rock that makes up much of the lunar highlands. The ending "-osis" is also Greek and usually marks a process or a state of change, as in "necrosis" or "fibrosis." Anorthosis therefore names the process by which calcium-rich plagioclase is altered into sodium-rich minerals under shifting temperature and fluid conditions. It is a technical coinage built from classical parts, not a term used by ancient writers.