Pigeonite

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Polarized light microscope image of part of a grain of orthopyroxene containing exsolution lamellae of augite (long dimension 0.5 mm, Bushveld igneous complex). The texture documents a multistage history: (1) crystallization of twinned pigeonite, followed by exsolution of augite; (2) breakdown of pigeonite to orthopyroxene plus augite; (3) exsolution of augite parallel to the former twin plane of pigeonite.
Polarized light microscope image of part of a grain of orthopyroxene containing exsolution lamellae of augite (long dimension 0.5 mm, Bushveld igneous complex). The texture documents a multistage history: (1) crystallization of twinned pigeonite, followed by exsolution of augite; (2) breakdown of pigeonite to orthopyroxene plus augite; (3) exsolution of augite parallel to the former twin plane of pigeonite.

Pigeonite is a mineral in the clinopyroxene group. It has a general formula of (Ca,Mg,Fe)(Mg,Fe)Si2O6 The calcium cation fraction can vary from 5% to 25%, with iron and magnesium making up the rest of the cations.

Pigeonite crystallizes in the monoclinic system, as does augite, and a miscibility gap exists between the two minerals. At lower temperatures, pigeonite is unstable relative to augite plus orthopyroxene. The low-temperature limit of pigeonite stability depends upon Fe/Mg in the mineral and is hotter for more Mg-rich compositions; for Fe/Mg of about 1, the temperature is about 900 °C. The presence of pigeonite in an igneous rock thus provides evidence for the crystallization temperature of the magma, and hence indirectly for the water content of that magma.

Pigeonite is found as phenocrysts in volcanic rocks on Earth and as crystals in meteorites from Mars and our moon. In slowly cooled intrusive igneous rocks, pigeonite is rarely preserved, but textural evidence of its breakdown to orthopyroxene plus augite may be present, as shown in the accompanying microscopic image.

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