Polarized light microscope image of part of a grain of orthopyroxene containing exsolution lamellae of augite 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. (long dimension 0.5 mm, Bushveld igneous complex)
Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene 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 the Fe/Mg ratio in the mineral and is hotter for more Mg-rich compositions; for a Fe/Mg ratio 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 the Moon. In slowly cooled intrusiveigneous rocks, pigeonite is rarely preserved. Slow cooling gives the calcium the necessary time to separate itself from the structure to form exsolutionlamellae of calcic clinopyroxene,[5] leaving no pigeonite present.[6] Textural evidence of its breakdown to orthopyroxene plus augite may be present, as shown in the accompanying microscopic image.
^Nesse, William (2012). Introduction to Mineralogy (Second ed.). Oxford University Press. p. 300.
^Winchell, Alexander N. (1900). "Mineralogical and petrographic study of the gabbroid rocks of Minnesota, and more particularly, of the plagioclasytes". The American Geologist. 26 (4): 197–245.
