7 Photos
Pigeonite
Chemical Formula	(Ca,Mg,Fe)(Mg,Fe)Si2O6
Species	Silicates
Crystal System	Monoclinic
Mohs Scale	6
Specific Gravity	3.17-3.46
Color	Brown, greenish brown-black
Streak	Grey white
Luster	Vitreous, Dull
Refractive Index	n = 1.683 - 1.722 n = 1.684 - 1.722 n = 1.704 - 1.752
Diaphaneity	Semitransparent
Cleavage	Distinct/Good
Fracture	Conchoidal
Crystal Habit：Prismatic crystals, to 1 cm; granular, massive.

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 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 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.

Pigeonite is named for its type locality on Lake Superior's shores at Pigeon Point, Minnesota, United States. It was first described in 1900.