Pigeonite

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
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)
General
Category	Silicate mineral (pyroxene)
Formula (repeating unit)	(Ca,Mg,Fe)(Mg,Fe)Si2O6
Strunz classification	9.DA.10
Dana classification	65.01.01.04
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 9.7, b = 8.95, c = 5.24 [Å]; β = 108.59°; Z = 4
Identification
Color	Brown, greenish brown-black
Crystal habit	Prismatic crystals, to 1 cm; granular, massive.
Twinning	Commonly twinned simply or multiply on {100} or {001}
Cleavage	Good on {110}, (110) ^ (110) ~87°
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	6
Luster	Vitreous to dull
Streak	Grey white
Diaphaneity	Semitransparent
Specific gravity	3.17 - 3.46 measured
Optical properties	Biaxial (+)
Refractive index	nα = 1.683 - 1.722 nβ = 1.684 - 1.722 nγ = 1.704 - 1.752
Birefringence	δ = 0.021 - 0.030
Pleochroism	Weak to moderate; X = colorless, pale green, brown; Y = pale brown, pale brownish green, brownish pink; Z = colorless, pale green, pale yellow
2V angle	0 - 30° measured
Dispersion	weak to distinct
References	[1][2][3]

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 intrusive igneous rocks, pigeonite is rarely preserved. Slow cooling gives the calcium the necessary time to separate itself from the structure to form exsolution lamellae of calcic clinopyroxene, leaving no pigeonite present.[4] 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.[3][5]