Pigeonite is a mineral in the clinopyroxene subgroup of the pyroxene ,Mg 6. 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 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 from Mars igneous rocks , pigeonite is rarely preserved. Slow cooling gives the calcium the necessary time to separate itself from the structure to form exsolution of calcic clinopyroxenehttp://sites.und.edu/dexter.perkins/opticalmin/cpx.htm, leaving no pigeonite present. 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