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 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.