Pigeonite
Pigeonite is a low-calcium clinopyroxene that forms in quickly cooled basalts and is a hallmark of eucrites.
Pigeonite is a clinopyroxene with a modest amount of calcium, between the calcium-poor orthopyroxenes and calcium-rich augite. It is stable only at high temperatures, so it survives mainly in rocks that cooled fairly quickly, such as lava flows and shallow intrusions.
Pigeonite is a defining mineral of basaltic eucrites, the lava rocks from the crust of the asteroid Vesta, where it occurs alongside calcium-rich plagioclase. It is also common in shergottites from Mars and in lunar mare basalts.
When pigeonite cools slowly, it can no longer hold its calcium evenly. Thin layers of calcium-rich augite separate out inside the crystal, a process called exsolution. The width and pattern of these lamellae reveal how slowly the rock cooled, and many eucrites show them because they were reheated by later lava flows or impacts. In some, the pigeonite has inverted entirely to orthopyroxene with augite blebs.
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