Pigeonite

Low-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 – 20).

Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing clinopyroxene converts to low-Ca orthopyroxene. This change was actually what was used in the 1960s to define the type 4/5 boundary, in classic work by Van Schmus and Wood. This is still how to differentiate between types 4 and 5.