Apollo 12 clinopyroxenes: Exsolution and epitaxy

Clinopyroxenes from rocks 12021, 12037, and 12052 have been studied by electron microprobe and single-crystal X-ray diffraction techniques in order to characterize the crystal-chemical aspects of pyroxene epitaxy and exsolution. The combined chemical and crystallographic data plotted on a Di-Hed-En-Fs data display, based on the unit-cell parameters b and β, suggest a large miscibility gap at low temperature between Ca-rich augite and Ca-poor pigeonites over a large range of Fe/Mg ratios. We also find that in rocks 12021 and 12037 primary pigeonites exsolved augites on both (001) and (100) while primary augites exsolved pigeonite on (001). In rock 12052 the common plane shared by pigeonite cores and epitaxial augite rims is approximately parallel to (100) and little subsolidus exsolution is indicated.