An electron microscopic study of a lunar pyroxene

Ion-thinned samples of lunar rock 12052 have been examined in the electron microscope. Exsolution textures have been observed in the pyroxene on a finer scale than those found in Apollo 11 rocks, indicating a faster cooling rate. The early stages of the reaction involve modulations on both (001) and (100) which suggest that decomposition may occur by a spinodal mechanism. A two-stage exsolution structure in the augite rim of a phenocryst is consistent with a sudden increase in the cooling rate—possibly when the magma was extruded onto the surface of the moon.The presence of antiphase domains in all primary and precipitated pigeonites confirms that pigeonites of all compositions have the space group C2/c at high temperatures.