Abstract

Pigeonite crystals from the Moore County meteorite show oriented exsolutions of two augites parallel to (001) and (100). The host and the exsolved phases indicate two-dimensional agreement not only in orientation but also in cell dimensions along their boundary planes. On the assumption that the two augites have an identical chemical composition and the difference between the two augites in the cell dimensions is due to lattice distortions in the exsolution, the cell dimensions corrected for the lattice distortions were estimated by using the elastic constants. The strain energies were estimated to be 0.02 and 0.04 kcal/mole for augites exsolved parallel to (001) and (100) respectively on the basis of the changes of the cell dimensions,

Pigeonite crystals from the Skaergaard intrusion were also studied for comparison. The exsolved augite is observed only in one orientation, which is not perfectly parallel to (001) of the pigeonite, and shows its intrinsic cell dimensions free from lattice distortions. This strongly suggests that pigeonite from the Moore County meteorite represents an intermediate stage in the process of exsolution of augite.

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