Exsolution relationships in a clinopyroxene of average composition Ca_0.43Mn_0.69Mg_0.82Si₂O₆: X-ray diffraction and analytical electron microscopy

A pyroxene of average composition Ca_0.43Mn_0.69Mg_0.82Si₂O₆ from Balmat, New York is exsolved to coexisting P2₁/c and C2/c phases. Structure refinements were performed in part to limit the compositions of the individual phases, as were plots of cell parameters and average M1–O and M2–O bond distances vs. composition. Site occupancies showed that the phases are manganoan diopside and kanoite. Transmission electron micrographs revealed that the phases are lamellar, share (001) as the interphase boundary, and are 2000Å wide. X-ray energy dispersive analysis of individual lamellae yielded Ca_0.68Mn_0.44Mg_0.88Si₂O₆ for the C2/c phase and Ca_0.12Mn_1.02Mg_0.86Si₂O₆ for the P2₁/c phase. Dark-field imaging revealed the presence of antiphase domain boundaries in the P2₁/c phase, indicating the existence of a C2/c–P2₁/c transition for the Ca-poor phase in the Ca–Mn–Mg pyroxene system. In situ heating experiments yield a temperature of the C2/c-P2₁/c inversion of 330±20°C. Observation of antiphase boundary (APB) positions before and after heating through the inversion temperature showed that the APB positions were essentially unchanged, indicating stabilization of APB’s by the concentration of Ca or some defect at the APB’s. Lattice fringe images across the lamellar boundary (001) indicate that the interface is semi- or completely coherent. The features of this exsolution texture are consistent with initial exsolution by spinodal decomposition or nucleation and subsequent slow cooling in a regionally metamorphosed phase.