The separate distributions for MgSiOa and FeSiO 3 in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe 2+ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe e+ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO a content but the corresponding Mg site occupancy appears largely independent of MgSi03. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxenc decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe :+ in the h~ (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in K D with composition resulting from intraerystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in K D between igneous and metamorphic pyroxenes.
The variation of the augite limb of the pyroxene solvus within the pyroxene quadrilateral is developed as a possible geothermometer.