Crystallographic studies of coexisting aluminan orthopyroxene and augite of high-pressure origin

Abstract

The cation distributions and the crystal structures of an aluminan bronzite and its coexisting augite have been refined by a least-squares technique using X-ray intensity data measured on a Picker single-crystal diffractometer. The intracrystalline cation distribution coefficient, k = (Fe/Mg)M1/(Fe/Mg)M2, of the augite is apparently larger than that of the bronzite, but the difference may not be significant because of the low concentration of Fe2+ and the presence of substantial amounts of Fe3+ and Al. Smaller M1 octahedra (M1−O = 2.070 A for the bronzite, M1−O = 2.057 A for the augite) reflect the presence of Al3+ and Fe3+ in the M1 sites and contribute to the high density characteristic of high-pressure minerals. The evidence of site preference of Al in the SiB site of orthopyroxene helps to explain why the concentration of Al in orthopyroxene is lower than it is in augite. The trend of continuous structural changes from calcic to subcalcic augites has been traced by comparing these pyroxenes with the coexisting aluminan pigeonite-augite pair from the Apollo 12 rock sample 12052. As Mg and Fe content in M2 increases, the distances between the M2 cation and the oxygens in the octahedral strip become shorter by a shift of the mean position of the M2 cation toward those oxygens, and the remaining M2−O distances become longer. The oxygens forming the shortest bond with M2 also approach M2. Interpretation of a structure at the extreme end of this trend offers a structural explanation of the presence of a miscibility gap. The apparent thermal ellipsoids, attributable to the positional disorder resulting from Ca-(Mg, Fe) disorder, trace the above trends.