The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415
A single crystal of untwinned orthopyroxene from lunar anorthosite sample 15415, with composition (Mg 1.14 Fe 0.80 Mn 0.02 Ca 0.04 )(Si 1.97 Al 0.03 )O 6 , has a unit cell in space group Pbca with a = 18.310(15)Å, b = 8.904(10)Å, c = 5.214(7)Å, containing 2 formula units. A set of 742 counter-measured intensity data made with MoK α radiation has been used to refine the crystal structure in isotropic thermal mode to R = 0.116. Anisotropic refinement led to R = 0.092, but thermal parameters are distorted by non-random errors resulting from poor crystal texture. The resulting structure is in close agreement with that obtained by Ghose [9] for a hypersthene from Greenland. A parameter q , which gives ( Mg q Fe 1−q ) for cation site M (1) and (Mg 1.14− q Fe q −0.18 Ca 0.04 ) for site M (2), was included in the least-squares analysis, yielding q = 0.90(1). This orthopyroxene has the high degree of cation order expected of pyroxenes subjected to Apollonian metamorphism at lower than 500–600°C. No evidence exists for a subsequent thermal event of sufficient intensity to disorder the pyroxene. On the basis of previous laboratory studies of argon-release patterns of lunar plagioclase and order-disorder kinetics of terrestrial pyroxenes, we attribute the reported isotopic age (3.9–4.1 AE) to cessation of metamorphism, perhaps caused by impact excavation.