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_1.14Fe_0.80Mn_0.02Ca_0.04)(Si_1.97Al_0.03)O₆, has a unit cell in space groupPbca witha = 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 toR = 0.116. Anisotropic refinement led toR = 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 parameterq, which gives (Mg_qFe_1−q) for cation siteM(1) and (Mg_1.14−qFe_q−0.18Ca_0.04) for siteM(2), was included in the least-squares analysis, yieldingq = 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.