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The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415

Earth and Planetary Science Letters
By: , and 

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Abstract

A single crystal of untwinned orthopyroxene from lunar anorthosite sample 15415, with composition (Mg1.14Fe0.80Mn0.02Ca0.04)(Si1.97Al0.03)O6, has a unit cell in space group Pbca with a = 18.310(15) A ??, b = 8.904(10) A ??, c = 5.214(7) A ??, containing 2 formula units. A set of 742 counter-measured intensity data made with MoK?? 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 (MgqFe1-q) for cation site M(1) and (Mg1.14-qFeq-0.18Ca0.04) 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. ?? 1978.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415
Series title Earth and Planetary Science Letters
Volume 37
Issue 3
Year Published 1978
Language English
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Earth and Planetary Science Letters
First page 476
Last page 484