Huebner J. Stephen
J.A. Konnert
H. T. Evans Jr.
1978
<p>A single crystal of untwinned orthopyroxene from lunar anorthosite sample 15415, with composition (Mg<sub>1.14</sub>Fe<sub>0.80</sub>Mn<sub>0.02</sub>Ca<sub>0.04</sub>)(Si<sub>1.97</sub>Al<sub>0.03</sub>)O<sub>6</sub>, has a unit cell in space group<i>Pbca</i><span> </span>with<i>a</i><span> </span>= 18.310(15)Å,<i>b</i><span> </span>= 8.904(10)Å,<i>c</i><span> </span>= 5.214(7)Å, containing 2 formula units. A set of 742 counter-measured intensity data made with MoK<sub>α</sub><span> </span>radiation has been used to refine the crystal structure in isotropic thermal mode to<i>R</i><span> </span>= 0.116. Anisotropic refinement led to<i>R</i><span> </span>= 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<i>q</i>, which gives (<i>Mg</i><sub>q</sub>Fe<sub>1−q</sub>) for cation site<i>M</i>(1) and (Mg<sub>1.14−<i>q</i></sub>Fe<sub><i>q</i>−0.18</sub>Ca<sub>0.04</sub>) for site<i>M</i>(2), was included in the least-squares analysis, yielding<i>q</i><span> </span>= 0.90(1).</p><p>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.</p>
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10.1016/0012-821X(78)90063-8
en
Elsevier
The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415
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