The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415
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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 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 MoKα 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 (MgqFe1−q) for cation siteM(1) and (Mg1.14−qFeq−0.18Ca0.04) 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.
| 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 |
| DOI | 10.1016/0012-821X(78)90063-8 |
| Volume | 37 |
| Issue | 3 |
| Year Published | 1978 |
| Language | English |
| Publisher | Elsevier |
| Description | 9 p. |
| First page | 476 |
| Last page | 484 |
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