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.
Suggested Citation
Evans, H.T., Stephen, H.J., Konnert, J., 1978, The crystal structure and thermal history of orthopyroxene from lunar anorthosite 15415: Earth and Planetary Science Letters, v. 37, no. 3, p. 476-484, https://doi.org/10.1016/0012-821X(78)90063-8.
| 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 |