^1,4Xiande Xie, ²Hexiong Yang, ³Xiangping Gu,²Robert T. Down
¹Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, 511 Kehua Street, Guangzhou, 510640, China
²Department of Geosciences, University of Arizona, 1040 East 4th Street, Tucson, Arizona 85721-0077, U.S.A.
³School of Geosciences and Info-Physics, Central South University, Changsha, Hunan, 410083, China
⁴Guangdong Key Laboratory of Mineral Physics and Materials, Guangzhou, 510640 China

Merrillite, ideally Ca9NaMg(PO4)7, is an important accessory phosphate mineral in many different groups of meteorites, including martian meteorites, and a major carrier of rare earth elements (REE) in lunar rocks. By means of electron microprobe analysis, single-crystal X-ray diffraction, and Raman spectroscopy, we present the first structure determination of merrillite with a nearly ideal chemical composition, Ca9.00Na0.98(Mg0.95Fe0.06)∑1.01 (P1.00O4)7, from the Suizhou meteorite, a shock-metamorphosed L6-chondrite. Suizhou merrillite is trigonal with space group R3c and unit-cell parameters a = 10.3444(3), c = 37.0182(11) Å, and V = 3430.5(2) Å3. Its crystal structure, refined to R1 = 0.032, is characterized by a structural unit consisting of a [(Mg,Fe)(PO4)6]16− complex anion that forms a “bracelet-and-pinwheel” arrangement. Such structural units are linked by interstitial complexes with a formula of [Ca9Na(PO4)]16+, which differs from that of [Ca9(PO3[OH])]16+, [Ca9(PO3F)]16+, [Ca9(Ca0.5□0.5)(PO4)]16+, or [(Ca9−xREE)x(Na1−x□x)(PO4)]16+ in terrestrial whitlockite, terrestrial/extraterrestrial bobdownsite, meteoritic Ca-rich merrillite, or lunar REE-rich merrillite, respectively. The Suizhou merrillite is found to transform to tuite at high pressures, pointing to the likelihood of finding REE-bearing tuite on the Moon as a result of shock events on REE-merrillite.

Reference
Xie X, Yang H, Gu X, Downs RT (2015) Chemical composition and crystal structure of merrillite from the Suizhou meteorite. American Mineralogist 100, 2753-2756
Link to Article [doi:10.2138/am-2015-5488]
Copyright: The Mineralogical Society of America

¹Colin, A., ¹Moreira, M., ²Gautheron, C., ³Burnard, P.
¹Institut de Physique du Globe de Paris, Université Paris Diderot, Paris, France
²Faculté des Sciences d’Orsay, Université Paris Sud, Orsay, France
³CRPG-CNRS, Université de Lorraine, Vandœuvre-lès-Nancy, France

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Reference
Colin A, Moreira M, Gautheron C, Burnard P (2015) Constraints on the noble gas composition of the deep mantle by bubble-by-bubble analysis of a volcanic glass sample from Iceland. Chemical Geology 417, 173-183.
Link to Article [DOI: 10.1016/j.chemgeo.2015.09.020]

¹Qi He, ¹Long Xiao, ²J. Brian Balta, ³Ioannis P. Baziotis, ⁴Weibiao Hsu, ⁵Yunbin Guan
¹Planetary Science Institute, School of Earth Sciences, China University of Geosciences, Wuhan, China
²Department of Geology and Planetary Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania, USA
³Agricultural University of Athens, Laboratory of Mineralogy and Geology, Athens, Greece
⁴Laboratory for Astrochemistry and Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
⁵Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California, USA

We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium-grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA-ICP-MS) analyses as a best-available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI-normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe-Ti oxide pairs is NNO-1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE-depleted reservoir and an oxidized, LREE-enriched reservoir. Stishovite, possibly seifertite, and dense SiO2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of ~30 GPa.

Reference
He Q, Xiao L, Balta JB, Baziotis IP, Hsu W, Guan Y (2015) Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12571]
Published by arrangement with John Wiley & Sons