^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