Chemical composition and crystal structure of merrillite from the Suizhou meteorite
2015; Mineralogical Society of America; Volume: 100; Issue: 11-12 Linguagem: Inglês
10.2138/am-2015-5488
ISSN1945-3027
AutoresXiande Xie, Hexiong Yang, Xiangping Gu, Robert T. Downs,
Tópico(s)Chemical Synthesis and Characterization
ResumoMerrillite, 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)Σ1.01 (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.
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