Chemical composition and crystal structure of merrillite from the Suizhou meteorite

Xiande Xie, Hexiong Yang, Xiangping Gu, Robert T Downs

Research output: Contribution to journalArticle

9 Scopus citations

Abstract

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(Mg0.95Fe0.061.01 (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(Ca0.50.5)(PO4)]16+, or [(Ca9-xREE)x(Na1-xx) (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.

Original languageEnglish (US)
Pages (from-to)2753-2756
Number of pages4
JournalAmerican Mineralogist
Volume100
Issue number11-12
DOIs
StatePublished - Nov 1 2015
Externally publishedYes

Keywords

  • crystal structure
  • Merrillite
  • Raman spectroscopy
  • Suizhou meteorite
  • whitlockite

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Geophysics

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