Chemical composition and crystal structure of merrillite from the Suizhou meteorite

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)Sigma(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) angstrom, and V = 3430.5(2) angstrom(3). Its crystal structure, refined to R-1 = 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 [Ca-9(PO3[OH])](16+), [Ca-9(PO3F)](16+), {Ca-9(Ca-0 5 square(0 5))(PO4)](16+), or [(Ca9-xREE)(x)(Na1-x square(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 BEE-bearing tuite on the Moon as a result of shock events on REE-merrillite.