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, Ca_9.00Na_0.98(Mg_0.95Fe_0.06)Σ_1.01 (P_1.00O₄)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) ų. 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(Ca_0.5□_0.5)(PO₄)]¹⁶⁺, or [(Ca_9-xREE)_x(Na_1-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.