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Chopinite, [(Mg, Fe)₃](PO₄)₂, a new mineral isostructural with sarcopside, from a fluorapatite segregation in granulite-facies paragneiss, Larsemann Hills, Prydz Bay, East Antarctica

¹ Department of Earth Sciences, University of Maine, 5790 Bryand Research Center, Orono, Maine 04469-5790, USA
² Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestrasse 3, 3012 Bern, Switzerland
³ Institut für Geowissenschaften/Mineralogie, Ruhr-Universität Bochum, 44780 Bochum, Germany
⁴ Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

^* Corresponding author, e-mail: esgrew{at}maine.edu

Chopinite, the Mg-dominant analogue of sarcopside, is a new mineral corresponding to synthetic Mg₃(PO₄)₂-II, a high-pressure polymorph of the meteoritic mineral farringtonite. A representative electron-microprobe analysis is SiO₂ 0.32, P₂O₅ 47.32, Al₂O₃ 0.05, MgO 30.35, MnO 0.15, FeO 20.99, CaO 0.35, F 0.02, Cl 0.01, Sum 99.54 wt%, which gives Ca_0.02Mg_2.20Fe_0.86Mn_0.01Si_0.02P_1.95O₈. Single-crystal X-ray diffraction gives monoclinic symmetry, P2₁/c, a = 5.9305(7) Å, b = 4.7583(6) Å, c = 10.2566(10) Å, = ß 90.663(9)°, V 289.41(6) ų, calculated density 3.34 g/cm³, Z = 2. Chopinite is of the olivine structure type, but with ordered vacancies and strongly distorted octahedra due to the valence 5+ for P, which results in marked ordering of Mg at M2, whereas Fe²⁺ concentrates at M1, most likely because of its axial symmetry. The strongest lines in the powder pattern [d in Å, (I_calc), (hkl)] are 5.92 (42) (100), 3.84(100) (102), 3.48(52) (11, 012, 111), 2.51(72) (11, 113), 2.44 (73) (21, 211). Chopinite is colorless and transparent, biaxial (–), 1.595(2), ß1.648(2), 1.656(2) (589 nm). 2V_x (meas.) = 40(2)°, 2V_x (calc.) = 41°; X //b, Z ^ a ~55°. Chopinite is found as four inclusions isolated in a fluorapatite segregation in a quartz mass in a paragneiss from Brattnevet, Larsemann Hills, East Antarctica. Grains are mostly anhedral and range from 0.1 x 0.3 mm to 0.2 x 0.6 mm in size. Minerals present in the chopinite-bearing specimen include wagnerite-Ma5bc, xenotime-(Y), stornesite-(Y), P-bearing K-feldspar and plagioclase, Ti-rich biotite, sillimanite, orthopyroxene, sapphirine, hercynite, and corundum. It is inferred to have formed as a result of high melt P concentrations by reaction of biotite with an anatectic melt in which P/Ca ratio exceeded that buffered by apatite saturation due to the very slow diffusion of P relative to Ca in anatectic melt.

Key-words: phosphate, new mineral, Antarctica, Larsemann Hills, electron microprobe, crystal structure, granulite facies, anatexis.

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