Chopinite, [(Mg, Fe) (sub 3) {square}](PO (sub 4) ) (sub 2) , a new mineral isostructural with sarcopside, from a fluorapatite segregation in granulite-facies paragneiss, Larsemann Hills, Prydz Bay, East Antarctica

Chopinite, the Mg-dominant analogue of sarcopside, is a new mineral corresponding to synthetic Mg (sub 3) (PO (sub 4) ) (sub 2) -II, a high-pressure polymorph of the meteoritic mineral farringtonite. A representative electron-microprobe analysis is SiO (sub 2) 0.32, P (sub 2) O (sub 5) 47.32, Al (sub 2) O (sub 3) 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 (sub 0.02) Mg (sub 2.20) Fe (sub 0.86) Mn (sub 0.01) Si (sub 0.02) P (sub 1.95) O (sub 8) . Single-crystal X-ray diffraction gives monoclinic symmetry, P2 (sub 1) /c, a = 5.9305(7) Aa, b = 4.7583(6) Aa, c = 10.2566(10) Aa, = beta 90.663(9) degrees , V 289.41(6) Aa (super 3) , calculated density 3.34 g/cm (super 3) , 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 Fe (super 2+) concentrates at M1, most likely because of its axial symmetry. The strongest lines in the powder pattern [d in Aa, (I (sub calc) ), (hkl)] are 5.92 (42) (100), 3.84(100) (102), 3.48(52) (111, 012, 111), 2.51(72) (113, 113), 2.44 (73) (211, 211). Chopinite is colorless and transparent, biaxial (-), alpha 1.595(2), beta 1.648(2), gamma 1.656(2) (589 nm). 2V (sub x) (meas.) = 40(2) degrees , 2V (sub x) (calc.) = 41 degrees ; X //b, Z - a approximately 55 degrees . 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 0.1X0.3 mm to 0.2X0.6 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.