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

摘要

Chopinite, the Mg-dominant analogue of sarcopside, is a new mineral corresponding to synthetic Mg-3(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 Ca0.02Mg2.20Fe0.86Mn0.01Si0.02P1.95O8. Single-crystal X-ray diffraction gives monoclinic symmetry, P2(1)/c, a = 5.9305(7) angstrom, b = 4.7583(6) angstrom, c = 10.2566(10) angstrom, beta = 90.663(9)degrees, V 289.41(6) angstrom(3), calculated density 3.34 g/cm(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 Fe2+ concentrates at M 1, most likely because of its axial symmetry. The strongest lines in the powder pattern [d in angstrom, (I-calc), (hkl)] are 5.92 (42) (100), 3.84(100) (102), 3.48(52) (11 (1) over bar, 012, 111), 2.51(72) (11 (3) over bar, 113), 2.44 (73) (21 (1) over bar, 211). Chopinite is colorless and transparent, biaxial (-), alpha 1.595(2), beta 1.648(2), gamma 1.656(2) (589 nm). 2V(x) (meas.) = 40(2)degrees, 2V(x) (calc.) = 41 degrees; X // b, Z(boolean AND) a similar to 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 0.1 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.