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 Ca0.02Mg2.20Fe0.86Mn0.01Si0.02P1.95O8. 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, = β 90.663(9)°, V 289.41(6) Å3, 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, (Icalc), (hkl)] are 5.92 (42) (100), 3.84(100) (102), 3.48(52) (111̅, 012, 111), 2.51(72) (113̅, 113), 2.44 (73) (211̅, 211). Chopinite is colorless and transparent, biaxial (−), α 1.595(2), β1.648(2), γ1.656(2) (589 nm). 2Vx (meas.) = 40(2)°, 2Vx (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 × 0.3 mm to 0.2 × 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.