Johntomaite, a new member of the bjarebyite group of barium phosphates: description and structure refinement

Abstract

Johntomaite, ideally BaFe2+2Fe3+2(PO4)3(OH)3, is the Fe2+-analogue of perloffite and represents a new member of the bjarebyite group of barium phosphates. It occurs as an extremely rare mineral on the dumps of the Spring Creek copper mine near Wilmington in the southern Flinders Ranges of South Australia, where it forms clusters of greenish black, monoclinic prisms 0.3 to 1 mm in length. It is associated with quartz, libethenite, pseudomalachite and mitridatite. It is opaque with a dark greyish green streak, it has a vitreous, greasy to subadamantine lustre, a perfect cleavage parallel to {100}, an irregular fracture, a Mohs hardness of 4½, and D(meas.) 4.05(1), D(calc.) 4.084 g/cm3. Optically, it is biaxial negative, with α 1.817(3), β 1.829(6), γ 1.837(3), 2Vα ∼ 80–85(5)° (meas.), 78.0° (calc.). Pleochroism is strong: X bluish green, Y dark brownish green, Z brownish; absorption Y > X ≥ Z; orientation Z = b. Dispersion is strong, (probably) r ≪ v. The mineral is monoclinic, P21/m, with a 9.199(9), b 12.359(8), c5.004(2) Aring;, β 100.19(6)°, V 559.9(5) Aring;3 and Z = 2. The strongest lines in the X-ray powder diffraction pattern are [d(I ) hkl]: 3.159 (100) 031,−221; 2.983 (50) 211; 2.749 (50b) 221,−311; 4.573 (40) 011; 3.091 (40) −131. Electron microprobe analysis yielded (wt.%): BaO 21.96, FeO 13.34, CaO 3.28, MnO 2.67, Na2O 0.07, MgO 0.05, CuO 0.07, ZnO 0.06, Fe2O3 22.62, Al2O3 0.25, P2O5 30.45, SiO2 0.13, F 0.36, H2O 3.73 (calc.), less O ≡ F − 0.15, sum 98.89. The empirical formula is Ba1.00 (Fe2+1.29Ca0.41Mn0.26Na0.02Mg0.01Cu0.01Zn0.01)Σ2.01(Fe3+1.97Al0.03)Σ2.00 [(P2.98Si0.02) O4]3.00(OH2.85F0.13)Σ2.98, based on three (P + Si) and the assumption that the Fe(2) site is completely filled with Fe3+ and Al. A single-crystal structure refinement (R = 4.92%) confirmed the bjarebyite-type structure of johntomaite. Both refinement and chemical analyses demonstrate that Ca and (Fe + Mn) are negatively correlated. Johntomaite is named after Mr. John Toma, amateur mineralogist and finder of the mineral. The type specimen is deposited at the South Australian Museum, Adelaide, Australia.