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

Johntomaite, ideally BaFe22+Fe23+(PO4)(3)(OH)(3), is the Fe2+-analogue of p erloffite and represents a new member of the bjarebyite group of barium pho sphates. 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, pseudomalac hite 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 4 1/2, and D(meas.) 4.05( 1), D(calc.) 4.084 g/cm(3). Optically, it is biaxial negative, with alpha 1 .817(3), beta 1.829(6), gamma 1.837(3), 2V(alpha) similar to 80-85(5)degree s (meas.), 78.0 degrees (calc.). Pleochroism is strong: X bluish green, Y d ark brownish green, Z brownish; absorption Y>X greater than or equal to Z; orientation Z = b. Dispersion is strong, (probably) r much less than nu. Th e mineral is monoclinic, P2(1)/m, with a 9.199(9), b 12.359(8), c 5.004(2) Angstrom, beta 100.19(6)degrees, V 559.9(5) Angstrom(3) and Z = 2. The stro ngest 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 22.6 2, Al2O3 0.25, P2O5 30.45, SiO2 0.13, F 0.36, H2O 3.73 (calc.), less O equi valent to F -0.15, sum 98.89. The empirical formula is Ba-1.00 (Fe1.292+Ca0 .41Mn0.26Na0.02Mg0.01Cu0.01Zn0.01)(Sigma 2.01)(Fe1.973+Al0.03)(Sigma 2.00) [(P2.98Si0.02) O-4](3.00)(OH2.85F0.13)(Sigma 2.98), 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 bjareby ite-type structure of johntomaite. Both refinement and chemical analyses de monstrate that Ca and (Fe + Mn) are negatively correlated. Johntomaite is n amed after Mr. John Toma, amateur mineralogist and finder of the mineral. T he type specimen is deposited at the South Australian Museum, Adelaide, Aus tralia.