Description and crystal structure of vanderheydenite, Zn (sub 6) (PO (sub 4) ) (sub 2) (SO (sub 4) )(OH) (sub 4) .7H (sub 2) O, a new mineral from Broken Hill, New South Wales, Australia

Vanderheydenite, Zn (sub 6) (PO (sub 4) ) (sub 2) (SO (sub 4) )(OH)4.7H (sub 2) O, is a new mineral from the Block 14 Opencut, Broken Hill, New South Wales, Australia. It occurs as aggregates of colourless crystals up to 0.5 mm across in voids of a sphalerite-galena matrix and is associated with anglesite, pyromorphite, sulfur, and liversidgeite. Crystals are pseudohexagonal blades up to 0.4 mm in length, flattened on {1 0 0} and exhibiting the forms {1 0 0}, {0 1 0}, and {0 2 1}. Cleavage was not observed. The Mohs hardness is estimated to be 3. The calculated density is 3.12 g/cm (super 3) from the empirical formula and 3.06 g/cm (super 3) from the ideal formula. The mineral is optically biaxial (-), with alpha = 1.565(4), beta = 1.580(4) and gamma = 1.582(4). The calculated 2V is 39.8 degrees . Chemical analysis by electron microprobe gave ZnO 55.63, CuO 0.07, FeO 0.11, MnO 0.06, P (sub 2) O (sub 5) 14.18, As (sub 2) O (sub 5) 4.33, SO (sub 3) 8.71, H (sub 2) O 18.31, total 101.40 wt%, with H (sub 2) O content derived from the refined crystal structure. The empirical formula calculated on the basis of 23 oxygen atoms is (Zn (sub 5.99) Cu (sub 0.01) Fe (sub 0.01) Mn (sub 0.01) ) (sub Sigma 6.02) [(PO (sub 4) ) (sub 1.75) (AsO (sub 4) ) (sub 0.33) ] (sub Sigma 2.08) (SO (sub 4) ) (sub 0.95) (OH) (sub 3.91) .6.96H (sub 2) O. The mineral is monoclinic, P2 (sub 1) /n, with a = 6.2040(12), b = 19.619(4), c = 7.7821(16) Aa, beta = 90.67(3) degrees , V = 947.1(3) Aa (super 3) . The five strongest lines in the X-ray powder diffraction pattern are [d(Aa), (I), (hkl)]: 9.826 (57) (0 2 0), 7.296 (20) (0 1 1), 6.134 (1 0 0) (0 2 1), 3.368 (10) (0 3 2, 1 5 0), 3.069 (9) (2 1 0, 0 4 2). The crystal structure of vanderheydenite (R1 = 0.0497 for 939 reflections with Fo > 4sigma F) contains chains of edge-sharing ZnO (sub 6) octahedra parallel to a that are linked by edge- and corner-sharing ZnO (sub 5) trigonal bipyramids and TO (sub 4) (T = P, As) tetrahedra forming zig-zag sheets parallel to {0 1 0}. Sheets are linked by half-occupied, distorted TO (sub 4) (T = P, S) tetrahedra in the [0 1 1] direction. Interstitial channels extend parallel to the a-direction and are occupied by strongly to weakly hydrogen-bonded H (sub 2) O groups.