Cardite, Zn5.5(AsO4)2(AsO3OH)(OH)3·3H2O, a new zinc arsenate mineral from Broken Hill, New South Wales, Australia

Cardite, Zn5.5(AsO4)2(AsO3OH)(OH)3·3H2O, is a new secondary mineral from the Block 14 Opencut, Broken Hill, New South Wales, Australia. Cardite occurs as radiating sprays of acicular crystals to 0.09 mm in length associated with anglesite, pyromorphite and köttigite. The mineral is white to pinkish white and pale tan with a white streak. Crystals are translucent with a vitreous lustre. The tenacity is brittle and the fracture is uneven. The calculated density is 4.02 g/cm3. Electron microprobe analyses (WDS mode) gave ZnO 43.39, CdO 7.26, CoO 1.24, MnO 0.59, FeO 0.12, As2O5 38.84, SO3 0.20, H2O (calc) 9.97, total 101.61 wt%. The empirical formula calculated on the basis of 18 oxygen atoms is (Zn4.75,Cd0.50,Co0.15,Mn0.07,Fe0.01)5.48[(AsO4)2.01(SO4)0.02]2.03(AsO3OH)(OH)2.89·2.98H2O. Cardite is orthorhombic, Cmcm, with unit-cell dimensions (100 K): a = 15.110(3), b = 15.492(3), c = 6.3850(13) Å, V = 1494.7(5) Å3 and Z = 4. The eight strongest lines in the X-ray powder diffraction pattern are [d(Å), (I), (hkl)]: 10.783 (100) (110), 7.564 (85) (200), 4.143 (48) (221), 3.328 (31) (041), 3.012 (20) (421), 2.763 (31) (222), 2.668 (24) (312), 2.451 (21) 260, 351, 461). The crystal structure of cardite (R1 = 0.067 for 13938 observed reflections with Fo > 4σ(Fo)) contains sheets of edge-sharing octahedra in which there is one vacancy for each four Zn cations in the sheet. Two orthogonal sheets lie parallel to (110) and to (1–10) and link along [001] by sharing common octahedra. AsO4 tetrahedra link to both sides of the sheet by corner-sharing. ZnO4 tetrahedra occupy channels in the structure and link to the sheets via corner-sharing AsO4 tetrahedra.