Description and crystal structure of a new mineral, edwardsite, Cu3Cd2(SO4)2(OH)6•4H2O, from Broken Hill, New South Wales, Australia

Edwardsite, Cu3Cd2(SO4)(2)(OH)(6)center dot 4H(2)O, is a new mineral from the Block 14 Opencut, Broken Hill, New South Wales, Australia. It occurs as druses of tabular and bladed crystals up to 0.06 mm in size, associated with niedermayrite and christelite. Edwardsite is pale blue, transparent with vitreous lustre and has excellent cleavage parallel to {100}. Density was not measured but the calculated density, from the empirical formula, is 3.53 g cm(-3) and the Mohs hardness is similar to 3. Optically, it is biaxial negative with alpha similar to 1.74, beta = 1.762(4), gamma similar to 1.77 and 2V(calc.) similar to +62 degrees. The optical orientation is X = b, Y similar to a, Z similar to c. Electron microprobe analysis gave (wt.%): CdO 32.43, CuO 28.06, ZnO 2.26, FeO 0.08, SO3 20.35, H2Ocalc. (from crystal-structure analysis) 14.14, totalling 99.32. The empirical formula, calculated on the basis of 18 oxygen atoms is Cu2.77Cd1.98Zn0.22Fe0.01(SO4)(2.00)(OH)(5.95)center dot 4.06H(2)O. Edwardsite is monoclinic, space group P2(1)/c, with a = 10.863(2) angstrom, b = 13.129(3) angstrom, c = 11.169(2) angstrom, beta = 113.04(3)degrees, V = 1465.9(5) angstrom(3) (single-crystal data) and Z = 4. The eight strongest lines in the powder diffraction pattern are [d (angstrom), (I/I-0), (hkl)]; 9.991, (90), (100); 5.001, (90), (200, 21 (1) over bar); 4.591, (45), (20 (2) over bar); 3.332, (60), (300, 032); 3.005, (30), (0 (2) over bar3); 2.824, (40), ((1) over bar(4) over bar2); 2.769, (55), (20 (4) over bar, 042, 10 (4) over bar); 2.670, (45), (2 (4) over bar(2) over bar). The crystal structure was determined by direct methods and refined to R1 = 3.21% using 1904. observed reflections with F-o > 4 sigma(F-o) collected using synchrotron X-ray radiation (lambda = 0.773418 angstrom). The structure is based on infinite sheets of edge-sharing Cu phi(6) (phi: O2-, OH) octahedra and Cd phi(7) (phi: O2-, H2O) polyhedra parallel to (100). The sheets are decorated on both sides by corner-sharing (SO4) tetrahedra, which also corner-link to isolated Cd phi(6) octahedra, thus connecting adjacent sheets. Moderate-strong to weak hydrogen bonding provides additional linkage between sheets.