Gillardite, Cu3NiCl2(OH)6, a new mineral from the 132 North deposit, Widgiemooltha, Western Australia

Gillardite, Cu3NiCl2(OH)6 (IMA 2006-041), is a new mineral from the 132 North deposit, Widgiemooltha, Western Australia, Australia. The name is in honour of Professor D. Gillard, in recognition of his contributions to the field of inorganic chemistry. It occurs as aggregates of equant crystals up to 0.5mm in size in a siicified ferruginous gossan, associated with a variety of secondary Ni and Cu minerals. Gillardite is rhombohedral, space group R3¯m, with single-crystal unit-cell parameters a=6.8364(1), c=13.8459(4)Ã3,Z=3,Dcalc=3.76g cm³. The ten strongest lines in the X-ray powder diffraction pattern [d in Ã(I)(hkl)] are 5.459(100)(101), 2.753(69)(113), 2.256(39)(204), 2.901(19)(201), 4.648(16)9003), 2.725(14)(202), 1.818(13)(303), 4.515(11)(102), 1.711(10)(220) and 3.424(8)(110). An average of 12 microprobe analyses (wt%) gave CuO, 55.6; 15.3; CoO, 0.2; FeO, 0.1; Cl, 17.3. One analysis (TGA) gave H2O, 13.1, less O=Cl, -3.9; total, 97.7. The derived empirical formula (based on 2 Cl pfu) is (Cu2.865Ni0.840Co0.006)Σ3.722Cl2(OH)5.960. Normalisation of the metal distribution to 4 metal ions pfu gives (Cu3.08Ni0.90Co0.01Fe0.01)Σ4.00Cl2(OH)5.96. Spot analyses show variation of Cu:Ni ratios and metal occupancies from (Cu3.135Ni0.853Co0.012) to (Cu2.922Ni1.058Co0.020). The simplified formula is thus Cu3.1Ni0.9Cl2(OH)6 or Cu3NiCl2(OH)6. The formula is entirely consistent with the results of a single-crystal X-ray structure analysis. Equant rhombohedral crystals showing the forms {101}, {021}, {0001} and {100} (probable) are dark green in colour and larger crystals are nearly black. No twinning was observed. Gillardite is non-fluorescent, has a green streak and is transparent with a vitreous lustre. Mohs hardness is 3, fracture is splintery and uneven, and cleavage is good on{101}. Gillardite is uniaxial (+), with ω=1.836 (0.002, ε=1.838 (0.002) (white light). No dispersion or pleochroism was observed. Gillardite is isomorphous with herbertsmithite, Cu3ZnCl2(OH)6, but can be conveniently distinguished from the latter by chemical analysis and a careful examination of X-ray powder diffraction data