Barlowite, Cu ₄FBr(OH) ₆, a new mineral isotructural with claringbullite: Description and crystal structure

The new mineral species barlowite, ideally Cu₄FBr(OH)₆, has been found at the Great Australia mine, Cloncurry, Queensland, Australia. It is the Br and F analogue of claringbullite. Barlowite forms thin blue, platy, hexagonal crystals up to 0.5 mm wide in a cuprite-quartz-goethite matrix associated with gerhardtite and brochantite. Crystals are transparent to translucent with a vitreous lustre. The streak is sky blue. The Mohs hardness is 2-2.5. The tenacity is brittle, the fracture is irregular and there is one perfect cleavage on {001}. Density could not be measured; the mineral sinks in the heaviest liquid available, diluted Clerici solution (D ≈ 3.8 g/cm³). The density calculated from the empirical formula is 4.21 g/cm³. Crystals are readily soluble in cold dilute HCl. The mineral is optically non-pleochroic and uniaxial (-). The following optical constants measured in white light vary slightly suggesting a small variation in the proportions of F, Cl and Br: ω 1.840(4)-1.845(4) and ε 1.833(4)-1.840(4). The empirical formula, calculated on the basis of 18 oxygen atoms and H₂O calculated to achieve 8 anions and charge balance, is Cu_4.00F_1.11Br_0.95Cl_0.09(OH)_5.85. Barlowite is hexagonal, space group P6₃/mmc, a = 6.6786(2), c = 9.2744(3) Å, V = 358.251(19) ų, Z = 2. The five strongest lines in the powder X-ray diffraction pattern are [d(Å)(I)(hkl)]: 5.790(100)(010); 2.889(40)(020); 2.707(55)(112); 2.452(40)(022); 1.668(30)(220).