Abstract
Ashburtonite, ideally HPb4Cu4Si4O12(HCO3)4(OH)4Cl, is a new mineral from Ashburton Downs, Western Australia. It occurs as clusters of clear blue, prismatic crystals up to 0.4 mm long. Associated minerals include diaboleite, duftite, beudantite, caledonite, plattnerite, cerussite, malachite, and brochantite. The mineral has a vitreous luster and light blue streak. It is brittle with a conchoidal fracture. Ashburtonite is uniaxial positive with ω = 1.786(3) and ∈ = 1.800(4). It is tetragonal, space group I4/m, with a = 14.234(7), c = 6.103(5) Å, and Z = 2. The strongest X-ray powder-diffraction lines are[d(Å), I, hkl] 10.2(100,110), 5.644(70,101), 4.495(100,310), 3.333(100,321), 3.013(90,411), 2.805(30,202), 2.611(50,222), 2.010(30,710,103,631), 1.656(30,642,503). Crystals are elongated on [001] with forms {110}, {100}, {001}, and {301}. The infrared spectrum indicates the presence of OH and CO3 groups. An electron-microprobe analysis gave PbO 52.2, CuO 18.7, SiO2 14.1, Cl 2.3, H2O(calc.) 4.22 and CO2(calc.) 10.31, −O = Cl = 0.51, total 101.20 wt%; Dcalc = 4.69 g/cm3. The structure has been refined to R = 3.5 and Rw = 2.4%. The four-membered, tetrahedral silicate rings are cross linked by ribbons of edge-shared Cu octahedra. The [HCO3] groups are loosely bonded to the sides of larye c axis channels by Cu octahedra and Pb in ninefold coordination. The Pb2+ has a stereo-chemically inactive lone pair of electrons, which causes a lowering of symmetry in the atomic structure.