Birchite a new mineral from Broken Hill New South Wales Australia: Description and structure refinement

Abstract

The new mineral species birchite, idealized formula Cd2Cu2(PO4)2 (SO4)-5H2O, Occurs on specimens from the Block 14 Opencut, Broken Hill, New South Wales, Australia, as sprays and aggregates of crystals to 0.75 mm across on a host rock composed of quartz, garnet, galena, chalcopyrite, and fluorapatite. It is a late-stage supergene mineral formed as part of a suite of secondary phosphate minerals under low-temperature conditions. Associated secondary minerals are covellite, cerussite, anglesite, plumbogummite-hinsdalite, pyromorphite, libethenite, and sampleite. Individual crystals are bladed to prismatic and acicular in habit, with a maximum length of 0.3 mm and width of 0.05 mm. The crystals are elongated along [001] and sometimes also flattened on (100). The crystal forms are major {100} and {010}, and minor {101} and {001}. Birchite is orthorhombic, space group Pnma, with unit-cell parameters refined from powder X-ray diffraction data, a = 10.489(6), b = 20.901(7), c = 6.155(5) Å, V= 1349.6(3) Å, and Z = 4. The eight strongest lines in the diffiaction pattern are [d(Å)(I)(hkl]: 10.451(100)(020); 5.146(28)(111); 4.223(38)(131); 3.484(39)(060); 2.902(70)(260); 2.719(33)(132); 2.652(32)(042); 1.919(80)(432). Birchite is translucent (masses) to transparent (crystals); pale blue with a vitreous luster. Optically, birchite is biaxial positive, with nα = 1.624(4), nβ = 1.636(5), nγ = 1.669(4), and 2Vcalc = +63°. The optical orientation is X = b, Y = a, Z = c; the optical axis plane lies within the {100} plane. Birchite shows very faint pleochroism, X = pale bluish, Z = pale greenish, absorption Z ≥ X. Birchite is brittle, has a conchoidal fracture and is nonfluorescent. Hardness (Mohs) is 3.5-4; the measured density is 3.61(4) g/cm3, and the calculated density is 3.647 g/cm3 (from the empirical formula). Average electron microprobe analysis (wt%): CdO 36.79, CuO 21.22, CaO 0. 17, MnO 0.17, ZnO 1.07, P2O5 20.21, SO3 9.70, H2O (from crystal-structure analysis) 12.37, total 101.70. The empirical formula, calculated on the basis of 17 O atoms and with H2O calculated to give 5H2O is (Cu1.94, ZN0.10) ε2.04 (Cd2.09, Ca0.2, Mn0.02) ε2.13 P2.07 S0.88 O12·5H2O. The crystal structure has been refined to an R index of 4.3% for 846 observed reflections measured with MoKα X-radiation. Alternating [CdO4(H2O)2] octahedra and [CUO3(H2O)2] square-pyramids share edges to form chains that extend along the α axis, which are linked by (PO4) tetrahedra to form [CdCu(PO4) (H2O)2O] sheets in the (010) plane. Two such sheets are linked via (PO4) tetrahedra vertices to form a layer in the (010) plane. Two layers, which are related by mirror symmetry, are linked via (SO4) tetrahedra vertices to form a heteropolyhedral framework structure. Interstitial channels within the framework extend along both the α and c axes and are occupied by a H2O group. The mineral is named for William D. Birch, Senior Curator of Geosciences at Museum Victoria, Australia.