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) Å3, and Z = 4. The eight strongest lines in the diffraction 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 ZX. 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)∑2.04(Cd2.09,Ca0.02,Mn0.02)∑2.13P2.07S0.88O12·5H2O. 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 a 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 a 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.

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