Birchite a new mineral from Broken Hill New South Wales Australia: Description and structure refinement
Date
2008
Authors
Elliott, P.
Brugger, J.
Pring, A.
Cole, M.
Willis, A.
Kolitsch, U.
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Journal article
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American Mineralogist: an international journal of earth and planetary materials, 2008; 93(5-6):910-917
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Peter Elliott, Joël Brugger, Allan Pring, Marcus L. Cole, Anthony C. Willis, and Uwe Kolitsch
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Abstract
The new mineral species birchite, idealized formula Cd<inf>2</inf>Cu<inf>2</inf>(PO<inf>4</inf>)<inf>2</inf> (SO<inf>4</inf>)-5H<inf>2</inf>O, 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), n<inf>β</inf> = 1.636(5), n<inf>γ</inf> = 1.669(4), and 2V<inf>calc</inf> = +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/cm<sup>3</sup>, and the calculated density is 3.647 g/cm<sup>3</sup> (from the empirical formula). Average electron microprobe analysis (wt%): CdO 36.79, CuO 21.22, CaO 0. 17, MnO 0.17, ZnO 1.07, P<inf>2</inf>O<inf>5</inf> 20.21, SO<inf>3</inf> 9.70, H<inf>2</inf>O (from crystal-structure analysis) 12.37, total 101.70. The empirical formula, calculated on the basis of 17 O atoms and with H<inf>2</inf>O calculated to give 5H<inf>2</inf>O is (Cu<inf>1.94,</inf> ZN<inf>0.10</inf>) <inf>ε2.04</inf> (Cd<inf>2</inf>.<inf>09</inf>, Ca<inf>0.2</inf>, Mn<inf>0.02</inf>) <inf>ε2.13</inf> P<inf>2.07</inf> S<inf>0.88</inf> O<inf>12</inf>·5H<inf>2</inf>O. The crystal structure has been refined to an R index of 4.3% for 846 observed reflections measured with MoKα X-radiation. Alternating [CdO<inf>4</inf>(H<inf>2</inf>O)<inf>2</inf>] octahedra and [CUO<inf>3</inf>(H<inf>2</inf>O)<inf>2</inf>] square-pyramids share edges to form chains that extend along the α axis, which are linked by (PO<inf>4</inf>) tetrahedra to form [CdCu(PO<inf>4</inf>) (H<inf>2</inf>O)<inf>2</inf>O] sheets in the (010) plane. Two such sheets are linked via (PO<inf>4</inf>) tetrahedra vertices to form a layer in the (010) plane. Two layers, which are related by mirror symmetry, are linked via (SO<inf>4</inf>) 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 H<inf>2</inf>O group. The mineral is named for William D. Birch, Senior Curator of Geosciences at Museum Victoria, Australia.
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Copyright © 2008 Mineralogical Society of America