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COBALTARTHURITE, Co²⁺Fe³⁺₂(AsO₄)₂(OH)₂·4H₂O, A NEW MEMBER OF THE ARTHURITE GROUP

John L. Jambor^1,, Joan Viñals², Lee A. Groat³ and Mati Raudsepp³

¹ Leslie Research and Consulting, 316 Rosehill Wynd, Tsawwassen, British Columbia V4M 3L9, Canada, and Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
² Department of Chemical and Metallurgical Engineering, University of Barcelona, E–08028 Barcelona, Spain
³ Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

E-mail address: jljambor{at}aol.com

Cobaltarthurite, of ideal end-member formula Co²⁺Fe³⁺₂(AsO₄)₂(OH)₂·4H₂O, is a new Co-dominant member of the arthurite group and is named to reflect that relationship. The new species occurs predominantly as coalesced pellet-like spheres, up to 1 mm across, that consist of straw-yellow to dark brown radial aggregates of fibrous crystals only a few micrometers in width. The mineral has a vitreous to silky luster, is brittle, translucent, nonfluorescent, has a hardness of 3 – 4, a white to light brown streak, and is readily soluble in HCl. Cobaltarthurite is biaxial positive, 1.741, ß 1.762, 1.797, 2V (calc.) 77°, with parallel extinction; the pleochroism is distinct, from almost colorless to medium yellow. Electron-microprobe analyses of yellow and dark brown material gave similar compositions, with the latter corresponding to (Co_0.50Mg_0.12Fe³⁺_0.11Mn_0.08Cu_0.010.11)_0.93Fe³⁺₂[(AsO₄)_1.95(PO₄)_0.04(SO₄)_0.01]_2.00(OH)_1.74·4H₂O, for which D_calc is 3.179 g/cm³ for Z = 2; the measured density is 3.22(2) g/cm³. Indexing of the X-ray powder pattern, which is similar to those of the other members of the arthurite group, gave a monoclinic cell with a 10.27, b 9.72, c 5.545 Å, ß 94.46°. The strongest six lines of the powder pattern [d in Å(I)(hkl)] are: 10.2(95)(100), 7.04(100)(110), 4.81(65)(001), 4.24(60)(111), 2.89(25)(221), and 2.87(55)(11). The mineral is associated with pharmacosiderite, olivenite, conichalcite, jarosite, arseniosiderite, and several other products of oxidation in a near-surface, sulfide-depleted lens near Mazarrón, in the Province of Murcia, southeastern Spain.

Keywords: cobaltarthurite, arthurite group, new mineral species, arsenate, X-ray data, infrared spectra, Raman spectra, Mazarrón, Spain.

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