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IDENTIFICATION OF SCORODITE IN FINE-GRAINED, HIGH-SULFIDE, ARSENOPYRITE MINE-WASTE USING MICRO X-RAY DIFFRACTION (µXRD)

Roberta L. Flemming^1,, Kristin A. Salzsauler^2,*, Barbara L. Sherriff² and Nikolay V. Sidenko²

¹ Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada
² Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada

E-mail address: rflemmin{at}uwo.ca

Secondary phases precipitated during oxidation of sulfides in mine wastes can be very fine-grained and poorly crystalline, making accurate identification difficult. As part of a study to examine arsenic mobility within the arsenopyrite residue stockpile, Snow Lake, Manitoba, arsenic-rich secondary phases were examined by a combination of micro X-ray diffraction (µXRD) and electron-probe micro-analysis (EPMA). With EPMA, we found one of the secondary As-containing phases to have an Fe:As ratio of 1:1. Examination of µXRD data allowed positive identification of this phase as scorodite (FeAsO₄· 2H₂O). Two texturally distinct occurrences of scorodite were identified in specific areas of the polished thin sections. Type-1 scorodite occurs around grains of primary arsenopyrite, and Type-2 scorodite is disseminated throughout an amorphous iron sulfo-arsenate (AISA) matrix in highly altered material. Our observations suggest that µXRD can be used to routinely identify phases comprising <1% of the bulk sample, provided that these phases are positioned under the X-ray beam. All of the scorodite examined is very fine-grained polycrystalline material, displaying homogeneous Debye powder rings in the two-dimensional (2D) General Area Diffraction Detector System (GADDS) image, using either a 500 or 50 µm X-ray beam diameter. The homogeneous texture of fine-grained secondary scorodite makes it easily discernable from relatively coarse-grained primary minerals, which give large discrete diffraction-spots or discontinuous "grainy" Debye rings in the GADDS image.

Keywords: scorodite, arsenic, mine waste, micro X-ray diffraction, Snow Lake, Manitoba.

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