Identification of Scorodite in Fine-Grained, High-Sulfide, Arsenopyrite Mine-Waste Using Micro X-Ray Diffraction ( xrd)

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.