Vibrational spectroscopy study of hydrothermally produced scorodite (FeAsO4•2H2O), ferric arsenate sub-hydrate (FAsH; FeAsO4•0.75H2O) and basic ferric arsenate sulfate (BFAS; Fe[(AsO4)1-x(SO4)x(OH)x]•wH2O)

Three crystalline ferric arsenate phases: (1) scorodite; FeAsO4 center dot 2H(2)O,(2) ferric arsenate sub-hydrate [FAsH; FeAsO4 center dot 0.75H(2)O) and (3) basic ferric arsenate sulfate (BFAS; Fe[(AsO4)(1-x)(SO4)(x)(OH)(x)]center dot wH(2)O) synthesized by hydrothermal precipitation (175-225 degrees C) from Fe(III)-ASO(4)(3-)-SO42- solutions have been investigated via Raman and infrared spectroscopies. The spectroscopic nature of these high-temperature Fe(III)- AsO43- -SO42- phases has not been extensively studied despite their importance to the hydrometallurgical industrial processing of precious metal (Au and Cu) arsenic sulfidic ores. It was found that scorodite, FAsH and BFAS all gave rise to very distinct arsenate, sulfate and hydroxyl vibrations. In scorodite and FAsH, the distribution of the internal arsenate modes was found to be distinct, with the factor effect being more predominant in the crystal system. For the crystallographically unknown BFAS phase, vibrational spectroscopy was used to monitor the arsenate <-> sulfate solid solution behavior that occurs in this phase where the molecular symmetry of arsenate and sulfate in the crystal structure is reduced from an ideal T-d to a distorted T-d or C-2/C-2v symmetry. With the new collected vibrational data of the pure phases, the use of attenuated total reflectance infrared (ATR-IR) spectroscopy was finally extended to investigate the nature of the arsenate in an industrial residue generated by pressure oxidation of a gold ore, where it was found that the arsenate was present in the form of BFAS. Copyright (C) 2009 John Wiley & Sons, Ltd.