The geochemistry of arsenic in acid mine drainage: the role of pharmacosiderite

One of the most interesting discoveries of the well-studied Sunny Corner acid mine drainage (AMD) site in the central west of New South Wales, Australia, is the presence of pharmacosiderite. The detection of this ‘rare’ iron arsenate mineral by chance in the hard host-rock geology triggered several questions on its occurrence, formation, solubility and ultimately – how it influences arsenic geochemistry. Pharmacosiderite group minerals (AFe4(OH)4(AsO4)3·nH2O) have been identified at the oxidised ores of Sunny Corner Mine This group of minerals are considered rare and are typically identified in discrete quantities such as a 3 mm seam on a quartz substrate (Hager, 2013). The synthesis of pure pharmacosiderite minerals has proven difficult and the hydronium endmember is the most elusive (Majzlan et al. 2019).