Sulfide weathering in As- and Sb-extreme environments: tackling arsenic in pyrochlore-supergroup antimonates

Supervisor: Tamara Đorđević (Department of Mineralogy and Crystallography) in collaboration with Uwe Kolitsch (Natural History Museum)

Funding Situation: potentially via a VISESS PhD fellowship

Project outline: In our recent projects dealing with the mineralogy and geochemistry of mining wastes, we often encounter a number of metastable minerals originating at low temperatures. They can form slowly from solid, disordered precursors. A good example of such processes is the weathering of sulfides and arsenosulfides. In the initial stages of such weathering, X-ray amorphous, nanocrystalline products are formed. They are unstable and can slowly convert to crystalline minerals. Recently, our group has investigated the weathering of Fe-poor and realgar-rich tailings (60 wt% AsS), containing significant amounts of stibnite (up to 13.5 wt%), where the main weathering products were mixtures of Fe3+-rich nanocrystalline pyrochlore-type antimonates (PGA) (general formula, A2Sb2O6Y) and amorphous As-dominant PGA. The crystallographic position of arsenic in these samples is not fully clear. Although in few cases As5+ may occupy the octahedrally coordinated B-position, it strongly prefers a tetrahedral coordination. The distorted cubic A-position is too large for the As(V). These crystal-chemical preferences explain that As is hosted mostly in the X-ray amorphous phases. However, it was clearly documented that the PGA may incorporate considerable arsenic. In the proposed project we aim to investigate the modes of arsenic incorporation in PGA at low temperatures (co-precipitation and hydrothermal experiments) in order to clarify the mechanisms that control As and Sb geochemistry during initial stages of weathering. Furthermore, we will examine natural PGA samples for the presence of As using methods with high-detection limits such as transmission electron microscopy and X-ray absorption spectroscopy.