Tripuhyite and schafarzikite: two of the ultimate sinks for antimony in the natural environment

Abstract Studies of the stability of the oxides schafarzikite, FeSb 2 O 4 , and tripuhyite, FeSbO 4 , have been undertaken to clarify the roles these secondary minerals may have in determining the dispersion of antimony in oxidizing environments. Solubilities were determined at 298.15 K in aqueous HNO 3 , and these data were used to calculate values of Δ G f ϴ at the same temperature. The derived Δ G f ϴ (s, 298.15 K) values for FeSb 2 O 4 and FeSbO 4 are – 959.4±4.3 and – 836.8±2.2 kJ mol –1 , respectively. These results have been compared with electrochemically derived data, extrapolated from 771–981 K. The present study shows conclusively that although the mobility of Sb above the water table is limited by simple Sb(III) and Sb(V) oxides and stibiconite-group minerals, depending upon the prevailing redox potential and pH, tripuhyite is an important ultimate sink for Sb in the supergene environment. It is highly insoluble even in strongly acidic conditions and its anomalous stability at ambient temperatures causes the common mineral goethite, FeOOH, to react to form tripuhyite at activities of Sb(OH) 5 (aq) as low as 10 –11 . The comparatively limited numbers of reported occurrences of tripuhyite in the supergene zone are almost certainly due to the fact that its physical properties, especially colour and habit, are remarkably similar to those of goethite. In contrast, the small number of reported occurrences of schafarzikite can be related to its decomposition to tripuhyite as redox potentials rise at the top of the supergene zone and the fact that it decomposes to sénarmontite, Sb 2 O 3 , in acidic conditions, releasing Fe 2+ ions into solution. In general, the findings confirm the immobility of Sb in near-surface conditions. Geochemical settings favouring the formation of the above minerals have been assessed using the results of the present study and data from the literature.