Abstract
Abstract. In this work, we have determined or evaluated thermodynamic properties of synthetic Sb2O5, MgSb2O6 (analogue of the mineral byströmite), Mg[Sb(OH)6]2⋅6H2O (brandholzite), and natural chapmanite [(Fe1.88Al0.12)Sb(Si2O5)O3(OH)]. Enthalpies of reactions, including formation enthalpies, were evaluated using reference compounds Sb, Sb2O3, Sb2O5, and other phases, with high-temperature oxide melt solution calorimetry in lead borate and sodium molybdate solvents. Heat capacity and entropy were determined by relaxation and differential scanning calorimetry. The best set of ΔfHo (kJ mol−1) and So (J mol−1 K−1) is byströmite -1733.0±3.6, 139.3±1.0; brandholzite -5243.1±3.6, 571.0±4.0; and chapmanite -3164.9±4.7, 305.1±2.1. The data for chapmanite give ΔfGo of -2973.6±4.7 kJ mol−1 and logK=-17.10 for the dissolution reaction (Fe1.88Al0.12)Sb(Si2O5)O3(OH) + 6H+→ 1.88Fe3+ + 0.12Al3+ + 2SiO20 + Sb(OH)30 + 2H2O. Analysis of the data showed that chapmanite is finely balanced in terms of its stability with schafarzikite (FeSb2O4) and tripuhyite (FeSbO4) under a specific, narrow range of conditions when both aqueous Fe(III) and Sb(III) are abundant. In such a model, chapmanite is metastable by a narrow margin but could be stabilized by high SiO20(aq) activities. Natural assemblages of chapmanite commonly contain abundant amorphous silica, suggesting that this mechanism may be indeed responsible for the formation of chapmanite. Chapmanite probably forms during low-temperature hydrothermal overprint of pre-existing Sb ores under moderately reducing conditions; the slightly elevated temperatures may help to overcome the kinetic barrier for its crystallization. During weathering, sheet silicates may adsorb Sb3+ in tridentate hexanuclear fashion, thus exposing their chapmanite-like surfaces to the surrounding aqueous environment. Formation of chapmanite, as many other sheet silicates, under ambient conditions, is unlikely.
Highlights
Antimony is an element that enters into both quite soluble and quite insoluble minerals as it moves through the aqueous environment
Using the same interpretation of our TG data, a weight loss of 9.28 wt % is attributable to H2O, and the remaining weight loss of 0.55 wt % is due to the partial reduction of the Sb2O5
Thermodynamic data suggest that chapmanite is finely balanced in its stability with tripuhyite and schafarzikite under a narrow range of conditions when ferric (Fe3+) and antimonous (Sb3+) species predominate
Summary
Antimony is an element that enters into both quite soluble and quite insoluble minerals as it moves through the aqueous environment. The solubility of such reservoirs was previously quantified by Filella and May (2003), Diemar et al (2009), Leverett et al (2012), Roper et al (2015), and others using thermodynamic data. The discrepancies between the observations of antimony being soluble at some sites but insoluble at other ones were addressed and resolved by Majzlan et al (2016). Two insoluble minerals, considered to be the “ultimate sinks” of antimony, are tripuhyite (FeSb5+O4) and schafarzikite (FeSb32+O4) (Leverett et al, 2012). Tripuhyite has been identified at a number of sites polluted by Sb (see Majzlan, 2021), but schafarzikite is rare, restricted to a few localities where it seems to be primary and not secondary
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