Chapmanite [Fe₂Sb(Si₂O₅)O₃(OH)]: Thermodynamic properties and formation in lowerature environments

In this work, we have determined or evaluated thermodynamic properties of synthetic Sb2O5, MgSb2O6 (analogue of the mineral byströmite), Mg[Sb(OH)6]26H2O (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 higherature 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-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 lowerature 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.