Chapmanite [Fe ₂Sb(Si ₂O ₅)O ₃(OH)]: thermodynamic properties and formation in low-temperature environments

In this work, we have determined or evaluated thermodynamic properties of synthetic Sbinline-formula₂Oinline-formula₅, MgSbinline-formula₂Oinline-formula₆ (analogue of the mineral byströmite), Mg[Sb(OH)inline-formula₆]inline-formula₂⋅6Hinline-formula₂O (brandholzite), and natural chapmanite [(Feinline-formula_1.88Alinline-formula_0.12)Sb(Siinline-formula₂Oinline-formula₅)Oinline-formula₃(OH)]. Enthalpies of reactions, including formation enthalpies, were evaluated using reference compounds Sb, Sbinline-formula₂Oinline-formula₃, Sbinline-formula₂Oinline-formula₅, 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 inline-formulaΔ_fH^o (kJ molinline-formula⁻¹) and inline-formulaS^o (J molinline-formula⁻¹ Kinline-formula⁻¹) is byströmite inline-formula $M26inlinescrollmathml-\mathrm{normal\; 1733.0}\pm \mathrm{normal\; 3.6}$ 70pt10ptsvg-formulamathimgaf2315255921e9dc5354978d7ba6413f ejm-33-357-2021-ie00001.svg70pt10ptejm-33-357-2021-ie00001.png , inline-formula139.3±1.0; brandholzite inline-formula $M28inlinescrollmathml-\mathrm{normal\; 5243.1}\pm \mathrm{normal\; 3.6}$ 70pt10ptsvg-formulamathimgd072a84b837f42e451ca96c985d07f27 ejm-33-357-2021-ie00002.svg70pt10ptejm-33-357-2021-ie00002.png , inline-formula571.0±4.0; and chapmanite inline-formula $M30inlinescrollmathml-\mathrm{normal\; 3164.9}\pm \mathrm{normal\; 4.7}$ 70pt10ptsvg-formulamathimg7c9084c5be21f7618af00cdbd0ae1c63 ejm-33-357-2021-ie00003.svg70pt10ptejm-33-357-2021-ie00003.png , inline-formula305.1±2.1. The data for chapmanite give inline-formulaΔ_fG^o of inline-formula $M33inlinescrollmathml-\mathrm{normal\; 2973.6}\pm \mathrm{normal\; 4.7}$ 70pt10ptsvg-formulamathimg851a0584ad75df9ba722baaabdfe5a65 ejm-33-357-2021-ie00004.svg70pt10ptejm-33-357-2021-ie00004.png  kJ molinline-formula⁻¹ and inline-formula $M35inlinescrollmathml\log K=-\mathrm{normal\; 17.10}$ 74pt12ptsvg-formulamathimg42f4e6e0368209c7c9f4f268122f9f97 ejm-33-357-2021-ie00005.svg74pt12ptejm-33-357-2021-ie00005.png for the dissolution reaction (Feinline-formula_1.88Alinline-formula_0.12)Sb(Siinline-formula₂Oinline-formula₅)Oinline-formula₃(OH) inline-formula+ 6Hinline-formula $M42inlinescrollmathml{}^{+}\to$ 23pt12ptsvg-formulamathimg7fa05e9209416092ec38e92dc4978c1c ejm-33-357-2021-ie00006.svg23pt12ptejm-33-357-2021-ie00006.png  1.88Feinline-formula³⁺ inline-formula+ 0.12Alinline-formula³⁺ inline-formula+ 2SiOinline-formula $M47inlinescrollmathml{}_{\mathrm{normal\; 2}}^{\mathrm{normal\; 0}}$ 7pt17ptsvg-formulamathimga12a2918e5be0404890d1c023439f7c4 ejm-33-357-2021-ie00007.svg7pt17ptejm-33-357-2021-ie00007.png  inline-formula+ Sb(OH)inline-formula $M49inlinescrollmathml{}_{\mathrm{normal\; 3}}^{\mathrm{normal\; 0}}$ 7pt17ptsvg-formulamathimg1178ba26908e030eed5f2bb1c26a2a29 ejm-33-357-2021-ie00008.svg7pt17ptejm-33-357-2021-ie00008.png  inline-formula+ 2Hinline-formula₂O. Analysis of the data showed that chapmanite is finely balanced in terms of its stability with schafarzikite (FeSbinline-formula₂Oinline-formula₄) and tripuhyite (FeSbOinline-formula₄) 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 SiOinline-formula $M55inlinescrollmathml{}_{\mathrm{normal\; 2}}^{\mathrm{normal\; 0}}$ 7pt17ptsvg-formulamathimgbb934b68e64057844c3a6b69d2c0c15e ejm-33-357-2021-ie00009.svg7pt17ptejm-33-357-2021-ie00009.png (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 Sbinline-formula³⁺ 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.