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

Majzlan, Juraj; Kiefer, Stefan; Lilova, Kristina; Subramani, Tamilarasan; Navrotsky, Alexandra; Dachs, Edgar; Benisek, Artur

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-formula1.88Alinline-formula0.12)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-formulaΔfHo (kJ molinline-formula−1) and inline-formulaSo (J molinline-formula−1 Kinline-formula−1) is byströmite inline-formula M26inlinescrollmathml - normal 1733.0 ± 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 - normal 5243.1 ± 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 - normal 3164.9 ± 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-formulaΔfGo of inline-formula M33inlinescrollmathml - normal 2973.6 ± 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 = - normal 17.10 74pt12ptsvg-formulamathimg42f4e6e0368209c7c9f4f268122f9f97 ejm-33-357-2021-ie00005.svg74pt12ptejm-33-357-2021-ie00005.png for the dissolution reaction (Feinline-formula1.88Alinline-formula0.12)Sb(Siinline-formula2Oinline-formula5)Oinline-formula3(OH) inline-formula+ 6Hinline-formula M42inlinescrollmathml + 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 normal 2 normal 0 7pt17ptsvg-formulamathimga12a2918e5be0404890d1c023439f7c4 ejm-33-357-2021-ie00007.svg7pt17ptejm-33-357-2021-ie00007.png inline-formula+ Sb(OH)inline-formula M49inlinescrollmathml normal 3 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 normal 2 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.

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Majzlan, Juraj / Kiefer, Stefan / Lilova, Kristina / et al: Chapmanite [Fe2Sb(Si2O5)O3(OH)]: thermodynamic properties and formation in low-temperature environments. 2021. Copernicus Publications.

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