Stabilities of byströmite, MgSb₂O₆, ordoñezite, ZnSb₂O₆ and rosiaite, PbSb₂O₆, and their possible roles in limiting antimony mobility in the supergene zone

Abstract

In order to clarify the roles that secondary minerals may have in determining the extent of dispersion of Sb in the supergene environment, syntheses and stability studies of the Sb(V) oxides byströ mite, MgSb 2 O 6 , ordoñ ezite, ZnSb 2 O 6 and rosiaite, PbSb 2 O 6 , have been undertaken. Solubilities in aqueous HNO 3 were determined at 298.2 K and the data obtained used to calculate values of DG f at the same temperature. The derived DG f (s, 298.2 K) values for MgSb 2 O 6 (À1554.1 Ô3.6 kJ mol À1 ), ZnSb 2 O 6 (À1257.0 Ô2.6 kJ mol À1 ) and PbSb 2 O 6 (À1154.2 Ô2.6 kJ mol À1 ) have been used in subsequent calculations to determine their relative stabilities and relationships with other secondary Sb minerals.

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

“…The agreement between our calorimetric data and published solubility data (Diemar et al, 2009;Roper et al, 2015) is less than optimal, although differences of similar magnitude are encountered in many systems. The values are summarized for easier comparison in Table 8.…”

“…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 Maj-zlan et al (2016).…”

The stability of the rare sodium antimonate, brizziite, and it's role in Sb mobility

“…Thus, the thermodynamically stable phase at 298.2 K is brizziite and kinetic effects must account for the existence of mopungite, unless the stabilities of one or both of the minerals are influenced profoundly by temperature. In line with our previous findings of the Mg antimonate phases brandholzite and byströmite (Roper et al ., 2014), it must simply be the case that the rate of transformation of mopungite to brizziite is very slow at 298.2 K. The lack of brizziite in any of the six remaining localities hosting mopungite supports this notion. …”

“…Competing cations must be essentially absent from mineralizing solutions, save for Mg 2+ (aq). This is certainly the case for Pb 2+ (aq) (Roper et al ., 2014). Furthermore, local Ca 2+ (aq) concentrations would need to be quite low to prevent the formation of minerals of the roméite group, such as Ca 2 Sb 2 O 7 , and at lower pH values than used in the solution experiments, Fe 3+ (aq) must be negligible to prevent the formation of tripuhyite, FeSbO 4 (Leverett et al .…”

“…Brizzi and Meli (1990) also reported mopungite from La Selva Mine, Tuscany, Italy in a siliceous matrix containing metallic Sb. The chemical similiarities of brizziite and mopungite suggest a potential kinetic stability for one of the phases at ambient temperatures, similar to recent findings concerning byströmite and brandholzite, Mg[Sb(OH) 6 ] 2 ·6H 2 O, (Roper et al ., 2014). The investigation of this possibility is reported below.…”