Thermodynamic properties of mansfieldite (AlAsO₄·2H₂O), angelellite (Fe₄(AsO₄)₂O₃) and kamarizaite (Fe₃(AsO₄)₂(OH)₃·3H₂O)

Abstract

Thermodynamic data for the arsenates of various metals are necessary to calculate their solubilities and to evaluate their potential as arsenic storage media. If some of the less common arsenate minerals have been shown to be less soluble than the currently used options for arsenic disposal (especially scorodite and arsenical iron oxides), they should be further investigated as promising storage media. Furthermore, the health risk associated with arsenic minerals is a function of their solubility and bioavailability, not merely their presence. For all these purposes, solubilities of such minerals need to be known. In this work, a complete set of thermodynamic data has been determined for mansfieldite, AlAsO4·2H2O; angelellite, Fe4(AsO4)2O3; and kamarizaite, Fe3(AsO4)2(OH)3·3H2O, using a combination of high-temperature oxide-melt calorimetry, relaxation calorimetry, solubility measurements, and estimates where possible and appropriate. Several choices for the reference compounds for As for the high-temperature oxide-melt calorimetry were assessed. Scorodite was selected as the best one. The calculated Gibbs free energy of formation (all data in kJ·mol–1) is –1733.4 ± 3.5 for mansfieldite, –2319.2 ± 7.9 for angelellite and –3056.8 ± 8.5 for kamarizaite. The solubility products for the dissolution reactions are –21.4 ± 0.5 for mansfieldite, –43.4 ± 1.5 for angelellite and –50.8 ± 1.6 for kamarizaite. Available, but limited, chemical data for the natural scorodite–mansfieldite solid-solution series hint at a miscibility gap; hence the non-ideal nature of the series. However, no mixing parameters were derived because more data are needed. The solubility of mansfieldite is several orders of magnitude higher than that of scorodite. The solubility of kamarizaite, on the other hand, is comparable to that of scorodite, and kamarizaite even has a small stability field in a pH-pε diagram. It is predicted to form under mildly acidic conditions in acid drainage systems that are not subject to rapid neutralization and sudden strong supersaturation. The solubility of angelellite is high, and the mineral is obviously restricted to unusual environments, such as fumaroles. Its crystallization may be enhanced via its epitaxial relationship with the much more common hematite. The use of the scorodite–mansfieldite solid solution for arsenic disposal, whether the solid solution is ideal or not, is not practical. The difference in solubility products of the two end-members (scorodite and mansfieldite) is so large that almost any system will drive the precipitation of essentially pure scorodite, leaving the aluminium in the aqueous phase.

Mansfieldite (AlAsO4·2H2O): a new white pigment in Himalayan artwork

“…21 The hydrated aluminum arsenate is part of the variscite mineral group, and one end of the solid solution series between mansfieldite and scorodite, the iron analog. That scorodite and mansfieldite are a solid solution series also means iron and aluminum (as Fe 3+ and Al 3+ ) more easily swap for each other in the mineral structure; in mineral systems this substitution often has a thermodynamic limit, as yet undefined here, 22 but significant Fe substitution is possible in a particular mansfieldite mineral grain. The iron detected alongside arsenic in the white pigment studied here as evidenced by SEM-EDS and MA-XRF mapping suggests that the pigment found in the thangkas is not high enough in iron to significantly alter the structure of the atoms from mansfieldite, as specific techniques like Raman spectroscopy and XRD are sensitive to these changes and the results here are consistent with the white material being primarily mansfieldite.…”

“…The relative solubility of mansfieldite is decreased in a solution with even minor amounts of Al 3+ based on calculations using available thermodynamic data. 22 Although sources of mansfieldite in Tibet have not to the authors’ knowledge been specifically reported in the literature, it is conceivable that the mineral was procured locally from primary hydrothermal ore deposits or from the abundant deposits of weathered arsenic sulfides, likely associated with at least some aluminum-bearing minerals. According to the early 1900s Richmond, Virginia newspaper reports concerning the Stone collection of Himalayan paintings, 3,4 the artworks were procured in Chamdo in Tibet.…”