Prediction and observation of formation of Ca–Mg arsenates in acidic and alkaline fluids: Thermodynamic properties and mineral assemblages at Jáchymov, Czech Republic and Rotgülden, Austria

Abstract

The underground spaces of the former mines in Jáchymov, Czech Republic and Rotgülden, Austria house diverse assemblages of secondary minerals, mostly arsenates. The formation conditions and processes involved with the secondary minerals were deciphered using field observations, sampling of both solids and co-existing aqueous solutions, calorimetric measurements on CaMg arsenates, forward thermodynamic modeling, and X-ray absorption spectroscopy. In Jáchymov, the sampled solutions have pH values from 0.3 up to 7.4 and are moderately to strongly oxidized. The acidic solutions precipitate arsenolite and kaatialaite, and originate by the weathering of native arsenic and sulfides. When these solutions encounter hydrothermal carbonates they are slowly neutralized and precipitate the CaMg arsenates picropharmacolite, pharmacolite, haidingerite, brassite, and rösslerite. Measured enthalpies of formation and standard entropies were combined to Gibbs free energies of formation (all in kJ/mol) for picropharmacolite [Ca4Mg(AsO3OH)2(AsO4)2·11H2O, −8139.2 ± 12.4], pharmacolite [Ca(AsO3OH)·2H2O, −1762.0 ± 3.1], haidingerite [Ca(AsO3OH)·H2O, −1522.8 ± 3.1], rösslerite [Mg(AsO3OH)·7H2O, −2849.7 ± 2.5], and brassite [Mg(AsO3OH)·4H2O, −2139.9 ± 2.7]. Forward thermodynamic modeling and the calculation of temperature-relative humidity phase diagrams using these values agreed with field observations. In Rotgülden, secondary CaMg minerals are restricted to gypsum and hörnesite, and no aqueous solutions were sampled there. The Mg arsenates brassite and rösslerite are typical for systems which initially develop substantial acidity and metal load in the aqueous phase, and afterwards are neutralized, whereas the Mg arsenate hörnesite is typical for systems which maintain circumneutral or mildly basic pH, and are slowly enriched in As. Evaluation of the saturation indices in almost 900 chemical analyses of pristine water and acidic/neutral mine drainage showed that they are all undersaturated with respect to CaMg arsenates. Therefore, these minerals control the As solubility only in specific (carbonate-rich, Fe-poor, S-poor) mine drainage systems that are progressively neutralized.