LAUSR

Abstract The underground spaces of the former mines in Jachymov, Czech Republic and Rotgulden, 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 Ca Mg arsenates, forward thermodynamic modeling, and X-ray absorption spectroscopy. In Jachymov, 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 Ca Mg arsenates picropharmacolite, pharmacolite, haidingerite, brassite, and rosslerite. 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], rosslerite [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 Rotgulden, secondary Ca Mg minerals are restricted to gypsum and hornesite, and no aqueous solutions were sampled there. The Mg arsenates brassite and rosslerite are typical for systems which initially develop substantial acidity and metal load in the aqueous phase, and afterwards are neutralized, whereas the Mg arsenate hornesite 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 Ca Mg arsenates. Therefore, these minerals control the As solubility only in specific (carbonate-rich, Fe-poor, S-poor) mine drainage systems that are progressively neutralized.