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 center dot)11H₂O, - 8139.2 ± 12.4], pharmacolite [Ca(AsO₃OH)center dot 2H₂O, - 1762.0 ± 3.1], haidingerite [Ca(AsO3OH)center dot H₄O, - 1522.8 ± 3.1], rosslerite [Mg(AsO₃OH)center dot 7H₂OO, - 2849.7 ± 2.5], and brassite [Mg(AsO₃OH)center dot 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.