Internally consistent thermodynamic data for metal divalent sulphate hydrates

Divalent metal sulphate hydrates (M2+SO4·nH2O) represent a characteristic mineral group for the surface environments of the Earth. Generally, they change their hydration state in response to the local temperature and humidity conditions — n decreases as the relative humidity is decreased or temperature is increased. Here, using mathematical programming (MAP) techniques, we derived standard thermodynamic values consistent both with calorimetric data taken from the literature and previously published temperature humidity brackets obtained by reversal experiments for several minerals of this group: rozenite (FeSO4· 4H2O), melanterite (FeSO4· 7H2O), bianchite (ZnSO4· 6H2O), goslarite (ZnSO4· 7H2O), retgersite (NiSO4· 6H2O), morenosite (NiSO4· 7H2O), moorhouseite (CoSO4· 6H2O), bieberite (CoSO4· 7H2O), bonattite (CuSO4· 3H2O), and chalcanthite (CuSO4· 5H2O).The most significant changes appeared in the Fe2+ system. Therefore, the available data and the resulting phase diagrams are discussed in more detail — the recommended standard values for melanterite and rozenite are:Rozenite:ΔfH0298=−2128.89kJmol−1,S0298=281.52Jmol−1K−1Melanterite:ΔfH0298=−3014.39kJmol−1,S0298=409.29Jmol−1K−1.

Research highlights
► Standard thermodynamic values for divalent sulphate hydrates are presented.
► The data were refined using mathematical programming techniques (MAP).
► They are consistent with calorimetric data and temperature humidity brackets.
► By means of these data phase diagrams for environmental studies can be calculated.