American Mineralogist, Volume 100, pages 615–627, 2015
Prediction of enthalpies of formation of hydrous sulfates Sophie Billon1,* and Philippe Vieillard1 CNRS-IC2MP-UMR 7285 Hydrasa, 5 Avenue Albert Turpain, 86022 Poitiers-Cedex, France
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Abstract A method for the prediction of the enthalpies of formation DH°f for minerals of hydrous sulfates is proposed and is decomposed in the following two steps: (1) an evaluation of DH°f for anhydrous sulfates based on the differences in the empirical electronegativity parameter DHO= Mz+(c) characterizing the oxygen affinity of the cation Mz+; and (2) a prediction of the enthalpy of hydration based on the knowledge of the enthalpy of dissolution for anhydrous sulfates. The enthalpy of formation of sulfate minerals from constituent oxides is correlated to the molar fraction of oxygen atoms bound to each cation and to the difference of the oxygen affinity DHO= Mz+(c) between any two consecutive cations. The DHO= Mz+(c) value, using a weighing scheme involving the electronegativity of a cation in a given anhydrous sulfate, is assumed to be constant. This value can be calculated by minimizing the difference between the experimental enthalpies and calculated enthalpies of formation of sulfate minerals from constituent oxides. The enthalpy of hydration is closely related to the nature of the cation in the anhydrous salt, to the number of water molecules in the chemical formula and to the enthalpy of dissolution for the anhydrous salt. The results indicate that this prediction method gives an average value within 0.55% of the experimentally measured values for anhydrous sulfates and 0.21% of the enthalpies of hydration or hydrous sulfates. The relationship between DHO= Mz+(sulfate), which corresponds to the electronegativity of a cation in a sulfate compound, and known parameter DHO= Mz+(aq) was determined. These determinations allowed the prediction of the electronegativity of some anhydrous transition metal double sulfate and contributed to the prediction of the enthalpy of formation for any hydrous double sulfate. With a simplified prediction of the entropy of a hydrous sulfate, calculations of Gibbs free energy of formation can be evaluated and contribute to the knowledge of the stability of some hydrous sulfates in different environmental conditions such as temperature or air moiety. Therefore, to check the reliability of the predictive model, stability fields for some hydrous ferric sulfates such as pentahydrate ferric sulfate, lawsonite, kornelite, coquimbite, and quenstedtite vs. temperature and relative humidity were studied and compared with experimental measurements. Keywords: Enthalpy of formation, hydrous double sulfate, entropy, enthalpy of hydration, double sulfates, sulfate, relative humidity, hydrous ferric sulfate, kornelite, lawsonite, coquimbite, quenstedtite, halotrichite, pickeringite, glauberite, picromerite, tamarugite, kalinite, syngenite, mendozite, tschermigite, krausite, goldichite, aphthitalite, bilinite, romerite, solubility product, Gibbs free energy, temperature
Introduction Sulfate minerals can be of economic interest (gypsum for manufacturing wallboard, Al-sulfates in the tanning and dying industries, barite in petroleum industry, jarosite in metallurgical industry, and in agriculture, etc.) and are of ecological interest too (sulfates are used to remove metals from polluted water), but they can also induce several environmental problems (Alpers et al. 2000). Indeed, the solubility of some sulfate minerals induces a provisional storage of metals and acidity, but when they dissolve, metals are released and water becomes very acidic, causing disastrous environmental consequences such as the death of aquatic organisms, destruction of plants, massive erosion of land, and the corrosion of anthropogenic infrastructure. Sulfate minerals occur in various natural environments (points 1, 2, 3, 5, and 6 below) and are sometimes modified later by human activities (point 2 below) or are only the result of human activities (point 4 below). Some examples of sulfate occurrences * E-mail:
[email protected] 0003-004X/15/0203–615$05.00/DOI: http://dx.doi.org/10.2138/am-2015-4925
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are presented below, and the usefulness of thermodynamic data of sulfate minerals are demonstrated in a few examples. (1) Evaporite deposits: the evaporation of seawater or continental water leads to sequences of mineral formation, including especially sulfate minerals. Spencer (2000) predicted various sequences of sulfate formation from modern marine or non-marine water using the thermochemical model of Harvie et al. (1984). Concurrently, Spencer (2000) performed careful petrographic studies (mineral texture and fabric, replacement features…) on evaporite rock, formed from the evaporation of modern seawater. His ultimate goal is to calibrate the model to perform the reverse modeling, i.e., find the chemical compositions of ancient sea waters from the mineralogy of evaporite deposits. Note that, in the mineralogical sequences predicted by Spencer (2000), few double sulfates occur and thermodynamic data of them are rarely available in literature. Do not take into account the multitude of potential intermediate phases that can form, can lead to errors in the prediction of sulfates formation. (2) Weathering (oxidation) of sulfide minerals in coal deposits
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or pyritiferous rocks (pyrite, marcasite) and in metallic sulfides ore deposits (galena, sphalerite, chalcopyrite, chalcocite, bornite, covellite, etc.) induces the formation of sulfate efflorescence. Many sulfide deposits are exploited, while other mine sites have been abandoned [500 000 inactive sites in the U.S. (Lyon et al. 1993)]. Efflorescences are found in open pits, on waste rock, and on tailing piles. Numerous and various sulfates can precipitate, for example, at the Comstock Lode (Nevada), the following were found: epsomite, pickeringite, gypsum, melanterite, goslarite, pentahydrite, copiapite, voltaite, and rhomboclase (Milton and Johnston 1938). Problems occurred when rainfall events induced the dissolution of sulfate efflorescence; the water is enriched with metals and acidifies. This is the case at Richmond Mine at Iron Mountain (California), where approximately 600 000 m3 of underground water have very low pH (1%) for arcanite (K2SO4) and negative deviation (
