NEW DATA AND OLD PROBLEMS IN THE THERMODYNAMICS OF ACID MINE DRAINAGE

We have measured the enthalpies of formation (DH^o_f) of two Fe sulfate minerals kornelite and paracoquimbite and a suite of Mg sulfates kieserite, sanderite, hexahydrite, and epsomite (Ackermann et al. 2009, Grevel and Majzlan 2009). The values measured by solution calorimetry (with either water or hydrochloric acid as the solvent) are (in kJ·mol^–1):

kornelite, Fe₂(SO₄)₃·7.75H₂O –4916.2±4.2

paracoquimbite, Fe₂(SO₄)₃·9H₂O –5295.4±4.2

epsomite, MgSO₄·7H₂O –3387.7±1.3

hexahydrite, MgSO₄·6H₂O –3088.1±1.1

sanderite, MgSO₄·2H₂O –1894.9±1.3

kieserite, MgSO₄·H₂O –1612.4±1.3

We are also working on measuring the formation enthalpies and standard entropies of a suite of jarosite samples. All these minerals are found in so-called acid mine drainage waters, that is, waters acidified by decomposition and oxidation of common sulfides, most commonly pyrite.

We have estimated the entropies of kornelite and paracoquimbite and critically evaluated the heat capacity and entropy data for the Mg sulfates. Afterwards, we calculated their Gibbs free energy of formation and constructed phase diagrams as a function of (1) temperature and relative humidity of air, and (2) molality of aqueous components in the coexisting aqueous solutions. In either case, the phase diagrams calculated from our and/or available thermodynamic data deviate, more or less, from the published experimental observations. The topology of the phase diagrams is very sensitive to the entropy estimates and the construction of a reliable phase diagram must await better constraints on entropy or Gibbs free energy of formation.We addressed this problem in a pilot study by applying the mathematical programming (MAP) techniques to the magnesium sulfate phases. This method, extensively applied to rock-forming silicates and oxides in the past, is capable of producing an internally consistent dataset, i.e., a set of thermodynamic values consisent with the calorimetric measurements and experimental observations. We were able to find a solution for the Mg sulfates and we intend to extend this approach to other systems.

Ackermann, S., Armbruster, T., Lazic, B., Doyle, S., Grevel, K.-D., Majzlan, J., 2009: American Mineralogist (in press)

Grevel, K.-D., Majzlan, J., 2009: Geochimica et Cosmochimica Acta (submitted)