A new method to calculate end‐member thermodynamic properties of minerals from their constituent polyhedra I: enthalpy, entropy and molar volume

Abstract

The thermodynamic properties of silicate minerals can be described as a linear combination of the fractional properties of their constituent polyhedra. In contrast, given the thermodynamic properties of these polyhedra, the thermodynamic properties of minerals can be estimated, where only the crystallography of the mineral needs to be known. Such estimates are especially powerful for hypothetical mineral end-members or for minerals where experimental determination of their thermodynamic properties is difficult. In this contribution the fractional enthalpy, entropy and molar volume for 35 polyhedra have been determined using weighted multiple linear regression analysis on a data set of published mineral thermodynamic properties. The large number of polyhedra determined, allows calculation of a much larger variety of phases than was previously possible and the larger set of minerals used provides more confident fractional properties. The OH-bearing minerals have been described by partial and total hydroxide coordinated components, which gives better results than previous models and precludes the need of a S-V term to improve estimates of entropy. However, the fractional thermodynamic properties only give adequate results for silicate minerals and double oxides, and should therefore not be used to estimate the properties of other minerals. The thermodynamic properties of ÔnewÕ minerals are calculated from a linear stoichiometric combination of their constituent polyhedra, resulting in estimates generally with associated uncertainty of <5%. The quality of such data appears to be of sufficient accuracy for thermodynamic modelling as shown for meta-bauxites from the Alps and the Aegean, where the effect of Zn on the P-T stability of staurolite can be both qualitatively and quantitatively reproduced.

Thermodynamic properties of illite, smectite and beidellite by calorimetric methods: Enthalpies of formation, heat capacities, entropies and Gibbs free energies of formation

“…6d) have been implemented and their results have been compared with experimental values. Holland (1989) and Van Hinsberg et al (2005) each provide two types of models, with (S-V) or without (S) taking into account a contribution of the molar volume of minerals. Fig.…”

“…Finally the properties coming from these solution experiments for 2:1 clay minerals have not been included in the usual thermodynamic databases for geochemical codes. Estimation methods (Chermak and Rimstidt, 1989;Vieillard, 2000Vieillard, , 2002van Hinsberg et al, 2005) remain the usual source of thermodynamic data for 2:1 clay minerals. However, these methods have been parameterized with minerals of various structures and are consequently not specifically adapted to clay mineral structures.…”

Thermodynamic and fully-coupled reactive transport models of a steel–bentonite interface

“…Of particular concern is that the model gives ∆G f values for aluminosilicates such as zeolites and clays that result in their solubilities being underestimated . In addition, the choice of polyhedral components in Chermak and Rimstidt's model has been questioned by Van Hinsberg et al, (2005). Also, the models presented by Wilson et al (2006a) do not include a consideration of the stability of Fe(III)-bearing 1:1 minerals such as cronstedtite and odinite which have subsequently been observed in experiments on ironbentonite interactions .…”

“…Furthermore, the choice of polyhedral components in this model has been questioned by Van Hinsberg et al (2005).…”

“…Since the models described by Wilson et al (2006a) were published, the accuracy of the model they used to generate estimates of Gibbs free energy of formation (∆G f ) for 1:1 layer silicates ) has been questioned . In addition, other aspects of the model have been criticised (Van Hinsberg et al, 2005). Therefore, new estimates have been generated as part of the work presented here (described in Appendix A).…”