A simple method for evaluating thermodynamic properties of minerals

n° 393 - May-June 2001


In geochemistry and environmental monitoring, knowledge of thermodynamic properties of minerals is often necessary for predicting their long-term behavior in response to changes in surrounding medium, pressure, temperature, humidity or chemical environment.

The work of Philippe Vieillard, director of research at the CNRS "Laboratoire d'Hydrogéologie, argiles, sols et alterations" (HYDRA.ASA, Hydrogeology, Clays, Soils and Weathering Laboratory, in Poitiers) has produced a simple method for evaluating minerals' thermodynamic properties. For simpler minerals these are usually determined from physical parameters interrelated using simple mathematical formulae founded on classical thermodynamic laws. For complex compounds, prediction of geochemical changes, although possible, is difficult, costly and time-consuming. Unlike for simple minerals, thermodynamic values for minerals containing several coexisting elements are not known.

In order to compensate for the lack of vital geochemical data, Philippe Vieillard developed a calculation procedure for thermodynamic values of any mineral from its crystalline structure at the molecular scale. Each cation in the mineral, with its particular charge, is surrounded by oxygen, the main anion. He had previously shown that a mineral's free energy of formation from oxides is proportional to the number of oxides it contains and to the difference in electronegativity between the cations around a common oxygen.

Philippe Vieillard took as example smectites, chemically diverse clay minerals which are poorly crystalline but have a homogeneous, three-site structure. Assuming that a cation occupying a given site has constant electronegativity and using clay-mineral solubility measurements, he established an electronegativity scale for the cations in smectites and determined the thermodynamic values of clay phases containing toxic polluting transition metals such as cadmium, nickel, zinc or cobalt.

This research has applications to modeling geochemical balances for environmental monitoring and forecasting long-term changes to natural systems. The devised calculation method also allows assessment of stability of clay minerals used in containment barriers for nuclear waste storage sites.

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