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.
