Clays and Clay Minerals, Vol. 59, No. 3, 215–232, 2011.
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Authors
Rozov, K.B.
Berner, U.
Kulik, D.A.
Diamond, L.W.
Issue Date
2011
Type
Article
Language
Keywords
Periodicals , Geology , Aqueous Solubility , Hydrotalcite , LDH , Molar Volume , Pyroaurite , Solid Solutions , Thermodynamic Modeling
Alternative Title
Solubility And Thermodynamic Properties Of Carbonate-bearing Hydrotalcite-Pyroaurite Solid Solutions With A 3:1 Mg/(al+fe) Mole Ratio
Abstract
The naturally occurring layered double hydroxides (LDH, or anionic clays) are of particular interest in environmental geochemistry because of their ability to retain hazardous cations and especially anions. However, incorporation of these minerals into predictive models of water-rock interaction in contaminant environments, including radioactive-waste repositories, is hampered by a lack of thermodynamic and stability data. To fill part of this gap the present authors have derived properties of one of the complex multicomponent solid solutions within the LDH family: the hydrotalcite-pyroaurite series, Mg3(Al1-xFex)(OH)8(CO3)0.5·2.5H2O. Members of the hydrotalcite-pyroaurite series with fixed MgII/(AlIII+FeIII) = 3 and various FeIII/(FeIII+AlIII) ratios were synthesized by co-precipitation and dissolved in long-term experiments at 23B2ºC and pH = 11.40B0.03. The chemical compositions of co-existing solid and aqueous phases were determined by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, and liquid scintillation counting of 55Fe tracers; X-ray diffraction and Raman were used to characterize the solids. Based on good evidence for reversible equilibrium in the experiments, the thermodynamic properties of the solid solution were examined using total-scale Lippmann solubility products, SPT. No significant difference was observed between values of SPT from co-precipitation and from dissolution experiments throughout the whole range of Fe/Al ratios. A simple ideal solid-solution model with similar end-member SPT values (a regular model with 0 < WG < 2 kJ mol-1) was sufficient to describe the full range of intermediate mineral compositions. In turn, this yielded the first estimate of the standard Gibbs free energy of the pyroaurite end member, Go 298,Pyr = -3882.60B2.00 kJ/mol, consistent with Go 298,Htlc = -4339.85 kJ/mol of the hydrotalcite end member, and with the whole range of solubilities of the mixed phases. The molar volumes of the solid-solution at standard conditions were derived from X-ray data. Finally, Helgeson’s method was used to extend the estimates of standard molar entropy and heat capacity of the end members over the pressure-temperature range 0-70ºC and 1-100 bar.
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Citation
Clays and Clay Minerals, Vol. 59, No. 3, 215–232, 2011.
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