Solid solutions of trace Eu(III) in calcite: Thermodynamic evaluation of experimental data over a wide range of pH and pCO2

Details Solid solutions of trace Eu(III) in calcite: Thermodynamic evaluation of experimental data over a wide range of pH and pCO2 Solid solutions of trace Eu(III) in calcite: Thermodynamic evaluation of experimental data over a wide range of pH and pCO2

Apr 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005gecoa..69.1721c&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 69, Issue 7, p. 1721-1737.

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The thermodynamics of dilute Eu-calcite solid solutions formed under widely different pH-pCO2 conditions at T = 25°C and p = 1 bar were investigated using three sets of Eu(III) uptake experiments, two of which were taken from the literature: (a) recrystallization in synthetic cement pore water at pH ˜ 13 and pCO2 ˜ 10-13 bar (this work); (b) coprecipitation in 0.1 M NaClO4 at pH ˜ 6 and pCO2 ˜ 1 bar; (c) coprecipitation in synthetic seawater at pH ˜ 8 and pCO2 ranging from 3 × 10-4 to 0.3 bar. Solid solution formation was modeled using the Gibbs energy minimization (GEM) method. In a first step (“forward” modeling), we tested ideal binary solid solution models between calcite and the Eu end-members Eu2(CO3)3, EuNa(CO3)2, Eu(OH)CO3 or Eu(OH)3, for which solids with independently measured solubility products exist. None of these four binary solid solutions was capable of reproducing all three experimental datasets simultaneously. In a second step (“inverse” modeling), ideal binary solid solutions were constructed between calcite and the candidate Eu end-members EuO(OH), EuH(CO3)2 and EuO(CO3)0.5, for which no independent solubility products are available. For each single data point and each of these end-members, a free energy of formation with inherent activity coefficient term (Gα* = Gαo + RT lnγα) was estimated from “dual thermodynamic” GEM calculations. The statistical mean of Gα* was then calculated for each of the three datasets. A specific end-member was considered to be acceptable if a standard deviation of ± 2 kJ mol-1 or less resulted for each single dataset, and if the mean Gα*-values calculated for the three datasets coincided. No binary solid solution with any of the seven above mentioned end-members proved to satisfy these criteria. The third step in our analysis involved consideration of ternary solid solutions with CaCO3 as the major end-member and any two of the seven considered Eu trace end-members. It was found that the three datasets can only be reproduced simultaneously with the ternary ideal solid solution EuH(CO3)2 EuO(OH) CaCO3, setting GEuH(CO3)2* = -1773 kJ mol-1 and GEuO(OH)* = -955 kJ mol-1, whereas all other end-member combinations failed. Our results are consistent with time-resolved laser fluorescence data for Cm(III) and Eu(III) indicating that two distinct species are incorporated in calcite: one partially hydrated, the other completely dehydrated. In conclusion, our study shows that substitution of trivalent for divalent cations in carbonate crystal structures is a more complex process than the classical isomorphic divalent-divalent substitution and may need consideration of multicomponent solid solution models.

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