Thermochemistry of mixing strontianite [SrCO{sub 3}(s)] and aragonite [CaCO{sub 3}(s)] to form Ca{sub x}Sr{sub 1-x}CO{sub 3}(s) solid solutions
- Univ. of California, Davis, CA (United States)
- Princeton Univ., NJ (United States)
Enthalpies of mixing [{Delta}H{sub 298}{sup mix}] of aragonite and strontianite to form Ca{sub x}Sr{sub 1-x}CO{sub 3}(s) solid solutions were obtained at 298 K from drop-solution enthalpies [{Delta}H{sub 975}{sup ds}] of pellets of the respective solid solutions into molten 2PbO{center_dot}B{sub 2}O{sub 3} at 975 K. The measured {Delta}H{sub 298}{sup mix} values are positive for all measured values of x, are nearly symmetric around x = 0.50, and reach a maximum value of +3.82 {plus_minus} 0.94 kJ mol{sup {minus}1}. Previous electrochemical studies have reported that {Delta}G{sub 298}{sup ex} values are also positive over the range 0.0 < x < 0.9 and reach a maximum value of +3.0({plus_minus}1.6) kJ mol{sup {minus}1} at x {approx} 0.7. The general similarity between the {Delta}H{sub 298}{sup mix} and the {Delta}G{sub 298}{sup ex} values indicates that the excess entropy of mixing is small or zero, consistent with the regular-solution treatment. Within this regular-solution treatment, the interaction parameter is W = 13.5 ({plus_minus}1.3) kJ mol{sup {minus}1}, which yields a very narrow range of stable miscible compositions at Earth surface conditions. Compositions of aragonite or strontianite with even a few percent impurity are not stable and will unmix to form a mechanical mixture of Ca-rich strontianite and a Sr-rich aragonite. It is, in general, difficult to accurately estimate the compositions of coexisting, miscible solids such as the Ca{sub x}Sr{sub 1-x}CO{sub 3} solids at Earth-surface conditions. The predictions are quite sensitive to small uncertainties in the data and such estimates are probably beyond the capabilities of either electrochemical cell measurements of calorimetry. 37 refs., 7 figs., 1 tab.
- DOE Contract Number:
- FG03-92ER14307; FG02-85ER13437
- OSTI ID:
- 281136
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 6 Vol. 60; ISSN 0016-7037; ISSN GCACAK
- Country of Publication:
- United States
- Language:
- English
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