Gibbs energies of formation of metal-carbonate solid solutions. Part 3: The Ca{sub x}Mn{sub 1{minus}x}CO{sub 3} system at 298 K and 1 bar
- Univ. of California, Davis, CA (United States)
New thermodynamic data are presented for the mixing of calcite (CaCO{sub 3(s)}) and rhodochrosite (MnCO{sub 3(s)}) to form Ca{sub x}Mn{sub 1{minus}x}CO{sub 3(s)} solutions. A reversible equilibrium was achieved between the aqueous solution and the metastable solid solution using electrochemical cells and a new electrode of the third kind. New values of {Delta}{sub f} G{degree} [PbCO{sub 3(s)}] and {Delta}{sub f} G{degree} [MnCO{sub 3(s)}] were determined as {minus}622.61 {+-} 0.79 kJ/mol and {minus}809.89 {+-} 0.73 kJ/mol, respectively, using the more thoroughly studied value for the Gibbs energy of formation of calcite as a reference. The Ca{sub x}Mn{sub 1{minus}x}CO{sub 3} system was evaluated using these new Gibbs energy values and was found to have thermodynamically stable compositions for 0.186 < x < 0.734, which is consistent with the compositions of natural minerals. X-ray analysis of the solid solutions, before and after the reaction, shows very little evidence of unmixing for time periods in excess of 45 days. The excess Gibbs energy function derived for this system at 25 C is: {Delta}G{degree}{sub ex} = [x (1 {minus} x)][(27.84 {+-} 5.12) {minus} (132.62 {+-} 23.47)x + (142.65 {+-} 23.68) x{sup 2}] where x represents the mole fraction of calcium in the solid solution. The {Delta}G{degree}{sub ex} values found in this study show a strong resemblance to high temperature {Delta}H{degree}{sub ex} values reported by Capabianco and Navrotsky (1987). This similarity suggests that the Ca{sub x}Mn{sub 1{minus}x}CO{sub 3} system has very small {Delta}S{degree}{sub ex} values. However, this conclusion cannot be easily reconciled with the phase equilibrium studies at high temperatures. These phase equilibrium studies indicate a large miscibility gap in intermediate calcium concentrations at high temperatures, in the region where the authors find evidence for miscibility at 25 C.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- FG03-92ER14307
- OSTI ID:
- 316233
- Journal Information:
- Geochimica et Cosmochimica Acta, Journal Name: Geochimica et Cosmochimica Acta Journal Issue: 16 Vol. 62; ISSN GCACAK; ISSN 0016-7037
- Country of Publication:
- United States
- Language:
- English
Similar Records
A new method for determining Gibbs energies of formation of metal-carbonate solid solutions: 1. The Ca[sub x]Cd[sub 1-x]CO[sub 3](s) system at 298 K and 1 bar
Solid-solute phase equilibria in aqueous solution. 5: The system CdCO sub 3 -CaCO sub 3 -CO sub 2 -H sub 2 O