Manganese carbonate formation from Amorphous and nanocrystalline precursors: Thermodynamics and geochemical relevance

The thermodynamic stabilities of different manganese carbonate phases at ambient conditions were determined by acid solution and water adsorption calorimetry. Amorphous manganese carbonate precursor provides a low energy pathway for MnCO₃ crystallization analogous to that observed in (Ca-Mg-Fe)CO₃ systems where crystallization enthalpies appear to be controlled by cation size (become less exothermic with increase in ionic radius). The surface energy of nanophase MnCO₃ (0.64 ± 0.08 J/m² for hydrous and 0.94 ± 0.12 J/m² for anhydrous surface) is lower than that of nano-calcite and MnCO₃ binds surface water less strongly (-65.3 ± 3 kJ/mol) than calcite (-96.26 ± 0.96 kJ/mol). This probably refects the greater basicity of CaO compared to MnO. Substantial particle size driven shifts in the MnCO₃-manganese oxide Eh-pH and oxygen fugacity-CO₂ fugacity diagrams were calculated using the measured surface energies. These shifts expand the stability field of hausmannite, Mn₃O₄, in both aqueous and anhydrous environments. The particle size driven (caused by differences in surface energy) shifts in oxidation potential (Eh, oxygen fugacity) and pH of phase boundaries could affect stability, a electrochemical and catalytic properties and hence influence geochemical and technological processes. Manganese oxides (mainly hausmannite) dominate at the nanoscale in aerated environments, while manganese carbonate is favored in coarse-grained materials and reducing environments. In supercritical CO₂, the expansion of the MnCO₃ stability field leads to significant reduction of the Mn₃O₄ stability field.