Mixing thermodynamics of the calcite-structured (Mn,Ca)CO3 solid solution: a computer simulation study.

Area of Science:

• Geochemistry
• Materials Science
• Computational Chemistry

Background:

• Calcite (CaCO(3)) and rhodochrosite (MnCO(3)) form solid solutions with varying cation arrangements.
• Previous experimental studies reported conflicting thermodynamic data, particularly for Mn-rich compositions.

Purpose of the Study:

• To investigate the thermodynamics of mixing in (Ca,Mn)CO(3) solid solutions using atomistic simulations.
• To reconcile discrepancies between theoretical predictions and experimental measurements of mixing enthalpies.
• To elucidate the role of cation ordering on the stability of these carbonate solid solutions.

Main Methods:

• Atomistic simulation techniques were employed to model the thermodynamic properties.
• Calculations focused on enthalpies of mixing across the entire composition range.
• Simulations incorporated varying degrees of cation ordering.

Main Results:

• Fully disordered solid solutions exhibit positive enthalpies of mixing, consistent with recent experiments.
• Incorporating a degree of cation ordering significantly improves agreement with experimental mixing enthalpies.
• The most stable configuration for equimolar (Ca,Mn)CO(3) is the ordered kutnahorite structure, with alternating cation layers.
• Simulations predict kutnahorite disordering temperatures that align better with experimental values when partial ordering is considered.

Conclusions:

• The "disordered" (Mn,Ca)CO(3) solid solutions are likely partially ordered, favoring lower-energy cation arrangements.
• Kinetic factors may favor the formation of these partially ordered structures.
• Atomistic simulations provide a powerful tool for understanding cation ordering and thermodynamics in mineral solid solutions.