Carbonates as (meta)stable solid solutions: Thermodynamic and kinetic insights

Carbonates are ubiquitous in Earth surface systems, such as sediments and cements. Contents of minor cations in carbonates can be considered as proxies of environment of formation involving stages of nucleation, growth and transformation. Thermodynamic models of carbonate solid solutions can help with this, also setting reference levels for kinetics and for interpreting the measurable variations of their composition (R_d values of metals) in time (growth rate) and in space (zoning). This message is illustrated on a few topics from my past studies facilitated by GEMS codes (https://gems.web.psi.ch).

Authigenic rhodochrosites in anoxic sediments of Baltic Sea deeps [1]. A (Mn, Ca, Mg, Sr, Ba, Fe)CO₃ solid solution model was refined using the sediment profiles data and Gibbs Energy Minimization (GEM) “dual thermodynamic” (DualTG) approach to estimate all binary regular interaction parameters, consistent with the  predictions in (Lippmann 1980). In the underlying thermodynamic model, porewater pH, pe, alkalinity, dissolved Mn, Fe, and S levels were controlled by equilibrium with rhodochrosite-mackinawite-greigite mineral buffer. The model matched well the observed porewater- and carbonate composition, predicting its non-linear response to variations in Mn loading, alkalinity and salinity of the sediment-porewater system.

Eu^III coprecipitation in calcite under widely different conditions (R_d datasets for high pCO₂; normal seawater; high-pH solutions) [2]. No binary solid solution with any of seven Eu^III endmember candidates could reproduce all three datasets. This was only possible with a ternary EuH(CO₃)₂ – EuO(OH) – CaCO₃ ideal solid solution constructed with DualTG approach, and consistent with TRLFS data.

Sr in calcite and Ca in strontianite [3]. (Ca,Sr)CO₃ solid solution system with non-isostructural endmembers was investigated in a stepwise approach from atomistic to thermodynamic modelling. Binary solid solution phases with calcite- or aragonite structure have nearly symmetric moderate non-ideality. However, calculations of equilibria including both phases resulted in strongly asymmetric ‘‘miscibility gap” with ~0.3% Sr in calcite and ~3.0% Ca in strontianite. The same picture was obtained using a DQF binary solid solution model in GEM calculations of Lippmann diagrams.

Growth rate dependence of uptake of divalent ions (R_d) in calcite [4]. These facts cannot be explained by equilibrium aqueous – solid solution partitioning, and need to consider intricate relations between speciation, particle growth, adsorption, surface entrapment, and solid solution formation. Two existing Growth Surface Entrapment- (Watson 2004) and Surface Reaction Kinetics (DePaolo 2011) models could be merged into a simple Unified Uptake Kinetics equation implemented and used in GEMS.

These studies benefited from DualTG calculations that use capabilities of GEM to compute chemical potentials of elements in (meta)stable systems [5]. Aspects of DualTG “streamlining” to obtain saturation index SI of solid solutions are discussed.

References

[1] Kulik D.A., Kersten M., Heiser, U., Neumann T. (2000): Aquat. Geochem. 6, 147-199.

[2] Curti E., Kulik D.A., Tits J. (2005): Geoch. Cosmoch. Acta 69, 1721-1737.

[3] Kulik D.A., Vinograd V.L., Paulsen N., Winkler B. (2010): Phys. Chem. Earth 35, 217-232.

[4] Thien B.M.J., Kulik D.A., Curti E. (2014): Appl. Geochem. 41, 135-150.

[5] Kulik D.A. (2006): Chem. Geol. 225, 189 – 212.