nai

Project Progress

Carbonate minerals with continuous compositional variation occur in a Martian meteorite (ALH 84001). Mechanisms for causing this are still under debate. We propose that sulfate-reducing bacteria may result in dolomite formation. We investigated the role of dissolved hydrogen sulfide in controlling precipitation of high-magnesian calcite and dolomite at low temperature. The origin of dolomite and high-Mg calcite remain unsolved problems in geochemistry that bear on the evolution of surface conditions on the early Earth and Mars. We found that both the concentration of the dissolved sulfide and temperature affect the amount of MgCO₃ incorporation into carbonate crystals, such that very high-Mg calcite (VHMC) may be produced. It is proposed that dissolved HS-, a Lewis soft base, interacts with H⁺ in the water dipoles that bind to Mg²⁺ ions at VHMC or dolomite surfaces, which weakens chemical bonds between the water dipoles and surface Mg²⁺ ions. We suggest that this weakening lowers the kinetic energy barrier for the dehydration of surface Mg²⁺-water complexes, and therefore enhances Mg²⁺ incorporation into VHMC and dolomite. In contrast, dissolved sulfate has been considered to inhibit dolomite formation. However, our experiments show that VHMC and disordered dolomite can grow in solution almost saturated with gypsum only if aqueous sulfide is present, which emphasizes the significance of dissolved sulfide in dolomite formation.

Ca-Mg ordering occurred after annealing the disordered dolomite at 300 °C for 140 hours (Fig. 1). This means, in natural environments, either very slow crystallization or further aging of disordered dolomite in aqueous sulfide-bearing pore water systems will eventually result in ordered or partially ordered dolomite during diagenesis.

Based on our synthesized carbonate minerals in the calcite and disordered dolomite series, we have proposed a new approach for measuring composition in the calcite – disordered dolomite solid solution series based on XRD, because X-ray diffraction has been used widely in analyzing Ca-Mg carbonates. Compositions of biogenic and inorganic (Ca,Mg)CO³ crystals are often calculated by comparing measured d₁₀₄ value with published empirical curves. However, the deficiency of these curves prevented their application to very high-Mg calcite and disordered dolomite. Based on synthesized high-Mg calcite and disordered dolomite, a new empirical curve between magnesian calcite d₁₀₄ values and MgCO₃ content in the calcite – disordered dolomite solid solution series is constructed. This new curve revealed the significant cell parameter changes accompanying the Mg-Ca cation disorder in dolomite, and it can help the characterization of the MgCO₃ content of both natural and synthetic magnesian calcite and disordered dolomite, especially for the mineral mixtures which are not suitable for other analysis methods.