Kinetic inhibition of dolomite precipitation: Insights from Raman spectroscopy of Mg2 +–SO42 − ion pairing in MgSO4/MgCl2/NaCl solutions at temperatures of 25 to 200 °C

Abstract

The origin of dolomite has been an issue for hundreds of years, and its kinetic inhibition is a critical aspect of this issue. Dissolved sulfate is regarded as an inhibitor for dolomite formation because it can bind Mg2+ to form tight ion pairs and thus prevent the incorporation of Mg2+ into dolomite. Using Raman spectroscopy, we investigated the Mg2+–SO42− association in vapor-saturated aqueous MgSO4/MgCl2/NaCl solutions at temperatures of 25 to 200°C. The Mg2+–SO42− association is highly temperature and concentration dependent: the fractions of contact ion pairs (CIPs) and triple ion pairs (TIs) increase with increasing temperature and MgSO4 concentration. The presence of MgCl2 increases the Mg2+/SO42− ratio and favors Mg2+–SO42− interactions to produce CIPs and TIs, whereas the presence of NaCl exerts a negative effect on Mg2+–SO42− interactions, particularly at high temperatures (i.e., ≥150°C). The primary sulfate species in concentrated MgSO4 solutions at high temperatures (i.e., ≥2mol/kg, 200°C) are various contact ion pairs, whereas those in diluted solutions at Earth surface temperature appear to be unassociated SO42− and weakly associated solvent-separated and solvent-shared ion pairs. We propose that dissolved sulfate can inhibit the incorporation of Mg2+ into dolomite crystals by attracting Mg2+ to form tight contact ion pairs under hydrothermal conditions. However, thermochemical sulfate reduction (TSR) can effectively remove sulfate and free Mg2+ to enhance the precipitation of hydrothermal dolomite from sulfate-bearing fluids. The inhibiting effect of dissolved sulfate on the formation of massive low-temperature dolomite appears to have been overestimated. Removal of sulfate by anaerobic bacterial sulfate reduction (BSR) may not be responsible for the formation of microbial dolomite at surface temperatures. These new understandings also have implications for the study of thermochemical sulfate reduction because the formation of CIPs can increase the activity of sulfate in reactions with hydrocarbons.