Evaluating the role of dissolved silica for dolomite formation in evaporitic environments: Insights from prolonged laboratory experiments

Abstract

The mineral Dolomite CaMg(CO3)2 is a common constituent of sedimentary rocks. Despite centuries of research, the mechanism of its formation remains elusive and debated. Recent studies have shown the presence of silica in solution promote the incorporation of Mg into the carbonate mineral, forming crystal phases that may be precursors to dolomite. The goal of this study was to evaluate with laboratory experiments whether dissolved silica may play a role for dolomite formation in sabkha (i.e., salt flats) environments. Several models for dolomite formation are based on the studies of sabkhas, which are often cited as modern analogue for ancient dolomite-rich sedimentary sequences. We performed long-incubation time (i.e., up to 600 days) laboratory precipitation experiments at 30 °C with solution mimicking the sabkha pore waters (characterized by a salinity of 23 % and Mg: Ca ratio of 15) to which we added different concentrations of Si. Our results revealed a positive correlation (p-value <0.001) between Si concentration in solution and the mol% Mg of the carbonate minerals forming in the experiment. With 2 mM of Si, the bulk precipitate was comprised of 90 % stoichiometric dolomite with possible signs or ordering. Moreover, the rhombohedral morphology of the crystals is analogue to that of natural dolomite previously described from sabkha sediments. Our results suggest that dissolved Si may play an important role for dolomite formation in evaporitic environments.