FROM PROMOTION TO INHIBITION: EFFECTS OF SILICA ON CARBONATE MINERAL GROWTH (Invited Presentation)

The formation mechanism of sedimentary dolomite (CaMg(CO₃)₂) has been challenging to decipher. The kinetic barrier of dehydrating surface Mg²⁺-water complexes at Earth’s surface temperatures has hindered laboratory synthesis in the past and compromised our understanding of dolomite precipitation in geologic settings. Recent laboratory experiments suggest that high concentration of dissolved silica could promote precipitation of high Mg-calcite (HMC) and disordered dolomite, which are precursor phases for sedimentary dolomite. The silica-driven abiotic pathway for dolomite precipitation dwindled in modern ocean due to the effective removal of dissolved silica by diatoms and other siliceous organisms, but was observed in many modern evaporative hypersaline lake systems. While silica appears to address many aspects of the temporal and spatial distributions of dolomite through geologic time, the effect of silica on sedimentary dolomite during diagenesis is still largely unclear. We aims to understand the effect of dissolved silica on dolomite mineralogy and geochemical signatures during early diagenesis through a combination of natural sample observation and annealing experiments using in-house and synchrotron techniques. Our results suggest that the oscillations of submillimeter lamellae in the early Silurian dolomites result from variation of dolomite crystal sizes rather than changes in mineral assemblage. Substantial silica (>200 ppm) was present in the micritic early Silurian dolomite, and crystal sizes of dolomite are inversely proportional to the amount of silica in the crystals. Annealing experiments of calcite, dolomite and high magnesium calcite (HMC) seeds with varying amounts of dissolved silica were performed to validate the trends observed in natural samples between dolomite crystal size and silica content. Synchrotron X-ray diffraction results show that adsorbed silica would substantially inhibit carbonate crystal size growth during recrystallization. This inhibition effect of the adsorbed silica on carbonate growth provides a new explanation for the well-preserved Paleozoic and Precambrian micritic dolomite. The retention of silica in carbonate rocks also suggests carbonate mineral formation has an overlooked effect in the silica cycle.