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Dypingite, a hydrated Mg-carbonate mineral, was precipitated from high-pH, high salinity solutions to investigate controls on carbon fixation and to identify the isotopic characteristics of mineral sequestration in mine tailings. δ¹³C values of dissolved inorganic carbon content and synthetic dypingite are significantly more negative than those predicted for equilibrium exchange of CO₂ gas between the atmosphere and solution. The measured δ¹³C of aqueous carbonate species is consistent with a kinetic fractionation that results from a slow diffusion of atmospheric CO₂ into solution. During dypingite precipitation, dissolved inorganic carbon concentrations decrease and δ¹³C values become more negative, indicating that the rate of CO₂ uptake into solution was outpaced by the rate of carbon fixation within the precipitate. This implies that CO₂ gas uptake is rate-limiting to CO₂ fixation. δ¹³C of carbonate mineral …