Carbon neutral or negative mining can potentially be achieved by integrating carbon mineralization processes into the mine design, operations, and closure plans. Brucite Mg­(OH)2 is a highly reactive mineral present in some ultramafic mine tailings with the potential to be rapidly carbonated and can contain significant amounts of ferrous iron Fe­(II) substituted for Mg; however, the influence of this substitution on carbon mineralization reaction products and efficiency has not been thoroughly constrained. To better assess the efficiency of carbon storage in brucite-bearing tailings, we performed carbonation experiments using synthetic Fe­(II)-substituted brucite (0, 6, 23, and 44 mol % Fe) slurries in oxic and anoxic conditions with 10% CO2. Additionally, the carbonation process was evaluated using different background electrolytes (NaCl, Na2SO4, and Na4SiO4). Our results indicate that carbonation efficiency decreases with increasing Fe­(II) substitution. In oxic conditions, precipitation of ferrihydrite Fe10 IIIO14(OH)2 and layered double hydroxides {e.g., pyroaurite Mg6Fe2 III(OH)16CO3·4H2O} limited carbonation efficiency. Carbonation in anoxic environments led to the formation of Fe­(II)-substituted nesquehonite (MgCO3·3H2O) and dypingite Mg5(CO3)4(OH)2·∼5H2O, as well as chukanovite Fe2 IICO3(OH)2 in the case of 23 and 44 mol % Fe­(II)-brucite carbonation. Carbonation efficiencies were consistent between chloride- and sulfate-rich solutions but declined in the presence of dissolved Si due to the formation of amorphous SiO2·nH2O and Fe–Mg silicates. Overall, our results indicate that carbonation efficiency and the long-term fate of stored CO2 may depend on the amount of substituted Fe­(II) in both feedstock minerals and carbonate products.