Solubility of the hydrated Mg-carbonates nesquehonite and dypingite from 5 to 35 °C: Implications for CO ₂ storage and the relative stability of Mg-carbonates

Hydrated Mg-carbonate minerals form during the weathering of ultramafic rocks, and can be used to store atmospheric CO ₂ to help combat greenhouse gas-fueled climate change. Optimization of engineered CO ₂ storage and prediction of the composition and stability of Mg-carbonate phase assemblages in natural and engineered ultramafic environments requires knowledge of the solubility of hydrated Mg-carbonate phases, and the transformation pathways between these metastable phases. In this study, we evaluate the solubility of nesquehonite [MgCO ₃ ·3H ₂ O] and dypingite [Mg ₅ (CO ₃ ) ₄ (OH) ₂ ·(5 or 8)H ₂ O] and the transformation from nesquehonite to dypingite between 5 °C and 35 °C, using constant-temperature, batch-reactor experiments. The logarithm of the solubility product of nesquehonite was determined to be: −5.03 ± 0.13, −5.27 ± 0.15, and −5.34 ± 0.04 at 5 °C, 25 °C, and 35 °C, respectively. The logarithm of the solubility product of dypingite was determined to be: −34.95 ± 0.58 and −36.04 ± 0.31 at 25 °C and 35 °C, respectively, with eight waters of hydration. This is the first reported dypingite solubility product. The transformation from nesquehonite to dypingite was temperature-dependent, and was complete within 57 days at 25 °C, and 20 days at 35 °C, but did not occur during experiments of 59 days at 5 °C. This phase transformation appeared to occur via a dissolution-reprecipitation mechanism; external nesquehonite crystal morphology was partially maintained during the phase transformation at 25 °C, but was eradicated at 35 °C. Together, our results facilitate the improved evaluation of Mg-carbonate mineral precipitation in natural and engineered ultramafic mineral weathering systems that sequester CO ₂ , and for the first time allow assessment of the saturation state of dypingite in aqueous solutions.