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Hydrated Mg-carbonate minerals form during the weathering of ultramafic rocks, and can be used to store atmospheric CO2 to help combat greenhouse gas-fueled climate change. Optimization of engineered CO2 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 IMgCO3 center dot 3H(2)O] and dypingite [Mg-5(CO3)(4)(OH)(2)center dot(5 or 8)H2O] and the transformation from nesquehonite to dypingite between 5 degrees C and 35 degrees 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 degrees C, 25 degrees C, and 35 degrees 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 degrees C and 35 degrees 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 degrees C, and 20 days at 35 degrees C, but did not occur during experiments of 59 days at 5 degrees 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 degrees C, but was eradicated at 35 degrees C. Together, our results facilitate the improved evaluation of Mg-carbonate mineral precipitation in natural and engineered ultramafic mineral weathering systems that sequester CO2, and for the first time allow assessment of the saturation state of dypingite in aqueous solutions.