Natural hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O) dissolution and precipitation experiments were performed in closed-system reactors as a function of temperature from 22.5 to 75ºC and at 8.6 < pH < 10.7. The equilibrium constants for the reaction Mg 5 (CO 3 ) 4 (OH) 2 ·4H 2 O + 6H + = 5Mg 2+ + 4HCO 3 À + 6H 2 O were determined by bracketing the final fluid compositions obtained from the dissolution and precipitation experiments. The resulting constants were found to be 10 33.7Ô0.9 , 10 30.5Ô0.5 and 10 26.5Ô0.5at 22.5, 50 and 75ºC, respectively. Whereas dissolution rates were too fast to be determined from the experiments, precipitation rates were slower and quantified. The resulting BET surface areanormalized hydromagnesite precipitation rates increase by a factor of~2 with pH decreasing from 10.7 to 8.6. Measured rates are approximately two orders of magnitude faster than corresponding forsterite dissolution rates, suggesting that the overall rates of the low-temperature carbonation of olivine are controlled by the relatively sluggish dissolution of the magnesium silicate mineral.
The solubility of natural dolomite was investigated from 50 to 175 °C in 0.1 mol/kg NaCl solutions using a hydrogen electrode concentration cell (HECC). The resulting apparent solubility products (Qsp-dol) for the reaction Dolomite = Ca 2+ + Mg 2-+ 2 CO3 2were extrapolated to infinite dilution to generate the equilibrium constants for this reaction (Ksp°-dol),. Retrieved equilibrium constants are based on a constant standard state isobaric heat capacity (o rp C ) of-789.832 J/mol/K for this dolomite hydrolysis reaction and can be accurately described using log10 Ksp°-mgs = a + b/T (K) + cT (K), where a = 18.1777, b =-4400.8736 K and c(fixed)=-0.06919 K-1. Resulting constants are nearly equal to those determined from SUPCRT92 (Johnson et al., 1992) at 200 C, but as much as 1.8 orders of magnitude more negative at 50 °C, suggesting that dolomite is somewhat more stable than previously assumed. Standard equilibrium constants generated in this study were used together with corresponding properties for the aqueous species to calculate revised standard state thermodynamic properties for dolomite.
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