Hydromagnesite solubility product and growth kinetics in aqueous solution from 25 to 75 °C

Hydromagnesite Mg-5(CO3)(4)(OH)(2)center dot 4H(2)O is the most widespread form of hydrated Mg-carbonate minerals. To better understand the factors controlling the precipitation of hydrated Mg-carbonates, we measured hydromagnesite solubility product at 25 and 50 degrees C and its growth rate between 25 and 75 degrees C, using natural hydromagnesite from a cave as seed material. The solubility products values derived in this study, Ksp-Hmgs = -37.08 +/- 0.50 and -38.90 +/- 0.54 at 25 and 50 degrees C, respectively, are in the upper range of published values. Hydromagnesite growth rate normalized to the BET surface area at 8 <= pH <= 10 is consistent with the direct and reversible attachment of the reactants at the solid surface being rate-limiting. It may be described by: R-Hmgs = A(0) . e(-Ea/RT) (Omega(1/5)(Hmgs)-1) where A(0), the pre-exponential factor, and Ea, the activation energy, are equal to 5.12 x 10(-7) mol/cm(2)/s and 45.5 +/- 9 kJ/mol, respectively, and Omega(Hmgs) stands for the saturation state of the solution with respect to hydromagnesite. Comparison of hydromagnesite growth rates with recently published magnesite growth rates (Saldi et al., 2009, 2012) show that: (1) hydromagnesite apparent growth activation energy is lower by more than 100 kJ/mol compared to the activation energy for magnesite obtuse step advancement, and (2) hydromagnesite growth rate constant extrapolated to 90 degrees C is 2.5 orders of magnitude higher than corresponding magnesite growth rate constant. Thus, our results confirm the long-standing hypothesis that the slow dehydration kinetics of the Mg2+ cation is responsible for the sluggish magnesite formation at low temperature, and that the kinetic barrier for hydromagnesite growth is much lower. Nevertheless, simulation of hydromagnesite and magnesite growth rates as a function of solution composition at 50 and 90 degrees C, and pH 7 and 9 reveal that, because of its much higher solubility, hydromagnesite would grow more quickly than magnesite in natural or industrial environments only at 50 degrees C and pH 9. This is consistent with the formation of hydromagnesite during the surface alteration of ultramafic rocks. (C) 2014 Elsevier Ltd. All rights reserved.