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Lindner, Michael; Saldi, Giuseppe D.; Stumpp, Dominik; Schott, Jacques; Jordan, Guntram (2019): On the effect of aqueous strontium on magnesite growth. In: Chemical Geology, Vol. 510: pp. 1-9
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Strontium was found to be a growth-promoting additive during the precipitation of calcite from aqueous solution. This raises the question of whether growth of the isostructural magnesite is promoted by aqueous strontium ions as well. For this, the influence of Sr2+ on magnesite growth was investigated by macroscopic mixed-flow reactor experiments and hydrothermal atomic force microscopy at 100 °C. It was found that neither growth rates nor molecular step morphologies are affected by the presence of strontium. The partition coefficient of strontium between magnesite and aqueous solution Kd Sr mgs =(XSr/XMg)/(mSr/mMg) was determined to be <0.21±0.09. In solutions with Sr concentrations >50 μM and Mg:Sr ratios ≤60, strontianite precipitation limited Sr incorporation into growing magnesite to ~0.5 mol%. Similar amounts of Sr incorporation into calcite were reported in literature, although the partition coefficient of Sr between calcite and solution is lower than between magnesite and solution. The slow growth rates of magnesite, however, imply the use of high solution supersaturations. These conditions inevitably promote strontianite precipitation depleting the solution of Sr. In the case of calcite, fast growth rates can be achieved at low supersaturations. Such solutions are permissive of high Sr concentrations without strontianite precipitation. The magnesite growth rates measured here complete the literature data, now spanning a range of supersaturation states of more than one order of magnitude (Ω ~ 20-240). The completed data allow for a refinement of the kinetic constants of magnesite precipitation. The fit of all rates to the function R = k(Ω−1)^n yielded the reaction order n = 1.76±0.03 and the growth rate constant k = 0.015±0.002 nmol m-2 s-1.