The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

Oelkers, Eric H.; Berninger, Ulf Niklas; Perez-Fernandez, Andrea; Chmeleff, Jérôme; Mavromatis, Vasileios

doi:10.1016/j.gca.2017.11.004

Cited by 35 publications

(32 citation statements)

References 102 publications

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“…The latter can be explained by a similar mechanism as proposed for the preferential removal of Mg under acidic conditions; under alkaline conditions, the Si-O bonds break relatively fast compared to the Mg-O bonds. Such a mechanism is consistent with the relative dissolution rates of amorphous SiO 2 compared to those of brucite (Mg(OH) 2 ) at pH >10, with the dissolution rates of brucite, reflecting the rate of breaking Mg-O bonds, being slower than those of amorphous SiO 2 at these alkaline conditions (Fraysse et al, 2006;Oelkers et al, 2018;Pokrovsky and Schott, 2004). The difference in dissolution rates between brucite and amorphous SiO 2 could be due to the protonation of the mineral surfaces as a function of pH, as the point of zero charge of brucite lies at pH 11 (Pokrovsky and Schott, 2004), while that of amorphous SiO 2 lies at pH 2 (Parks, 1967).…”

Section: Non-stoichiometric Dissolution At Alkaline Conditionssupporting

confidence: 70%

“…The dissolution rates based on either Mg or Si release of sepiolite as measured in the flow through experiments are comparable to those of talc, anthophyllite and chrysotile, but are considerably lower than the dissolution rates of olivine minerals and Ca-bearing silicates (see Figure 8). Oelkers (2001) and Schott et al (2009) attributed the relatively rapid dissolution rates of the olivines at acidic conditions to their structure (see also Oelkers et al, 2018). The dissolution of olivine requires only the breaking of relatively weak divalent metal-oxygen bonds, whereas the dissolution of phyllosilicates requires the additional breaking of Si-O bonds.…”

Section: Dissolution Rates and Mechanisms As A Function Of Phmentioning

confidence: 99%

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An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

Mulders

Oelkers

2020

Geochimica et Cosmochimica Acta

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The steady-state dissolution rates of sepiolite were measured as a function of pH from mixed-flow reactor experiments at 25 o C in both citrate bearing and citrate-free aqueous solutions. Dissolution at pH ≤5.5 and pH ≥9.18 was found to be non-stoichiometric, with relatively more Mg being released at low pH, while Si was preferentially released at high pH. The steady-state dissolution rate of sepiolite at 25 o C, based on Si release rates at far from equilibrium conditions, r + in mol/cm 2 /s at pH ≥4.44 can be described bywhere a i refers to the activity of the subscripted aqueous species. This rate equation is based on a dissolution mechanism by which interstitial Mg is exchanged for 2 H + , followed by the rate limiting release of partially detached Si-tetrahedra. At pH ≤4.44 and in citrate-free aqueous solutions, sepiolite dissolution rates based on Si release are similar to those measured for amorphous SiO 2 despite the fact that the bulk fluid phase was undersaturated with respect to this solid. In contrast, in citrate bearing aqueous solutions, the Si release rate increases with decreasing pH following a trend consistent with the corresponding higher pH values. This difference is attributed to the increase of amorphous SiO 2 dissolution rates in citrate bearing aqueous fluids, preventing the formation of substantial SiO 2 -rich surface layers. To further examine the extent of nonstoichiometric dissolution and its implications on solid phase transformations, batch experiments were performed in citrate bearing aqueous solutions at pH 2.57, 3.40 and 4.31. During these longer term experiments the fluid phase attained saturation with respect to amorphous SiO 2 . From mass balance calculations, Energy-Dispersive X-ray spectroscopy (EDS) and X-Ray powder Diffraction (XRD), it can be inferred that after 688 hours, 98% of the Mg was removed from the solid structure during pH 2.57 batch experiments, while SiO 2 was retained in an amorphous solid phase, demonstrating the independence of the Mg compared to Si release to the aqueous solution.

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Section: Non-stoichiometric Dissolution At Alkaline Conditionssupporting

confidence: 70%

Section: Dissolution Rates and Mechanisms As A Function Of Phmentioning

confidence: 99%

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

Mulders

Oelkers

2020

Geochimica et Cosmochimica Acta

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“…In this study, continuing our efforts to deciphering the mechanisms controlling the isotopic fractionation of major and trace elements/impurities in carbonate minerals (e.g. Mavromatis et al, 2012;2017c;Oelkers et al, 2018;Saldi et al, 2018;Pearce et al, 2012;Tang et al, 2012;Shirokova et al, 2013), we examine the effect of mineral growth kinetics on Ba isotope fractionation between calcite and aragonite and their forming aqueous solutions. This experimental approach is complemented by first-principles calculations of Ba isotope fractionation at equilibrium.…”

Section: Introductionmentioning

confidence: 99%

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

Mavromatis

Zuilen

Blanchard

et al. 2020

Geochimica et Cosmochimica Acta

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Barium isotope fractionation during calcite and aragonite inorganic precipitation was studied in mixed flow reactors as a function of precipitation rate at 25°C and pH = 6.3±0.1. The measured Ba isotope fractionation that occurs between calcite and the forming fluid in the investigated range of calcite growth rates (10-8.0 ≤ R p(calcite) ≤ 10-7.3 mol/m 2 /s) is insignificant. Barium isotope fractionation between aragonite and the fluid decreases with increasing precipitation rate from Δ 137/134 Ba aragonite-fluid = + 0.25±0.06 ‰ for R p(aragonite) ≤ 10-8.7 mol/m 2 /s to-0.10±0.08‰ for R p(aragonite) =10-7.6 mol/m 2 /s, thus reflecting preferential incorporation of either heavy or light Ba isotopes in aragonite at slow and fast growth rates, respectively. The dependence of Ba isotope fractionation on aragonite growth rate is well described by the surface reaction kinetic model developed by DePaolo (2011) when the values + 0.27 ‰ and-2.0±0.2‰ are used for the equilibrium and kinetic Ba isotope fractionation factor, respectively. The enrichment of aragonite in the heavier Ba isotopes is consistent with the equilibrium fractionation factor of + 0.34‰, calculated here between Ba-substituted aragonite and Ba 2+ (aq), from first-principles calculations. This positive fractionation is related to a shorter average Ba-O bond length in the aragonite structure while the coordination number does not change much (i.e. 9). The lack of isotope fractionation between the Ba aquo ions and the 6-coordinated Ba in calcite likely suggests that the coordination reduction required for the incorporation in the lattice of Ba adsorbed at calcite growing sites proceeds without isotope fractionation with the fluid. Otherwise the precipitated calcite should have been enriched in heavy isotopes by ~0.17‰, as predicted by first-principles calculations. These results are the first experimental measurements of Ba isotope fractionation during inorganic calcite and aragonite mineral formation and set the basis for understanding the mechanisms controlling Ba isotope composition in CaCO 3 minerals that is an essential perquisite for application of this isotopic system in natural samples.

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“…The morphology of these magnesium carbonate hydrate particles produced by talc carbonation was visualized by SEM, and the representative images are presented in Figure 1. As can be observed, the magnesium carbonate hydrate particles exhibited four different microscopic morphologies [22][23][24]. In Figure 1a, the needle-like particles were produced, and the axis diameter was in the range of 0.3-0.8 µm.…”

Section: Production Of Magnesium Carbonate Hydrate With Different Mormentioning

confidence: 93%

Synthesis of magnesium carbonate hydrate from natural talc

et al. 2020

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Various morphologies of magnesium carbonate hydrate had been synthesized without using any organic additives by carefully adjusting the reaction temperature and time during the talc carbonation process. At lower temperatures, magnesium carbonate hydrate was prone to display needle-like morphology. With the further increase of the carbonation temperature, the sheet-like crystallites became the preferred morphology, and at higher aging temperatures, these crystallites tended to assemble into layer-like structures with diverse morphologies, such as rose-like particles and nest-like structure. The reaction time had no effect on the crystal morphology, but it affected the particle size and situation of the crystal growth. X-Ray diffraction results showed that these various morphologies were closely related to their crystal structure and compositions. The needle-like magnesium carbonate hydrate had a formula of MgCO3·3H2O, whereas with the morphological transformation from needle-like to sheet-like, rose-like, and nest-like structure, their corresponding compositions also changed from MgCO3·3H2O to 4MgCO3·Mg(OH)2·8H2O, 4MgCO3·Mg(OH)2·5H2O, and 4MgCO3·Mg(OH)2·4H2O.

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The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

Cited by 35 publications

References 102 publications

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

Synthesis of magnesium carbonate hydrate from natural talc

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The temporal evolution of magnesium isotope fractionation during hydromagnesite dissolution, precipitation, and at equilibrium

Cited by 35 publications

References 102 publications

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

Experimental and theoretical modelling of kinetic and equilibrium Ba isotope fractionation during calcite and aragonite precipitation

Synthesis of magnesium carbonate hydrate from natural talc

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An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

An experimental study of sepiolite dissolution rates and mechanisms at 25 °C