Exchange and fractionation of Mg isotopes between epsomite and saturated MgSO4 solution

Magnesium isotope fractionation during hydrous magnesium carbonate precipitation with and without cyanobacteria

“…the aqueous fluid), meaning the lighter, less stable isotopes will be favored in the solid phase (Criss, 1999). In contrast, (Li et al, 2011) reported that epsomite preferentially incorporates heavy Mg from aqueous solution. These authors attribute this contrasting behavior to the difference in strength and configuration of bonds in sulfate versus carbonate minerals.…”

The control of carbonate mineral Mg isotope composition by aqueous speciation: Theoretical and experimental modeling

“…The importance of Mg in global and past geochemical processes has also motivated experimental studies aimed at quantifying Mg isotope fractionation during the formation of calcite (Immenhauser et al, 2010;Li et al, 2012;Mavromatis et al, 2013), magnesite , dolomite (Li et al, 2015), nesquehonite/ dypingite (Mavromatis et al, 2012;Shirokova et al, 2013), brucite (Wimpenny et al, 2014;Li et al, 2014), and epsomite (Li et al, 2011). These studies indicate that 24 Mg is preferentially incorporated into all precipitated carbonate minerals, such that the minerals have a lower 26 Mg/ 24 Mg ratio than in the fluid phase.…”

“…Criss, 1999;Schauble, 2004;Li et al, 2014). For example, the enrichment of brucite, magnesite and dolomite in 24 Mg is consistent with the longer average Mg-O distance in these minerals (2.100-2.093 Å in brucite, Catti et al, 1995;Chakoumakos et al, 1997;2.101 and 2.115 Å in magnesite and dolomite, respectively, Schauble, 2011;Antao et al, 2004) compared to aqueous Mg 2+ (2.08 Å; Pavlov et al, 1998), whereas the shorter Mg-O distance in epsomite (2.065 Å; Baur, 1964) can explain the preferential incorporation of 26 Mg into its crystal lattice (Δ 26 Mg epsomitefluid = +0.6‰; Li et al, 2011). The strong fractionation of Mg 2+ between calcite and aqueous solution (Δ 26 Mg calcite-fluid = −3.5‰) compared to Mg-carbonates likely results from the strain induced by the substitution of the relatively small magnesium cation into the octahedral calcium sites of this mineral.…”

The effect of Al on Si isotope fractionation investigated by silica precipitation experiments

“…Indeed, attaining Si isotopic equilibrium in experimental settings is virtually impossible due to the extremely low exchange rates between solids and fluids in low-temperature processes, especially in the SiO 2 -H 2 O system. Li et al (2011) suggested that recrystallization (or re-organization) induced by "Ostwald ripening", the dissolution of small particles and the re-deposition of the dissolved species on the surfaces of larger particles in a saturated solution, is the only way to induce an isotope exchange at low temperature that is not overprinted by kinetic processes. To test whether equilibrium has indeed been attained, experimentalists use the addition of isotopicallyenriched species in one of the two compartments (Johnson et al, 2002;Welch et al, 2003;Schuessler et al, 2007).…”