Magnesium isotope fractionation between brucite [Mg(OH)2] and Mg aqueous species: Implications for silicate weathering and biogeochemical processes

Brucite, with its octahedral structure, has a lattice configuration that is similar to the Mg-bearing octahedral layers in phyllosilicates. Understanding stable Mg isotope fractionation between brucite and aqueous solution therefore bears on interpretation of Mg isotope data in natural weathering systems. In this study, we experimentally determined Mg isotope fractionation between brucite and two Mg aqueous species, the free Mg aquo ion ([Mg(OH2)6]2+) and EDTA-bonded Mg (Mg-EDTA2−). Results from recrystallization and brucite synthesis experiments suggest mild preferential partitioning of light Mg isotopes into brucite compared to Mg aquo ions at low temperatures, where measured Δ Mg brucite - Mg 2 + 26 fractionation increased from ca. − 0.3 ‰ at 7 °C, to ca. − 0.2 ‰ at 22 °C, to ca. 0‰ at 40 °C. MgO hydrolysis experiments in EDTA-bearing solutions suggest that the Δ Mg brucite - Mg - EDTA 26 fractionation is ⩾ + 2.0 ‰ at 22 °C, indicating that light Mg isotopes strongly partition into Mg-EDTA complex relative to brucite, as well as relative to Mg aquo ions. Magnesium atoms in brucite, Mg aquo ions, and Mg-EDTA complexes are all octahedrally coordinated, and the measured Mg isotope fractionations correlate with average bond lengths for Mg. Aqueous Mg ions have the shortest bond length among the three phases, and enrich heavy Mg isotopes relative to brucite and Mg-EDTA. In contrast, Mg-EDTA has the longest average bond length for Mg, and enriches light Mg isotopes relative to Mg aquo ions and brucite; the relatively long Mg-EDTA bond suggests that organically bound Mg may commonly have low 26Mg/24Mg ratios, which may explain proposed “vital” effects for stable Mg isotopes. Such relations between bond length and Mg isotope fractionation could be extended to other phyllosilicates such as serpentine- and clay-group minerals where Mg is also octahedrally coordinated.