Abstract
Magnesium in biogenic aragonite skeletons is widely used to infer the geochemical conditions under which the minerals precipitated. However, because molecular speciation of Mg2+ in the aragonite crystal structure remains uncertain, questions are often posed about the reliability and utility of Mg as a paleoenvironment proxy. In this study, we investigated the Mg2+ coordination in abiogenic aragonite by applying first-principles density functional theory (DFT). We report for the first time a Mg2+-site disordered structure model of aragonite: substituted Mg2+ is five-fold coordinated and induces substantial disorder in the bond distances that depends on the relative positions of Mg2+ sites. Using the site disorder model, DFT qualitatively reproduces the key features of experimental Mg K-edge X-ray absorption near-edge structure (XANES) spectra of aragonite. DFT also resolves a long-standing conflict between experimental and theoretical studies pertaining to the enrichment of 26Mg in aragonite relative to calcite. The Boltzmann-averaged 26Mg/24Mg reduced partition function ratios were calculated in the order of aqueous Mg2+ > Mg-doped aragonite > Mg-doped calcite. The disorder structure model is likely applicable to the substitution of cations with radii smaller than the radius of Ca2+ ions in aragonite. Our study provides atomistic insight into metal substitution effects on the metal content and isotope fractionation in biogenic aragonite, which are critical for developing a reliable paleoenvironment proxy.
Published Version
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