Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
10144975 | Geochimica et Cosmochimica Acta | 2018 | 60 Pages |
Abstract
Although adsorption at the solid surface is the first step controlling boron incorporation in the crystal lattice during the standard growth mechanism of calcite and aragonite, little is known about the identity, structure and isotopic composition of the boron complexes formed at the CaCO3-solution interface. To generate this important information, we investigated experimentally the boron chemical and isotopic fractionation during adsorption at the calcite-water interface as a function of pH (6.5-11.7) at 4 and 20â¯Â°C in 0.01 and 0.1â¯M NaCl aqueous solutions. The surface complexation modeling of B adsorption and isotopic composition data showed that boron is sorbed at the calcite surface as a tetrahedral complex (>CaB(OH)40) formed by reaction of borate ions with Ca-protonated surface sites (log Kint0â¯=â¯1.54â¯Â±â¯0.37 at 20â¯Â°C) and excluded the formation of trigonal B surface complexes (>CaB(OH)3+). The B isotopic composition of >CaB(OH)40 is â¼5â° and 2â° heavier than that of aqueous B(OH)4â in 0.01 and 0.1â¯M NaCl solution, respectively. Consistently, these values suggest that adsorbed borate ions have isotopic compositions intermediate between those of aqueous borate and structural tetrahedral species in calcite, which have been recently predicted to be â¼12â° heavier than aqueous borate using quantum mechanical calculations (Balan et al., 2018). The good agreement between the isotopic composition of adsorbed boron measured in this study and boron experimentally co-precipitated with calcite in the 8-9 pH range at close to equilibrium conditions (i.e. via ion-by-ion attachment at advancing steps) (Noireaux et al., 2015) indicate that the isotopic composition of structural boron can be inherited from the boron surface complexes formed at the calcite/water interface. The results of this study contradict the assumption of no isotopic fractionation between tetrahedral boron in calcite and the borate ion that sustains the boron paleo-pH proxy, but confirm that trigonal B cannot be directly incorporated in the crystal structure during near equilibrium growth of calcite.
Keywords
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Physical Sciences and Engineering
Earth and Planetary Sciences
Geochemistry and Petrology
Authors
Giuseppe D. Saldi, Johanna Noireaux, Pascale Louvat, Laeticia Faure, Etienne Balan, Jacques Schott, Jérôme Gaillardet,