Mg/Ca Partitioning Between Aqueous Solution and Aragonite Mineral: A Molecular Dynamics Study

Abstract

We have calculated the concentrations of Mg in the bulk and surfaces of aragonite CaCO3 in equilibrium with aqueous solution, based on molecular dynamics simulations and grand‐canonical statistical mechanics. Mg is incorporated in the surfaces, in particular in the (001) terraces, rather than in the bulk of aragonite particles. However, the total Mg content in the bulk and surface of aragonite particles was found to be too small to account for the measured Mg/Ca ratios in corals. We therefore argue that most Mg in corals is either highly metastable in the aragonite lattice, or is located outside the aragonite phase of the coral skeleton, and we discuss the implications of this finding for Mg/Ca paleothermometry.

Properties of water confined in hydroxyapatite nanopores as derived from molecular dynamics simulations

“…The polarizable shell-model water potential [16] used in the present study has proven successful in describing the structure of liquid water in contact with biominerals such as calcite [23] and aragonite [24] (CaCO 3 ), and dolomite…”

Environmental and biological controls on Na∕Ca ratios in scleractinian cold-water corals

“…Reconstructing past salinities can be accomplished with several different techniques, e.g., diatom and dinoflagellate species composition (Zonneveld et al, 2001), morphology and the size of placoliths from Emiliania huxleyi (Bollmann et al, 2009), Ba/Ca ratios in foraminiferal calcite (Weldeab et al, 2007), the strontium isotope composition of bivalves (Israelson and Buchardt, 1999), the process length of di-noflagellate cysts (Mertens et al, 2009), the hydrogen isotope composition of alkenones (van der Meer et al, 2007;Schouten et al, 2006), or temperature-corrected (Mg/Ca, TEX 86 ) oxygen isotopes (Elderfield and Ganssen, 2000). While some of these proxies may yield reliable results (e.g., coupled Mg/Ca and oxygen isotopes - Elderfield et al, 2012;Lear et al, 2000) others suffer from rather large uncertainties introduced by modeled parameters or require a good knowledge of the regional oceanography (Wit et al, 2013).…”

A Computational Study of Magnesium Incorporation in the Bulk and Surfaces of Hydroxyapatite

“…The high value found for ε already indicates a strong energetic preference for the Mg ion to be in water instead of in the HA bulk. We can now estimate the concentration of Mg in HA in equilibrium with aqueous solution as a function of the activities a [Mg 2+ ] (aq) and a [Ca 2+ ] (aq) of the cations in solution, as proposed in x eq = a false[ Mg 2 + false] false( aq false) a false[ Ca 2 + false] false( aq false) nobreak0em0.25em⁡ exp ( − ε R T ) The exponential term at room temperature is 2.0 × 10 –10 , which means that at any reasonable ratio between the concentrations of Mg 2+ and Ca 2+ in solution, only a very small amount of Mg can be expected to be incorporated in the bulk, assuming equilibrium between the (Mg,Ca)HA solid solution and the aqueous solution.…”

“…The high value found for ε already indicates a strong energetic preference for the Mg ion to be in water instead of in the HA bulk. We can now estimate the concentration of Mg in HA in equilibrium with aqueous solution as a function of the activities a[Mg 2+ ] (aq) and a[Ca 2+ ] (aq) of the cations in solution, as proposed in35…”