The interaction of dolomite surfaces with metal impurities: a computer simulation study

Abstract

This study investigates the behaviour of selected, morphologically important surfaces of dolomite (CaMg(CO 3 ) 2 ), using computational modelling techniques. Interatomic potential methods have been used to examine impurity substitution at cationic sites in these surfaces. Environmentally prevalent cations were studied to this end, namely Ni 21 , Co 21 , Zn 21 , Fe 21 , Mn 21 and Cd 21 , all of which are also found as end-member carbonate minerals. Solidsolution substitution was investigated and showed that Cd and Mn will substitute from their end-member carbonate phase at either dolomite cation site. Mn is found to preferentially substitute at Mg sites, in agreement with experimental findings. For Ni 21 , Co 21 and Zn 21 , the magnitude of substitution energies is approximately equal for all surfaces, with the exception of the (10 14) surface. However, for the larger cations, a far greater disparity in substitution energies is observed. At a stepped surface, analogous substitutions were performed and it was found that substitution energies for all impurity cations were reduced, indicating that uptake is more viable during growth. The predominant surface, the (10 14), was solvated with a monolayer of water in order to investigate the influence of hydration on substitution energetics. The addition of water changes the relative preference for substitution of the different cations. Under aqueous conditions, the substitution energy is determined by three competing factors, the relative importance of which cannot be predicted without this type of computational investigation.

Defining reactive sites on hydrated mineral surfaces: Rhombohedral carbonate minerals

“…Extension of these Atomistic Simulations to other hydrated (10.4) carbonate surfaces, such as magnesite and dolomite, reveals that H 2 O also adsorbs associatively on these surfaces according to a 1:1 H 2 O:MeCO 3(surface) stoichiometry where each adsorbed H 2 O molecule interacts with one metal center (Mg for magnesite and Mg or Ca for dolomite) and at least one neighboring O surface atom Parker, 2001, 2002;Wright et al, 2001;Parker et al, 2003;Kerisit et al, 2005a;Austen et al, 2005). This information strongly suggests that, regardless of the specific orientation of adsorbed H 2 O molecules relative to the mineral surface, all (10.4) single-and mixed-metal carbonate surfaces are subjected to similar hydration processes where water remains undissociated upon adsorption.…”

Subsolidus phase relations in theCaCO3–MgCO3system predicted from the excess enthalpies of supercell structures with single and double defects

“…The last equation and its analog for the B limit can be now used to calculate the J's in the calcite-magnesite system. To calculate the enthalpies of the single-and double-defect structures, we used the force-field model of Austen et al 28 and the program GULP. 32,33 The excess enthalpies and the J's are given in Table II.…”

“…23 It has been shown 23 that the excess energies of various supercell structures in the calcite-magnesite system calculated with the potentials of Austen et al 28 correlate linearly with the excess energies of the same structures calculated with a density-functional theory ͑DFT͒ based model. 10 The choice in favor of the force-field model allows comparison of the results with those obtained with the random sampling strategy.…”

“…Although future applications will very likely be based on first-principles calculations, here we illustrate the method using a force-field model 28 which has been already successfully applied to simulate phase relations in the rhombohedral carbonates. 23 It has been shown 23 that the excess energies of various supercell structures in the calcite-magnesite system calculated with the potentials of Austen et al 28 correlate linearly with the excess energies of the same structures calculated with a density-functional theory ͑DFT͒ based model.…”

Zebra pattern in rocks as a function of grain growth affected by second-phase particles

“…The value of the surface free energy for limestone is 0.27 Jm −2 [46] whereas the value for dolomite is poorly known. We therefore based the distribution of the surface energy in our model on the work of Austen et al [47]. Although the simulated process is thought to be situated in a burial environment with an average depth of 2 km, we neglected the pore pressure and the stresses acting on the crystals assuming that these play only a minor role.…”