X-ray photoelectron spectroscopic studies of dolomite surfaces exposed to undersaturated and supersaturated aqueous solutions

Abstract

Cleaved surfaces of dolomite were studied using ex-situ X-ray photoelectron spectroscopy (XPS) following exposure of the surfaces to various experimental conditions. Dolomite samples exposed to air, to a highly undersaturated solution (0.1 M NaCl, pH = 9), and to solution with a supersaturation (ÀDl/kT) of 5.5 (pH = 9) were investigated with semiquantitative methods of analysis to ascertain the degree of non-stoichiometry resulting at the dolomite surface from reactive conditions. It was found that the dolomite cleavage surface in undersaturated solution was not altered significantly from the stoichiometric surface termination. The composition of the cleaved surface after exposure to supersaturated solution, a surface known to have self-limiting growth characteristics under similar conditions, was found to be Ca 2+ rich (Ca x Mg 2 À x (CO 3 ) 2 , 1.7 > x > 1.3). The observations, while underscoring differences in hydration/dehydration kinetics of the two alkaline earth cations, suggest that achievement of equilibrium at dolomite-water interfaces may be subject to significant barriers from both undersaturated and supersaturated solutions.

Structure and reactivity of the dolomite (104)–water interface: New insights into the dolomite problem

“…Additional observations of the interfacial structure after extended reaction times show (based on model-fits to the X-ray reflectivity data): (1) a well-defined structure for initial dolomite surfaces that is similar to that found previously for calcite (104); (2) formation of a self-limiting two-layer thick Ca-rich surface layer in contact with a supersaturated solution, (3) a surface termination in saturated solutions that is dependent on sample history, with the initial surface showing a stoichiometric surface termination while surfaces previously reacted with supersaturated solution having a Ca-rich second layer and a surface layer that may be Mgrich or incomplete. The observation of a Ca-rich film under supersaturated solution conditions with the Ca/Mg ratio of 3 that was found here (within the top two layers) is consistent with previous XPS data (Hu et al, 2006) which found a nominal surface composition of 2 < Ca/Mg < 6. Whereas previous AFM measurements did not explore the surface termination in equilibrium solutions after formation of the Ca-rich film, they showed rapid removal of this film when exposed to lower degrees of supersaturation and no evidence for the formation of partial layers.…”

“…For instance, the surface composition of dolomite was found to have a Mg/Ca ratio of 3:1 when placed in contact with seawater with [Mg]/[Ca] = 5 (Mucci and Morse, 1985). Comparison with the present results and recent XPS results (Hu et al, 2006) with a 1:1 Mg/Ca ratio suggests that the surface composition is controlled by solution composition. We can therefore speculate that with an appropriate solution composition, the surface might be unconstrained by strain limitation (having ideal stoichiometry), and therefore could grow to an arbitrary thickness.…”

“…Solutions were prepared and characterized as described previously Hu et al, 2006). Solution compositions are listed in Table 1.…”

“…Conventional techniques for characterizing surface layer composition (e.g., X-ray photoelectron spectroscopy) are difficult to apply to this system due to spectral interference from the substrate, which prevents accurate assessment of the surface layer composition. Measurements to date, however, suggest that the surface layer formed in a stoichiometric supersaturated solution is Ca-rich (Hu et al, 2006) with a stoichiometry of 2 < Ca/Mg < 6. Structural and compositional measurements of surface layers are best done in situ so that the properties of the surface are not perturbed by changes of sample environment.…”

A route for the direct crystallization of dolomite

“…This is particularly relevant because, as yet, inorganic dolomite has not been synthesized at ambient conditions directly from solution (Land 1998). Most experimental inorganic dolomite formation studies have quantified the "dolomitization" of calcite (Gaines 1977;Katz and Matthews 1977;Miura and Kawabe 2000;Nordeng and Sibley 1994;Kaczmarek and Sibley 2007), other Ca-Mg carbonates (Katz and Matthews 1977;Zempolich and Baker 1993;Sibley et al 1994), or followed proto-dolomite crystallization (Malone et al 1996) or dolomite growth (Gaines 1977;Arvidson and Mackenzie 1999;Higgins and Hu 2005;Hu et al 2005Hu et al , 2006 on various carbonate seed crystals. These experiments all mimic the diagenetic dolomite formation processes.…”