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Structure and reactivity of the dolomite (104)-water interface: New insights into the dolomite problem |
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| Authors: | P Fenter Z Zhang NC Sturchio XM Hu |
| Institution: | a Argonne National Laboratory, CHM/200, 9700 South Cass Avenue, Argonne, IL 60439, USA b Department of Earth and Environmental Sciences, University of Illinois at Chicago, IL 60607, USA c Department of Chemistry, Wright State University, 3640 Col. Glenn Hwy.Dayton, OH 45435, USA |
| Abstract: | The structure and reactivity of the dolomite (104)-water interface was probed in situ with high resolution X-ray reflectivity and surface force microscopy at room temperature. Measurements in stoichiometric solutions alternating between saturated and supersaturated (log IAP/K = 2.3) conditions show that the dolomite surface termination readily changes in response to solution composition, but these changes are self-limiting and partially irreversible. The freshly cleaved dolomite (104) surface in contact with the saturated solution has a stoichiometric termination, a distinct surface hydration layer and small surface structural displacements, similar to those observed previously at the calcite-water interface. After reaction with supersaturated solutions dolomite is terminated by a two-layer thick Ca-rich film with substantial structural displacements of the cations. With subsequent exposure to a saturated solution this surface was transformed to an interfacial structure different from the freshly cleaved surface, having a reduced density of the outermost surface layer and a Ca-rich second layer. These results provide new insight into the lack of dolomite growth in modern carbonate environments (i.e., the “dolomite problem”), suggesting that this behavior is associated with a combination of thermodynamic and kinetic factors, including (1) growth of compositionally modified epitaxial CaXMg2−X(CO3)2 layers having thicknesses limited by lattice strain, (2) slow incorporation of Mg during layer growth, and (3) partial irreversibility of surface reactions. |
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