Reaction rims of dolomite (CaMg [CO 3 ] 2 ) were produced by solid-state reactions at the contacts of oriented calcite (CaCO 3 ) and magnesite (MgCO 3 ) single crystals at 400 MPa pressure, 750-850 °C temperature and 3-146 h annealing time to determine the reaction kinetics. The dolomite reaction rims show two different microstructural domains. Elongated palisades of dolomite grew perpendicular into the magnesite interface with length ranging from about 6 µm to 41 µm. At the same time, a 5-71 µm wide rim of equiaxed granular dolomite grew at the contact with calcite. Platinum markers showed that the original interface is located at the boundary between the granular and palisade-forming dolomite. In addition to dolomite, a 12 to 80 µm thick magnesiocalcite layer formed between the dolomite reaction rims and the calcite single crystals. All reaction products show at least an axiotactic crystallographic relationship with respect to calcite reactant, while full topotaxy to calcite prevails within the granular dolomite and magnesio-calcite. Dolomite grains frequently exhibit growth twins characterized by a rotation of 180° around one of the 112 0 equivalent axis. From mass balance considerations it is inferred that the reaction rim of dolomite grew by counter diffusion of MgO and CaO. Assuming an Arrhenius type temperature dependence, activation energies for diffusion of CaO and MgO are E a (CaO) = 192 ± 54 kJ/mol and E a (MgO) = 198 ± 44 kJ/mol, respectively.

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Reaction rim growth experiments provide insight into mass transport phenomena, which are important for metamorphic rock-forming processes and deformation mechanisms. We investigated the formation of enstatite single rims between quartz and forsterite and of enstatite-forsterite double rims between quartz and periclase using porous polycrystalline starting materials. About 3 wt% water was added, acting as a catalyst for reactions.Experiments of mainly 4 and 23 h duration were performed in a Paterson-type deformation apparatus at 1000°C temperature, 400 MPa confining pressure and differential stresses between 0 and 46 MPa. The resulting reaction rim width varied between <1 µm and ≈ 23 µm, depending on duration and type of reaction product. At isostatic pressure conditions, our data indicate that rim growth is proportional to time, controlled by dissolution-precipitation at interfaces of interconnected fluid-filled pores. In contrast, under non-isostatic stress conditions the reaction rim thickness increases non-linearly with time, implying diffusioncontrolled growth. The magnitude of differential stress has no systematic influence on the reaction rate. Microstructural observations suggest that deformation-induced reduction of interconnected porosity causes this change in rate-controlling mechanism. For a natural MgO-SiO2 system, the results infer that fast interface-controlled reaction in the presence of high amounts of water is easily suppressed by concurrent deformation. Keywords Water  Rim growth  Differential stress  Mineral reaction  Deformation