We present a thermodynamic model for diffusion controlled growth of a reaction rim of phase between the phases and in a two component system. We investigate the case, where the reactant phase has planar, cylindrical and spherical geometry and is embedded in a matrix of phase . We find that for non planar geometry and for the general case, where the molar volumes of the reactant phases are different, the rim growth rate depends on the matrix-inclusion arrangement. The model is applied to growth of enstatite reaction rims that form at quartz-forsterite interfaces. Two different geometrical setups are considered, namely a spherical grain of forsterite in a quartz matrix and a spherical quartz grain in a forsterite matrix. The enstatite rims are polycrystalline and transfer of the MgO and SiO2 components across the growing rim occurs by a combination of volume and grain boundary diffusion. For enstatite rims that were grown at experimental conditions of 1000°C and 1 GPa (Milke and others, 2008) bulk mass transfer may be described by effective diffusion coefficients in the range of 1.8 1017m2s1 < DSiO2 < 1.1 10 16 m2s1 and 2.7 1017m2s1 < DMgO < 1.6 10 16 m2s1.