Diffusion‐controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experiment

Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion‐controlled. From the rate constants, the D′ δ‐values of the rate‐limiting species were derived, i.e. the product of grain boundary diffusion coefficient D′ and the effective grain boundary width, δ. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that ( D′ CaO δ)/( D′ SiO2 δ)≥1.33, and that SiO 2 diffusion controls rim growth. D′ SiO2 δ increases from about 10 −25 to 10 −23  m 3  s −1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol −1 . In runs at 1 GPa, performed in a piston‐cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion‐advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO 2 diffusion is rate‐limiting. D′ δ increases from about 10 −21 to 10 −20 m 3  s −1 as temperature increases from 1100 to 1200 °C. D′ SiO2 δ or D′ CaO δ in rims on calcite is c. 10 times larger than D′ SiO2 δ in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact‐metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine‐infiltrated metacherts to water‐bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact‐metamorphism as compared to our experimental results.