The mechanism of the reaction 1 tremolite+3 calcite+2 quartz =5 diopside+3 CO2+1 H2O: results of powder experiments

The mechanism of the reaction 1 tremolite +3 calcite+2 quartz=5 diopside+3 CO₂+1 H₂O was investigated at 2 and 5 kb, , using powder experiments lasting from 14 to 170 days. Because experiments were at high ratios of fluid to solids, the study identified the mechanism under surface-control conditions and thus establishes which reactant surface determines the kinetics. To achieve a diopside nucleation rate high enough to gain detectable reaction in the time of experimentation, the equilibrium boundary had to be overstepped by 30°–60° C at 5 kb. Experiments in which diopside successfully nucleated show that the reaction proceeds by a dissolution-crystallization mechanism. Experimentally-produced textures are presented in a series of SEM images and demonstrate that diopside nucleates and grows topotactically exclusively on tremolite. The mechanism of the forward reaction is modeled by a simplified scheme consisting of three processes, each comprising formation, transport and incorporation of 1) the Ca-, 2) the Mg-, and 3) the Si-bearing species in the fluid in response to dissolution of the reactants and crystallization of diopside. Using the dependence of the overall-reaction rate on the surface area of the reactants, it was experimentally determined that process 2) (dissolution of tremolite, transport of the Mg-bearing species in the fluid and crystallization of diopside) will be rate-limiting in most cases where metamorphism occurs in an internally controlled system. Due to the experimental design chosen, the dissolution of tremolite at the beginning of process 2) is rate-limiting in the experiments. The magnitude of the probable temperature-overstep necessary to achieve a significant nucleation rate during metamorphism is discussed on the basis of the experimental evidence and a simple nucleation rate model.