Kinetics and mechanisms of the reaction of zoisite to anorthite under hydrothermal conditions: reaction phenomenology away from the equilibrium region

The kinetics and mechanisms of the dehydration reactions of zoisite have been studied at 635°-792° C, 1-2 kbar. The equilibrium reaction does not occur and is replaced by metastable reactions involving the formation of gehlenite and a calcium tri-octahedral mica, instead of corundum: zoisite → anorthite+grossular+gehlenite +calcium 3T mica+H₂O. The experimental data can be interpreted by zero-order equations dX_An/dt=k (X_An=fractional extent of reaction, t=time, k=zero-order rate constant). These relations hold for variations in P, T, A_initial(the initial surface area of zoisite) and also in the presence of seed crystals, which enhance the reaction rate. No induction period is evident and only at advanced stages of reaction are sharp decreases observed in the rate, which are attributed to physical affects (shrinking particles, armouring). SEM studies show that dissolution of zoisite is anisotropic, occurring preferentially parallel to the crystallographic b-axis, with the result that a characteristic 'sawtooth' etch structure develops. Garnet grows as euhedral crystals located in cavities or on 'teeth' of dissolving zoisite, whereas anorthite forms as clusters of coalescing grains which spread over and enclose the zoisite. In seeded runs, garnet growth initiates on both seeds and on zoisite surfaces whereas anorthite growth is more closely tied to the seeds, resulting in the development of clusters of smaller grains. The experimental evidence favours dissolution and nucleation-controlled growth as rate-determining processes. The preservation of zero-order kinetics in the face of shrinking particles is attributed to the anisotropic dissolution mechanism, which effectively preserves a constant reaction interface. The rate effects of nucleation appear to accord with the classic model in which growth of crystal layers is initiated by the formation of coherent nuclei. The temperature dependence of rate constants reflects both thermally activated Arrhenius-type behaviour and the rate-depressing influences of approach to equilibrium. Similarly pressure affects on reaction rate can be interpreted in terms of competition between rate enhancement due to pressure increase and rate-depression accompanying the approach to equilibrium. Although the equilibrium-approach effects accord with current treatments of reaction kinetics, a problem exists in deriving an exact relation coupling dissolution and nucleation rate control. Consequently an overall-reaction rate equation, such as that of Fisher and Lasaga (1981), is only partially successful in interpreting the temperature dependence of rates. The data suggest that the surface reaction equation of Wood and Walther (1983) only applies when nuclei are present.