The influence of CaCl2 on the kinetics of the reaction 1 tremolite plus 3 calcite plus 2 quartz -> 5 diopside plus 3 CO2 plus 1 H2O. An experimental investigation

The effects of temperature and CaCl2 concentration on the kinetics of the m ineral reaction: 1 tremolite + 3 calcite + 2 quartz double right arrow 5 diopside + 3 CO2 1 H2O were studied experimentally in cold sealed pressure vessels. Results were o btained in 130 runs under conditions equivalent to medium pressure amphibol ite facies. Experiments were performed with and without CaCl2 dissolved in a supercritical CO2 - H2O mixture (CaCl2 = 0 to 0.5 mol/l water; XCO2 = 0.7 5 and 0.25). The pressure was 5 +/- 0.05 kb, and the temperatures ranged fr om 630 degrees to 720 degrees (+/- 3 degrees C), with six different tempera ture oversteps of 13 degrees up to 103 degrees C above the equilibrium temp erature of 617 degrees C, Percent conversion, measured by CO2 production as a function of time for a period of up to 34 days (816 hrs), is presented, and the appropriate reaction rate constants (up to k 5 x 10(-8) mol m(-2) s ec(-1)) as well as the apparent activation energies (E-app = 255 - 470 kJ/m ol) are extracted from the measured data. The observed asymmetric sigmoidal shape of all conversion versus time curve s indicates changes in the rate-limiting process during the reaction. The d evelopment of the overall kinetics as a result of the combination of the di fferent sub-processes (dissolution, nucleation, and growth of reactants and product) are discussed. Analysis of the experimental results shows that th e reaction rates obtained from a chloride-bearing fluid are an order of mag nitude larger than those obtained from chloride-free experiments. The impli cations for natural systems are quite important, because our results sugges t that reaction rates may indeed be much faster than originally expected. A dditionally, there seems to be evidence that nucleation starts earlier and is increased in the presence of a fluid that has even a low chloride conten t.