Experimental investigation of the kinetics of the reaction 1 tremolite+11 dolomite?8 forsterite+13 calcite+9 CO2+1 H2O

Abstract

The kinetics of the reaction of tremolite plus dolomite to give forsterite, calcite, CO2, and H2O were investigated by powder experiments at 500 MPa. In the first group of experiments, the mole fraction of CO2( $$X_{\operatorname{CO} _2 } $$ ) of the carbon dioxide-H2O mixture (present in all runs) lay in the range of 0.60–0.69. At this $$X_{\operatorname{CO} _2 } $$ -value, three series of experiments were performed at temperatures of 750°, 730°, and 720° C. The equilibrium temperature at 500 MPa and $$X_{\operatorname{CO} _2 } $$ is 690°±10° C. Throughout the second group of experiments, the $$X_{\operatorname{CO} _2 } $$ was 0.10–0.18 and the run temperatures of 3 series of experiments were 670°, 660°, and 650° C. The equilibrium temperature at 500 MPa and $$X_{\operatorname{CO} _2 } $$ is 620°±10° C. For each of the 6 run temperatures, conversion/time curves were determined. The group of runs performed in the presence of the CO2-rich fluid with a maximum conversion at about 55% consistently revealed smaller rates of reaction than the runs with the H2O-rich fluid. By contrast, total conversion was obtained in the H2O-rich experiments. The different maximum conversions observed in the two groups of experiments can be explained by the mechanisms of forsterite and calcite growth, which are a function of the CO2 concentration of the fluid. The time dependence of the experimentally determined reaction conversion U can be described by the simple equation dU/dt=k if one additionally takes into account the decrease of the dolomite surface and the approach of equilibrium conditions during the course of the experiments. Assuming that the reaction takes place far from equilibrium, the rate constants k of the overall reaction found for the conditions of the run experiments are given by the following two equations: $$\operatorname{l} \operatorname{n} k_1 = \frac{{610897(J)}}{{RT}} + 63.780{\text{ }}[X_{\operatorname{CO} _2 } = 0.60 - 0.69]$$ $$\operatorname{l} \operatorname{n} k_2 = \frac{{582824(J)}}{{RT}} + 67.687{\text{ }}[X_{\operatorname{CO} _2 } = 0.10 - 0.18]$$ The apparent activation energies are 611±150kJ ( $$X_{\operatorname{CO} _2 } $$ =0.6) and 583±170 kJ ( $$X_{\operatorname{CO} _2 } $$ =0.1). Allowing a maximum influence on kinetics due to approach to equilibrium, which is strongly dependent of the formulation of the mass-dependent ΔG of the reaction, the apparent activation energies are reduced to minimum values of 429 kJ ( $$X_{\operatorname{CO} _2 } $$ =0.6) and 409 kJ ( $$X_{\operatorname{CO} _2 } $$ =0.1). The rate data determined were applied to contact metamorphism by combining the experimental rate data with the heating rate of 1° C in 50 years. Assuming that at $$X_{\operatorname{CO} _2 } $$ =0.1 for nucleation of forsterite and calcite, the equilibrium temperature has to be overstepped by 10° C, the reaction is completed after 30 years under unfavourable conditions (dolomite grains = 10 mm and n=0.1), and the total overstepping of the equilibrium temperature during the conversion of 100% is 11° C.