THE REACTION-KINETICS OF LAUMONTITE UNDER HYDROTHERMAL CONDITIONS

Quantification of the rate of mass transfer in diagenetic and hydrothe rmal systems requires a knowledge of the kinetics of mineral-fluid rea ctions at elevated temperature and pressure and at chemical conditions close to equilibrium. This communication describes an experimental st udy of the reaction kinetics of laumontite, a Ca-zeolite, at 80, 120 a nd 150-degrees-C, 50 MPa using batch (close to equilibrium) and flow ( far from equilibrium) experimental techniques in the-pH range 7-10. Fl uid and mineral buffers were employed to constrain chemical conditions during the experiments. The dissolution of laumontite is non-stoichio metric, with Ca being preferentially removed from the laumontite struc ture. SEM observations of reacted laumontite grains are consistent wit h a surface controlled reaction mechanism. Dissolution rates obtained by the batch and flow techniques showed good agreement. The dissolutio n rate of laumontite is independent of hydrogen ion activity in the fl uid phase at 80-degrees-C, but increases slightly with increasing hydr ogen ion activity at 120 and 150-degrees-C. Measured dissolution rates range from 3x10(-11) (80-degrees-C) to 1x10(-9) (150-degrees-C) molm- 2s-1 and are 10-50 times greater than that of heulandite at 25-degrees -C. The activation energy of dissolution is almost-equal-to 58 kJmol-1 .