PRECIPITATION AND DISSOLUTION RATE CONSTANTS FOR CRISTOBALITE FROM 150-DEGREES-C TO 300-DEGREES-C

Experiments to measure the rates of dissolution and precipitation for cristobalite (xtb), SiO2(xtb) + 2H2O(1) k+ reversible k- Si(OH)4(aq), were carried out from 150-300-degrees-C. These experiments consisted o f monitoring the isothermal approach to chemical equilibrium of aqueou s solutions and synthetic cristobalite in a closed-system. The dissolu tion and precipitation rate constants for cristobalite, in water, are given by ln k+ = -0.9 - DELTAE(a+)/(R.T(K)) ln k- = -0.16 - DELTAE(a-) /(R.T(K)). The activation energy of dissolution, DELTAE(a+), is 68.9 /- 11 kJ . mol-1. The activation energy of precipitation, DELTAE(a-)(5 2.9 +/- 10 kJ . mol-1), is in agreement, within the given uncertainty, with published activation energies for other silica polymorphs. The s ame activation energy of precipitation for different silica polymorphs is predicted by Transition State Theory (TST). Using experiments that were allowed to most closely approach equilibrium, steady-state conce ntrations were approximated at 150-degrees-C from initially supersatur ated solutions and at 200-degrees-C from both supersaturated and under saturated solutions. From the resulting steady-state conditions, equil ibrium constants were derived for the above reaction, pK150 = 2.22 and pK200 = 2.10. These values are in close agreement with published data . The results show that cristobalite may precipitate from hydrothermal solutions if the concentration of Si(OH)4 exceeds that at cristobalit e saturation, and is less than that of amorphous silica saturation and if there are cristobalite nuclei present.