Precipitation and dissolution rate constants for cristobalite from 150 to 300°C

Details Precipitation and dissolution rate constants for cristobalite from 150 to 300°C Precipitation and dissolution rate constants for cristobalite from 150 to 300°C

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59...77r&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 59, Issue 1, pp.77-85

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Scientific paper

Experiments to measure the rates of dissolution and precipitation for cristobalite (xtb), were carried out from 150-300°C. These experiments consisted of monitoring the isothermal approach to chemical equilibrium of aqueous solutions and synthetic cristobalite in a closed-system. The dissolution and precipitation rate constants for cristobalite, in water, are given by (In) k + = -0.9 - E ( a + / R · T (K)) (In) k - = -0.16 - E ( a - / R ·(K)) . The activation energy of dissolution, E a+ , is 68.9 ± 11 kJ·mol -1 . The activation energy of precipitation, E a- (52.9 ± 10 kJ·mol -1 ), is in agreement, within the given uncertainty, with published activation energies for other silica polymorphs. The same 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 concentrations were approximated at 150°C from initially supersaturated solutions and at 200°C from both supersaturated and undersaturated solutions. From the resulting steady-state conditions, equilibrium constants were derived for the above reaction, pk 150 = 2.22 and pK 200 = 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 cristobalite saturation, and is less than that of amorphous silica saturation and if there are cristobalite nuclei present.

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