The kinetics of dissolution of smectite from the Cabo de Gata volcanic deposit was investigated in the present study. Assuming that the sample is composed solely of smectite, the structural formula of the treated smectite was calculated to be K0.19Na0.51Ca0.195Mg0.08(Al2.56Fe0.42Mg1.02)(Si7. 77Al0.23)O20(OH)4 Two types of experiments were carried out: batch experiments to obtain equilibrium data and stirred-flow-through experiments to measure the smectite dissolution rate. All experiments were carried out at a temperature of 80°C and pH of 8.8. After more than 2 yr smectite was still dissolving in the batch experiments, but at a very slow rate. The slow dissolution rate indicates that the system is reasonably close to equilibrium with respect to smectite dissolution. Therefore, the average ion activity product (5 ± 4 x 10-53), obtained from the last samples of the batch experiments, is used as a proxy for the equilibrium constant of the smectite dissolution reaction at 80°C given as Smectite + 20H2O → 0.51Na+ + 0.19K+ + 0.195Ca2+ + 1.1Mg2+ + 0.42Fe(OH)4- + 2.79Al(OH)4- + 7.77H4SiO4 + 0.08OH- In the flow-through experiments at steady state, the average Al/Si (0.33 ± 0.03) and Mg/Si (0.15 ± 0.03) ratios were in very good agreement with these molar ratios of the whole rock analysis (0.35 and 0.14, respectively). The major achievements and conclusions of the present study are as follows: For the first time we present a full stoichiometric dissolution of smectite (i.e., stoichiometric dissolution was observed for Al, Si, and Mg), and show that the obtained dissolution rate is a good measure of the smectite dissolution rate. Pretreatment of the smectite surfaces is necessary to obtain reliable and stoichiometric kinetic results. The dissolution rate of the sample reflects the dissolution rate of the montmorillonitic layers. Under the experimental conditions smectite dissolution rate is not inhibited by aluminum. The dissolution rate of smectite decreases as a function of the silicon concentration. This observation may be explained both by the effect of deviation from equilibrium on dissolution rate and by silicon inhibition, expressed as respectively. The current data set cannot be used to differentiate between these two possible reaction mechanisms. Copyright (C) 2000 Elsevier Science Ltd.
ASJC Scopus subject areas
- Geochemistry and Petrology