Rates and nonstoichiometry of vermiculite dissolution at 22°C

The dissolution kinetics of a trioctahedral vermiculite was studied in hydrochloric acid solution within the pH range 2 to 6.5 at 22°C (room temperature). Powdered samples dominated by particles of about 5μm size were reacted in dialysis reactors operating as open systems. High initial cation release rates reflect both rapid selective leaching of the exchangeable cations and fast dissolution of 2:1 layers. Rates decrease successively during approximately 1000h, whereafter the influence of time wanes in all experiments. Initially, output pH increases relative to input pH in all experiments, but becomes as a rule, equal to input pH after a couple of days. This is most likely due to protonation of the exchange sites, which balances charge losses due to leaching of interlayer Mg and K. At low pH the duration of this pH-elevation is much shorter than at pH > 3. All interlayer Mg in the pH 2 and 3 runs is completely lost within 500h. Thereafter the ratio of dissolved Mg/Si stabilises at a value slightly below the stoichiometric ratio of the bulk mineral. At pH 2 the 2:1 layers dissolve close to stoichiometrically, whereas lower than stoichiometric Al/Si release ratios occur at pH ≥ 3. Examination of the reacted material by XRD shows that these lower than stoichiometric ratios result from formation of Al-hydroxy interlayers. Formation of octahedrally coordinated Al in the experiments is confirmed by 27Al MAS NMR, since only four-coordinated Al is present in the raw vermiculite. The unreacted vermiculite contains phlogopite and interstratifications of 10 and 14Å layers. In the experiment with highest rates at pH 2, a regularly interstratified hydromica appears to be the most weathering resistant phase. At times when 10% of the respective sample has been dissolved, the logarithms of the rates are proportional to − 0.44 pH. It is irrelevant to compare rates measured at different pHs on an equal reaction time basis, as the changes of the sample composition are most rapid at low pH. The dissolution rates for the different pHs vary with time due to different aging effects. Comparable effects are also observed with a parallel phlogopite experiment.