THE DISSOLUTION OF BIOTITE AND CHLORITE AT 25-DEGREES-C IN THE NEAR-NEUTRAL PH REGION

We studied the dissolution of biotite and chlorite in laboratory syste ms with flow-through and batch reactors. The initial dissolution of bi otite in the near-neutral pH region, under N-2(g) atmosphere is highly non-stoichiometric. A slow linear release of iron during a period of weeks indicates a surface-chemical-reaction-controlled mechanism of re lease for iron. The release of potassium is much faster than that of i ron. A parabolic dependence of accumulated release with time suggests a diffusion-controlled mechanism of potassium release. The rates of ma gnesium, aluminium and silicon release are between those for potassium and iron and approach that of iron with time. There is no significant influence of (bi)carbonate or pH on biotite dissolution rate or stoic hiometry in the pH region 7 < pH < 8.5. The release rates of elements from chlorite are close to stoichiometric and similar to the iron rele ase rate from biotite. In closed batch reactors at near-basic pH the c omposition of test solutions in contact with biotite is apparently con trolled by gibbsite (Al), kaolinite (Si) and Fe(III)-(hydr)oxide. We e stimated a turn-over time (10(1)-10(2) yr) for molecular oxygen and a time scale (10 months) to develop characteristic Fe(II) concentrations for a granitic groundwater.