Three series of experiments were conducted at room temperature in a reactor where the mineral suspensions were separated from the solution using dialysis membrane. In the first series, biotite samples were treated in HCl and H 2 SO 4 solutions between pH 4 and 1. In the second and third series, the samples were all treated with HCl solution at pH 3, the former in the presence of different concentration of KCl solutions and the latter in the presence of 200 mu M strong solution of KCl, AlCl 3 and BaCl 2 . The composition of the fluid was monitored using wet chemical techniques and the solid residue was investigated using Mossbauer spectroscopy and transmission electron microscopy. Weathering is accompanied by oxidation of structural Fe and the degree of oxidation depends primarily on the extent of exfoliation, which in turn depends on both pH of the solution and on K selectivity of the solid biotite phase. For example, the oxidation ratio of iron increases with decreasing pH and decreasing activity of KCl. For K-free electrolytes such as AlCl 3 and BaCl 2 , however, the oxidation ratio is constant for a given pH. Moreover, weathering is a discontinuous process. This results in a quasi-periodic texture as shown in the HRTEM images and SAED pattern of the weathered products. There is a certain "critical" concentration of Fe and K in solution over which further weathering is blocked.