抄録
Micas and chlorites are often weathered to vermiculite and smectite minerals under surface and near-surface earth conditions. It has been seen by a number of observations that there is a certain weathering sequence of micas and chlorites toward smectite minerals which are ultimately hydrated 2:1 expandible layer silicate. This prompted us to undertake experimental alteration studies of micas and chlorites in order to gain a better understanding of factors controlling weathering processes.
Potassium was extracted from the 1-2μm fraction of 2 M1 muscovite by BaCl2 and from the 0.2-2μm fraction of 1 M phlogopite by CaCl2 to replace with hydrated interlayer cations. The release of K was initiated at certain preferential layers (perhaps structurally weak spots) and then followed from layers next to those where K had previsouly been released, so that there was a strong tendency to form a zonal structure. The experiments with coarser fractions of the phlogopite, however, showed a tendency to form a rather homogeneous mixed-layer structure at intermediate stage of hydration. Re-entry of K in the vermiculitized micas resulted in the return of original mica structures, 1M, 2M, 3T etc., suggesting that structural heredity exists.
Experimental alteration of chlorite to vermiculite required certain structural disruption in the interlayer hydroxide sheet of chlorite prior to its selective disso-lution. This can be achieved by dehydroxylation of the hydroxide sheet, or by oxidation in the case of Fe2+-bearing chlorites. A clinochlore sample was altered to vermiculite by treating a dehydroxylated sample with a weak acid in the presence of Na+. With the oxidation technique using saturated Br solution, a chamosite sample was converted into vermiculite whereas an Fe-clinochlore sample was altered into a high charge corrensite (1: 1 regular mixed layer of vermiculite and chlorite).
These experimental results were discussed in the light of observations on mineral weathering in nature and important problems desired to be solved in near future were presented.
