The transformation of mica‐like layer silicates to expansible three‐layer day mineralsFirst of all a preliminary survey is given of those authors who have so far carried out investigations on certain minerals about transformations of layer‐silicates to expansible three‐layer clay minerals.It can be deduced from these investigations that the most important presupposition for a transformation, is the setting free of potassium and the reduction of the layer‐charge.The knowledge of the presuppositions of a transformation was achieved mainly in model tests (deposit‐clays), which also explain the running off of the exchanges of the intermediate layers. To demonstrate transformation processes with natural clay fractions of soils, requires an extensive clay fractionizing which is accomplished by means of centrifuges.Personal investigations with fractionized clays of a Chernozem have shown, that in soils, the transformation of illite into expansible three‐layer‐minerals occurs at the same time as the decrease in size of grain. Although the illites which are in the process of transformation reach an expansion up to 18 A from about 0,2 μ granular size, a qualitative difference exists (layer‐charge, contraction tendency) to the genuine montmorillonites, that only becomes less at the finest fractions>0,01 μ.The investigations with other soils (‘gray brown‐soils’,‘acidic brown soil’) however, reveal that there is no suitable transformation scheme for all soils. Depending on the milieu in which the soil development has taken place, characteristic transformation sequences result. Because of the hitherto existing results three development‐models were introduced and discussed, each of whic