CLAY STABILITY IN CLAY-DOMINATED SOIL SYSTEMS

Seven samples from a chronosequence of soils developed in historically created polders on the Atlantic coast (Marais Poitevin, Vendee, Franc e) were investigated in order to illustrate the rate of mineralogical change in a clay-dominated system. The oldest polder was constructed i n 1665, the last one in 1912; thus the time span of soil evolution is from 80 to 330 years. All the samples had more than 50% clay (<2 mu m) . The most reactive, fine clay sub-fraction (<0.1 mu m) was investigat ed in detail by X-ray diffraction and chemical analysis. The observed mineralogical changes with increasing age followed the schematic react ion: smectite + mica = illite + mixed-layer minerals. The progress of reaction in time appears to be non-linear. This reaction seems to occu r in a chemically constant system, and the mineralogical change can be seen as a readjustment of species to a given chemical composition.