A Two-Cycle Concept on the Genesis of Soil Clay Minerals

Abstract

The clay mineralogical composition of soils could not be regarded as products of a single genetic process, but be done as those of any composite processes. It may be reasonable to induce from reliable examples of the clay mineralogical identification of Japanese soils that such composite genetical processes are able to be resolved into two different cycles of processes, that is, the geological cycle and the bio-hydrological cycle.
The geological cycle means a series of skelton forming processes through the weathering, denudation, transportation, deposition and diagenesis during the geological time-scale. These processes develop evolutionally as shown in Fig. 2. The essential clay minerals and their elemental lattice structures in soils are nothing else but those which have been formed and/or inherited through this cycle of processes, in other words, under the influences of Dokuchaev's time factor of soil formation.
The bio-hydrological cycle means a series of modificatory processes to the skeletons formed through the preceding cycle. It mainly consists of such processes as the partial collapse, ion-exchange in a broad sense and the effect of organic matter. The ion-exchange phenomena include the release of fixed cation such as K+ in illite, hydration and dehydration, as well as the formation of complex with hydroxyl-Al, organic compounds, etc., at the interlayer spaces of the three layered lattice minerals. Consequently, these minerals are turned into each other, passing through the transitional mixed-layered phases. It should be stressed that such interchange phenomena of three layered lattice minerals could serve as the most useful indicators to the recent pedogenesis, on account of their high sensibility to the change of environment.