Structural-mineralogical aspects of soil formation and lithogenesis in humid areas of the boreal and tropical climates

摘要

The concepts of textural differentiation in podzolic soils developing in humid areas of the boreal and tropical (and subtropical) climates are discussed. The composition of clay minerals in the podzolic horizons of these soils is analyzed. The main attention is paid to the minerals of the kaolinitic group, as they are the minerals that predominate in the clay fraction of the podzolic horizons. It is suggested that three different associations of kaolinitic minerals can be distinguished. The first association includes newly formed kaolinitic minerals; they are synthesized in soils of geosyncline zones under conditions of a humid tropical and subtropical climate and are represented by halloysite (metahalloysite) and kaolinite-smectite. The second association can be referred to as the group of inherited kaolinitic minerals. They are represented by various crystallochemical forms or by a mixture of different forms. This mineral association is typical of the vast continental platforms and is considered as the association of minerals inherited from the previous stage of soil formation and sedimentation under the conditions of a humid tropical or subtropical climate. The third association is represented by kaolinitic minerals of perfect crystallographic form; these minerals are involved into the zone of pedogenesis from the soil-forming rocks. In turn, they entered the composition of the soil-forming rocks from the ancient (Paleozoic and Precambrian) rocks that formed under conditions of humid tropical weathering. They are typical of the boreal plains. The enrichment of the upper part of the profile of podzolic soils in the minerals of the kaolinitic group leads to considerable changes in the physicochemical soil properties and, thus, affects the direction of soil formation.