Iron Minerals in Particle-Size Fractions of Forest-Zone Soils of the Russian Plain

摘要

The content and composition of iron minerals have been studied in particvle-sizer fractions separated from a series of forest soils of the Russian Plain, namely, soddy-podzolic soil on Permian red-earth deposits, soddy calcareous soil on moraine, and brown forest soil on calcareous moraine. It is shown that hematite and goethite tend to concentrate in the clay (<0.001 mm) fraction of all ghe soils. Ferroxyhyte is associated with montmorillonite and, together with agggregates of the latter, is fractionated between the sand-silt and clay fractions of the soddy-podzolic soil. The fraction of fine seand and silt (0.25-0.001 mm) contains spherules of pedogenic magnetite Fe_3O_4. In contrast to lithogenic magnetite, these spherules can be completely dissolved in a dithionite-citrate-bicarbonate solution. Particle-size fractionation of a sample from the plow horizon of brown forest soil leads to the transformation finitial iron-bound minerals-goethite and ferrihydrite-into lepidocrocite. This process can be due to a drop in the Eh of soil suspension in the course of soil pretreatment and dispersion (according to the Gorbunov procedure) and the formation of Fe(II). The dissolution of hematite and the substitution of goethite for magnetite take place in the plow horizon of soddy calcarous soil. The dissolution of hematite inherited from parent rock and the neosynthesis of ferroxyhyte are registered in the plow horizon of soddy-podzolic soil. Ferrihydrite has been detected in the plow horizon of brown forest soil.