Iron minerals in soils on red-colored deposits

摘要

The presence of hematite in humus horizons of soils on red-colored deposits in the European part of Russia is related to the stability of this mineral and points to a low contribution of hematite iron to pedogenic hydroxidogenesis. It is found that iron hydroxidogenesis develops in soddy-podzolic soils on Permian deposits, brown forest soils, and typical chernozem. This process is not typical of the soddy weakly podzolic soil formed on the local moraine. Iron hydroxides in soils on red-colored deposits are represented by acicular goethite, collomorphic hydrogoethite, ferroxyhyte, and protoferrihydrite. In the soddy-podzolic soil, the thermodynamically stable goethite appears to be of chemogenic origin, other iron hydroxides being biogenic. In brown forest soils, the thermodynamically unstable iron hydroxides are formed under automorphic conditions, and the chemogenic synthesis of hydroxides resulting in the formation of thermodynamically stable goethite (hydrogoethite) takes place under semihydromorhic conditions. Ferroxyhyte (especially Mn-ferroxyhyte) is more resistant to reductive solution and is, therefore, more common in forest zone soils than ferrihydrite.