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 hydroxidoge
nesis. It is found that iron hydroxidogenesis develops in soddy-podzolic so
ils on Permian deposits, brown forest soils, and typical chernozem. This pr
ocess is not typical of the soddy weakly podzolic soil formed on the local
moraine. Iron hydroxides in soils on red-colored deposits are represented b
y acicular goethite, collomorphic hydrogoethite, ferroxyhyte, and protoferr
ihydrite. 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 f
ormed under automorphic conditions, and the chemogenic synthesis of hydroxi
des resulting in the formation of thermodynamically stable goethite (hydrog
oethite) takes place under semihydromorhic conditions. Ferroxyhyte (especia
lly Mn-ferroxyhyte) is more resistant to reductive solution and is, therefo
re, more common in forest zone soils than ferrihydrite.