Diagenetic iron oxyhydroxides formed by the oxidation of iron(II) were
allowed to deposit onto inert collectors vertically inserted into the
sediments of eight lakes chosen to represent a range of lake pH value
s. Particles comprising the iron-rich deposits have been characterized
at macroscopic (composition; X-ray diffraction) and microscopic (tran
smission electron microscopy; energy dispersive spectroscopy; electron
diffraction) levels. The only crystalline forms of Fe(III) oxyhydroxi
des identified were poorly ordered ferrihydrite and lepidocrocite. Mor
phologies of lepidocrocite (lath-type) and presumably of ferrihydrite
(spherical and ellipsoidal particles) are similar to those reported pr
eviously as being formed in the water column of seasonally anoxic lake
s. Deposition of iron oxyhydroxides in the lake sediments also appears
to occur on bacterial cells and on their exopolymers. Silicon, sulfat
e. chloride, phosphate, manganese, calcium, and aluminum represented m
inor components of the diagenetic Fe(III) oxyhydroxides, whereas organ
ic carbon was a more abundant component. Calculations with a diffuse l
ayer version of the surface complexation model using the ambient disso
lved concentrations, and comparison of the predicted and measured comp
ositions of the iron-rich particles suggest that sulfate, phosphate, a
nd probably calcium were sorbed on the iron oxyhydroxides, whereas sil
icon was probably more firmly bound at the surface.