Occurrence of microcrystalline goethite in an unusual fibrous form

Abstract

Small fragments of intermeshed fibrous material of high electron density were observed by electron microscopy in the fine (< 0.5 μm) clay taken from an iron-rich cemented layer forming part of the B horizon of a Podzol (Typic Haplorthod). Each fibre was about 5 nm in width and about 100 nm in length. Similar material was also common in larger (0.5–2.0 μm) clay fractions but here it occurred as unbroken particles which resembled bacteria in shape and size. At high magnifications, individual fibres were seen to consist of very small (5 nm) particles aligned along the length of each fibre. Electron microanalysis of the fibrous material showed it to consist of iron, aluminium and silicon. The average composition (water-free basis) of seventeen bacteria-like particles was 66% Fe 2O 3, 22% Al 2O 3 and 11% SiO 2. The particles were positively identified as aluminium-substituted goethite by selected-area electron diffraction. The fibrous material was completely dissolved by dithionite-citrate-bicarbonate treatment but was only slightly affected, if at all, by ammonium oxalate, pH3, in darkness. Other extraction data and X-ray diffraction results showed that the clay also contained considerable amounts of material of short range order, probably in the form of ferrihydrite and allophane.