Surface area, crystal morphology and size

Because Fe oxides are intimately associated with other soil components, it is not easy to determine the specific surface area of soil Fe oxides. An approximation can be obtained by attributing the surface area difference from before and after selective re- 

The crystals of soil Fe oxides are usually less well developed than those of synthetic ones. Goethite crystals from soils are, like synthetic ones, acicular (Fig. 16.7a, e) and show defects, micropores and serrated edges. Stars composed of spindles (Fig. 16.7 b) 

d) Fibrous C7stals from a podzol, Scotland (Nakai SLYoshinaga, 1980 with permission). e) Small acicular c stals from a redoximorphic soil, Natal, South Africa (courtesy P. Self), f) Equidimensional crystals from an Oxisol, Brazil (Fontes et al., 1992 courtesy M.R. Fontes with permission). 

The crystal size of soil Fe oxides usually ranges from a few to several hundred nm. A survey of 256 goethites, 101 hematites and 72 lepidocrocites from soils around the world showed maxima in the mean coherent length (MCL) perpendicular to (101), of 15-20 nm for goethite and ca. 40 nm perpedicular to (110), for hematite (Fig. 16.10). These values have been deduced from XRD line broadening using the Scherrer for- 

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