Soil lepidocrocite

Schwertmann, U. Taylor, R.M. (1973) The in vitro transformation of soil lepidocrocite to goethite. Pseudogley Gley, Trans. Comm. V VI Int. Soc. Soil Sd. Stuttgart-Hohenheim 1971 45-54... 

The orange coloured lepidocrocite, y-FeOOH, is named after its platy crystal shape (lepidos scale) and its orange colour (krokus = saffron). It occurs in rocks, soils, biota and rust and is often an oxidation product of Fe ". It has the boehmite (y-AlOOH) structure which is based on cubic close packing (ccp) of anions. 

The solubility plots for lepidocrocite, ferrihydrite and hematite (Fig. 9.2) and for goethite, ferrihydrite and soil-Fe (Fig. 9.3) show only the total Fe activity. They were obtained in the same way as that for goethite using the appropriate constants from Tables 9.1, 9.2 and 9.4. 

As lepidocrocite is metastable relative to goethite, it can be expected that lepidocrocite may transform into goethite. As demonstrated in the laboratory, this transformation proceeds via solution (see Chap. 14). Electron micrographs from a redoxi-morphic soil in Australia indicate that the same process seems to occur in soils (Fig. 16.5). The lepidocrocite crystals show dissolution features and there are small, acicu-lar, goethite crystals in their neighbourhood. Feroxyhyte was reported in two allopha-... 

Fig. 16.4 I nverse relationship between the relative lepidocrocite and goethite content in soils of Kalimantan (Ohta et al., 1993 with permission).

Fig. 16.5 Electron micrographs of an association of lepidocrocite (Lp) with goethite (Gt) from a redoximorphic soil, Natal, South Africa (courtesy P. Self).

Fig. 16.9 Electron micrographs of soil lepidocro-cite. a) Large multidomainic lath-like crystal viewed perpendicularto [001] with laminar pores from a re-doximorphic soil, Natal, South Africa, b) Poorly crystalline grassy lepidocrocite crystals mixed with tiny ferrihydrite particles and pseudo-hexagonal kaolinite platelets. Origin as before (a. b courtesy P. Self), c) Small lepidocrocite crystal from a hydromorphic soil (with ferrihydrite) viewed perpendicularto [001] and showing (020) lattice fringes (see also Schwert-mann. Taylor, 1989,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-... 

Fig. 16.10 Frequency distribution of the corrected width at half height (WHH) ofgoethites (101), hematites (110) and lepidocrocites (002) from soils and other surface environments. The range ofWHH at the abcissa corresponds to about 10-100 nm (Schwertmann, 1988b with permission).

The omnipresence of aluminium in weathering environments results in most of the Fe oxides in soils, except lepidocrocite, being Al-substituted. The possible range of substitution as deduced from synthesis experiments (see Chap. 3) viz. up to Al/ (Fe Al) of ca. 0.33 in goethite and up to Al/(Fe Al) of ca. 0.16 in hematite is also found in soil goethites and hematites. Where the two oxides coexist on a small scale... 

YR to 10 R as the hematite/goethite ratio increased from 0 to 1 (Fig. 16.11), whereas hues of soils containing only goethite ranged between 7.5 YR and 2.5 Y. Soils with lepidocrocite and ferrihydrite covered the in-between-range of 5 YR-... 

U. Childs, C.W (1985) Occurrence and properties of lepidocrocite in some soils of New Zealand, South Africa and Australia. 

Lauer Jr, H.V (1998) Lepidocrocite to maghemite to hematite A pathway to magnetic and hematitic Martian soil. Meteoritics Planetary Sci. 33 743-751... 

Schwertmann, U. Fitzpatrick, R.W. (1977) Occurrence of lepidocrocite and its association with goethite in Natal soils. Soil Sci. Soc. 

Wang, H.D. White, G.N. Turner, F.T. Dixon, J.B. (1993) Ferrihydrite, lepidocrocite, and goethite in coatings from East Texas vertic soils. Soil Sci. Soc. Am. J. 57 1381-1386 Wang, L. Chin,Y.-P. Taina, S.J. (1997) Adsorption of (poly) maleic acid and an aquatic fulvic acid by goethite. Geochim. Cosmochim. Acta 61 5313-5323... 

Zhang, X. Zhang, F. Mao, D. (1999) Effect of iron plaque outside roots on nutrient uptake by rice Oryza sativa L.) Phosphate uptake. Plant and Soil 209 187-192 Zhang,Y Charlet, L. Schindler, P.W. (1992) Adsorption of protons, Fe(II) and Al(IIl) on lepidocrocite (y-FeOOH). Colloids Surfaces 63 259-268... 

Plate 16.1 a) Soil profile coloured by goethite (Ochrept, France), b) Soil profile coloured by hematite (Ultisol, Brazil), c) Soil profile coloured by lepidocrocite (Aquept, South Africa), d) Ferrihydrite formation by oxidation of Fe " in water seeping out of a Cley. 

In water logged soils radial oxygen loss from the root raises the redox potential in the rhizosphere as a consequence of which iron oxide plaques are seen to develop on root surfaces. Bacha and Hossner (1977) removed the coatings on rice roots growing under anaerobic conditions. The coatings were composed primarily of the iron oxide mineral lepidocrocite (y-FeOOH) as the only crystalline component. St-Cyr and Crowder (1990) studied the iron oxide plaque on roots of Phragmites and detected both Fe and Mn. The Fe Mn ratio of the plaque resembled the ratio of Fe Mn in substrate carbonates. The plaque material also contained Cu. 

Madrid, L. and Diaz-Barrientos, E., Description of Titration curves of mixed materials with variable and permanent surface charge by a mathematical model. 1. Theory. 2. Application to mixtures of lepidocrocite and montmorillonite, J. Soil Sci., 39, 215, 1988. 

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