Maghemite in soils

Curry, N.A. Johnston, G.B. Besser, P.J. Mor-rish, A.H. (1965) Neutron diffraction measurements on pure and doped synthetic hematite crystals. Phil. Mag. 12 221-228 Da Costa, A.C.S. Bigham, J.M. Rhoton, F.E. Traina, S.J. (1999) Quantification and characterization of maghemite in soils derived from volcanic rocks in Southern Brazil. Clays Clay Min. 47 466-473... 

Schwertmann, U. Eechter, H. (1984) The influence of aluminum on iron oxides. XI. Aluminum-substituted maghemite in soils and its formation. Soil Sci. Soc. Am. J. 48 1462-1463... 

Taylor, R.M. Schwertmann, U. (1974a) Maghemite in soils and its origin. 1. Properties and observations on soil maghemites. Clay Min. 10 289-298... 

Maghemite, y-Fe203, is a red-brown, ferrimagnetic mineral isostructural with magnetite, but with cation deficient sites. It occurs in soils as a weathering product of... 

Barron, V. Torrent, J.(2002) Evidence for a simple pathway to maghemite in Earth and Mars soils. Geochim. Cosmochim. Acta 66 2801-2806... 

Lepidocrocite is much less common than hematite and goethite, but it is not rare. Iron coatings around rice roots, formed of goethite and lepidocrocite, have been identified. Maghemite, magnetite, schwertmannite, and akaganeite are other Fe-oxides present in soil environments, which form under specific conditions (Cornell and Schwertmann, 1996). 

Solubility. The processes of dissolution and precipitation of Fe and Mn oxides control the mobility of Fe and Mn in soils and sediments and their availability to organisms. Dissolution of Fe and Mn oxides is a function of pH. Fe oxides decrease in solubility in the order Ferrihydrite (amorp) > maghemite > lepidocrocite > hematite > goethite. The solubility of Mn oxides decrease in the order pyrochroite > hausmannite > bixbyite > manganite > bimessite > nsutite > pyrolusite. 

Al substitution is commonly found in soil-related maghemite. The spectra are somewhat similar to that of Al-free maghemite, but the sextet is often accompanied by a doublet (Fig. 3.16), the contribution of which decreases at lower temperatures. This typical superparamagnetic behavior is a consequence of a combined effect of particle size and diamagnetic substitution [125, 126]. 

R.E. Vandenberghe, J.J. Hus, E. De Grave, Evidence from Mossbauer spectroscopy of neoformation of magnetite/maghemite in the soils of loess/palaeosol sequences in China. Hyp. Interact. 117, 359-369 (1998)... 

Da Motta Kampf, 1992), but no correlation has been found in others (Anand Gilkes, 1987 Zeese et ah, 1994). The ratio may depend on whether or not the two oxides were formed simultaneously in the same environment. Maghemites from tropical soils contained A1 up to an Al/(Fe -i- Al) ratio of ca. 0.15 as indicated by chemical analysis and reduction in unit cell size (Schwertmann Fechter, 1984 Fontes Weed, 1991). 

It is unlikely that the enhanced susceptibility of the Bone Chamber samples could reflect a source as a burnt soil. Although such burnt soils have been shown to have susceptibilities ranging as high as 9000 x 10 m kg (Dearing, 1999), they frequently contain maghemite (see Thompson and Oldfield, 1986, for a discussion), and would be expected to show other indications of burning, such as fragments of charcoal or baked clays. These were not observed in either the Bone Chamber or Reed Chamber samples. 

Maghemite y-Fe O, 38.8 to >40.4 Same conditions as lepidocrocite, but formed in tropical and subtropical climates, especially in highly weathered soils. 

The corrosion scales on the archaeological artefacts have been shown to consist primarily of siderite (FeC03) visible as large yellow-brownish crystals on the surface of some of the artefacts. Analysis of cross-sections of a few artefacts also showed that the inner corrosion layers consisted of siderite with some magnetite or maghemite. Importantly, only very low contents of sulfur were found, indicating that corrosion by sulfate-reducing bacteria is not a problem at this site. Siderite was also found on modern iron samples that had been exposed for a few years in the soil at Nydam. 

---