An occurrence of a fully-oxidized natural titanomaghemite in basalt JAOQUELINE E. M. ALLAN, J. M. D. COEY AND I. S. SANDERS Physics Department, Trinity College, Dublin 2, Ireland U. SCHWERTMANN Institut fiir Bodenkunde, Freising-Weihenstephan, West Germany G. FRIEDRICH AND A. WIECHOWSKI Institut for Mineralogie und Lagerst~ittenlehre, Aachen, West Germany Abstract Titanomaghemite occurs in a relatively fresh doleritic intrusion in an area of Precambrian gneiss in Minas Gerais, Brazil. It hosts exsolution lamellae of ilmenite and contains more than 90% of the iron in the ferric form. It is more resistant to weathering than the ilmenite and is inherited virtually unaltered by the resulting soils. Titanomaghemite, extracted as grains from a weathered rind of the rock, has lattice parameter a0 = 0.8348(3)nm and has a canted spin structure due to substitution of non-magnetic ions on tetrahedral and octahedral sites of the spinel structure. The average canting angle is 32 + 3 ~ and canting occurs predominantly on the octahedral iron sublattice. Its formula, based on microprobe analysis and M6ssbauer spectroscopy may be expressed as: [Feo.77Tio.22Zno.ol ] ( Fe t .19Tio.26Mno.02Alo.04[]0.49 } O4 where [] and (} denote ions on tetrahedral and octahedral sites, respectively. The spontaneous magnetization of the mineral is 36(3) J/T/kg. K EY W ORD S: titanomaghemite, ferric iron, gneiss, Minas Gerais, Brazil. Introduction MAGHEMITES are cation-deficient iron oxides with a structure related to that of spinel, which contain little or no ferrous iron. The ideal for- mula, y-Fe203 may be written [Fe 3+](ve~ 5 m~/~)o~ where [] and { } denote, respectively, tetrahedral (A) and octahedral (B) sites of the spinel struc- ture, and [] represents a vacancy. There is a solid- solution series between magnetite, Fe304, and maghemite, which converts to hematite, a-Fe203, on heating to about 300~ Both cation (e.g. AI,Ti) and anion (OH) substitutions are common in the structure, and a range of lattice parameters (0.830 ~<a0 ~<0.835 nm) is found in the literature (Lindsley, 1976). The term titanomaghemite refers to cation-deficient spinels in the Fe3+-rich field defined by Fe304-Fe2TiO4-FeTiO3-y- Fe203. Mineralogical Magazine, June 1989, Vol. 53, pp. 299-304 Copyright the Mineralogical Society Titanomaghemite occurs extensively in sea- floor basalts, where it usually forms by gradual undersea alteration of titanomagnetite produced at the mid-ocean ridge, with a time-constant of about 106 years (Johnson and Atwater, 1977). Partial oxidation of titanomagnetite may also take place at high temperatures and be retained as the basalt is quenched (Hauptmann, 1974). Occur- rences are also reported in seamounts (Kono et al., 1980) and subaerial volcanic rocks (Akimoto and Kushiro, 1960). Maghemite in soils can be formed from goethite by burning in the presence of biomass. Such goethite is frequently aluminous, and aluminium will then be incorporated into the structure of maghemite (Schwertmann and Fechter, 1984; Fitzpatrick, 1988). There is also some evidence of the possible formation of soil maghemite by a solution-precipitation process (Kopp and Lee, 1987). Highly magnetic soils, often reported to