An Occurrence of a Fully-Oxidized Natural Titanomaghemite in Basalt

Jacqueline E. M. Allan, J. M. D. Coey, I. S. Sanders, U. Schwertmann, G. Friedrich and A. Wiechowski
Physics Department, Trinity College, Dublin 2, Ireland
Institut für Bodenkunde, Freising-Weihenstephan, West Germany
Institut für Mineralogie und Lagerstättenlehre, 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 Mössbauer spectroscopy may be expressed as: [Fe0.77Ti0.22Zn0.01]{Fe1.19Ti0.26Mn0.02Al0.040.49}O4 where [] and {} denote ions on tetrahedral and octahedral sites, respectively. The spontaneous magnetization of the mineral is 36(3) J/T/kg.

Keywords: titanomaghemite • ferric iron • gneiss • Minas Gerais • Brazil

Mineralogical Magazine; June 1989 v. 53; no. 371; p. 299-304; DOI: 10.1180/minmag.1989.053.371.04
© 1989, The Mineralogical Society
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