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An occurrence of a fully-oxidized natural titanomaghemite in basalt

Published online by Cambridge University Press:  05 July 2018

Jaoqueline E. M. Allan ,
J. M. D. Coey ,
I. S. Sanders ,
U. Schwertmann ,
G. Friedrich  and
A. Wiechowski
Jaoqueline E. M. Allan
Affiliation:
Physics Department, Trinity College, Dublin 2, Ireland
J. M. D. Coey
Affiliation:
Physics Department, Trinity College, Dublin 2, Ireland
I. S. Sanders
Affiliation:
Physics Department, Trinity College, Dublin 2, Ireland
U. Schwertmann
Affiliation:
Institut für Bodenkunde, Freising-Weihenstephan, West Germany
G. Friedrich
Affiliation:
Institut für Mineralogie und Lagerstättenlehre, Aachen, West Germany
A. Wiechowski
Affiliation:
Institut für Mineralogie und Lagerstättenlehre, Aachen, West Germany
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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:

where [] and {} denote ions on tetrahedral and octahedral sites, respectively. The spontaneous magnetization of the mineral is 36(3) J/T/kg.

Keywords

titanomaghemiteferric irongneissMinas GeraisBrazil
Type
Non-silicate Mineralogy
Information
Mineralogical Magazine , Volume 53 , Issue 371 , June 1989 , pp. 299 - 304
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1989

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