Identification of naturally occurring non-stoichiometric titanomagnetites

Abstract

It has recently been shown that the samples on which the contours of equal magnetic properties for cation deficient spinels in the FeO  Fe 2O 3  TiO 2 ternary system are based are, in fact, two phase intergrowths. The contours cannot therefore be used as a means of identifying naturally occurring cation deficient or non-stoichiometric titanomagnetites. It has also been shown that the oxidation of titanomagnetites to cation deficient spinels is only possible under conditions not usually found in nature and the question arises as to whether such materials do occur naturally. In the present paper the usefulness of the possible methods of identification of such materials is discussed. These techniques include X-ray and electron probe analysis, calculation of the effect of diamagnetic impurities on Curie point, the effect of sustained heat treatment at moderate temperatures (∼ 350 °C) coupled with ore microscope observation, the application of hydrostatic pressure, and characteristics of the thermomagnetic curve. It appears that the most powerful single method for detecting non-stoichiometry is the carefully performed thermomagnetic curve and measurements by Creer (1969) on a variety of samples seem to confirm this. The present paper describes the application of many of the above techniques to two basalts containing homogeneous titanomagnetites. This detailed study indicates that the homogeneous titanomagnetite contained in one of the samples is highly non-stoichiometric. The implications of this result for palaeomagnetic and palaeointensity studies are discussed.