Method to Determine the Degree of Non-stoichiometry of Iron-Titanium Oxides

Abstract

Summary Electronprobe microanalyses are used to determine the degree of nonstoichiometry (n,) of an oxidized titanomagnetite as well as the molefraction (X,) of ulvospinel in solid solution with magnetite. This means that the associated chemical composition can be located on the FeO-Fe,O,-TiO, ternary diagram irrespectively of magnetic properties. Every point in the ternary diagram may be represented by extended spinel co-ordinates (ns, Xs), The same apply to rhombohedra1 (nr, X,) and orthorhombic co-ordinates (no, X,). Consequently, any of the three specified co-ordinate systems may be used to plot the chemical composition of an arbitrary iron-titanium oxide independent of its crystallographic structure. The effect of impurities is discussed. While the X-parameter depends on the distribution of impurities between iron and titanium positions, the n-parameter does not and can unambiguously be estimated from the amount of metallic ions present. As this may be obtained by the electronprobe technique on microscopic phases, the method becomes particularly useful to deal with naturally occurring iron-titanium phases. It provides a new way to study the effects of oxidation and form the basis of a procedure to test the phase uniformity of optically homogeneous cation deficient spinels.