Iron-titanium oxides in Some New Zealand ironsands

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Abstract
Optical, magnetic, and X–ray data are presented, together with chemical analyses, for selected North Island ironsand concentrates.Titanomagnetite in the form of magnetite–ulvospinel solid solutions is the predominant constituent, containing minor amounts of aluminous spinels, mainly in solid solution.The magnetic data indicate appreciable variation in composition within each sample. Ulvospinel and aluminous spinels in solid solution combine to depress the Curie temperature of the magnetite, but act in opposition on the lattice dimension, which therefore remains close to that of pure magnetite Progressive oxidation of the titanomagnetite in some specimens has caused exsolution of rhombohedral FeoCX–FeTiOn solid solutions. The end product of such oxidation is titanhematite, which constitutes the whole of some grains, and in others is accompanied by pseudobrookite.Oxidation of titanomagnetite grains with an exsolved aluminous spinel phase has been accompanied by re-solution of the latter and accommodation in the rhombohedral lamellae Data for ilmenite suggest that it contains a small amount of FeL0:i in solid solution A specimen of titanomagnetite separated from one of the potential volcanic source rocks is also discussed and shown not to differ significantly from the material of the ironsands. A nearly pure magnetite from a South Island ironsand has also been studied for comparison

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