Magneto-Chemical Study of the Generalized Titanomagnetite in Volcanic Rocks

Abstract

chemically analysed specimens of homogeneous titanomagnetites separated from various volcanic rocks were examined magnetically and crystallographically. Care was taken not to use the titanomagnetite carrying exsolved ilmenite-hematite series minerals. About 80 percent of the present specimens possesses the chemical composition situated in a magnetite-ulvospinel-ilmenite compositional field in a FeO-Fe2O3-TiO2 ternary system. Natural occurrence of the homogeneous titanomagnetite of which chemical composition enters into a magnetite-ilmenite-hematite compositional field was also found in some volcanic rocks. The concept of the generalized titanomagnetite, i.e. the spinel phase in the FeO-Fe2O3-TiO2 system with varying vacancy in the metal ion site of the crystal structure, was applied for the magneto-chemical study of these natural titanomagnetites. The magnetic and crystallographic properties of the present specimens are interpreted successfully by making use of the equal lattice parameter diagram and equal Curie temperature diagram on the FeO-Fe2O3-TiO2 system both of which have been determined from the synthetic experiments. A remarkable character that the titanomagnetite in some volcanic rocks is separable to an ensemble of the single phase grains of which chemical composition varies grain by grain nearly along the reduction-oxidation line was also found.