Microtextures and microintergrowths of irontitanium-aluminium oxide minerals in a magmatic tinaniferous iron ore from Attu island, southwestern Finland

Abstract

Microtextures of coexisting magnetite, ilmenite and aluminous spinel and their chemical composition have been studied in a magmatic titaniferous iron ore from Attu Island in south-western Finland. Primary ilmenite contains exsolution lamellae of magnetite and aluminous spinel, oriented along the basal plane (0001). The microtextures of magnetite have been formed by exsolution of aluminous spinel and by oxidation-exsolution of ilmenite along {100} and {111} — planes respectively. To some extent the exsolutions have been formed preferentially along planar structures like grain interfaces and intragranular high angle boundaries. Primary magnetite forms intergrowths with secondary ilmenite and aluminous spinel in border zones adjacent to primary ilmenite. These intergrowths partly show a similarity with exsolution textures of primary magnetite. The available data suggest that they have been formed by privileged growth of exsolved ilmenite and aluminous spinel in the magnetite border zones adjacent to ilmenite. Corrosion of magnetite by the adjacent ilmenite is considered to be less probable for the genesis of these intergrowths.