Titanite in carbonatitic rocks: Genetic dualism and geochemical significance

Abstract

Titanite is a relatively rare Ti silicate in carbonatitic rocks. It is a primary phase in alkalirich carbonatites, and may also occur in silicocarbonatites whose composition was modified by assimilation of wallrock silicate material. More typical is late-stage titanite that forms by reaction of a precursor Ti mineral with deuteric fluids. Both genetic types show significant variations in chemical composition arising mostly from the substitution of Ti with Al, Fe, Nb and Zr. Cationic substitutions at the Ca site are limited to several atomic per cent. Zoning in primary titanite typically involves a decrease in the proportion of Nb and Zr toward the rim, whereas deuteric crystals show the reverse zoning pattern.