Crystal chemistry and genesis of Nb-, V-, and Al-rich metamorphic titanite from Egypt and Greece

Abstract

Titanite CaTiOSiOa from calc-silicate rocks of Gebel Umm Shagir, Egypt, associated with garnet, p,'roxene, scapolite, calcite and quartz, contains up to 11 wt.9o Nb2O5, 6 wt.9o Al2O3 and2 *t.Vo V2Oj. Other elements present in minor amounts are Fe, Ta, Sn, and F. The main substiturions are2Ti : Nb + AI, Ti + O = Al + (OH,F), and V for Ti. Up to 50 mol.9o of Ti is substituted. The crystals show complex zoning, with Nb contents decreasing from core to rim. The grorth of the crystals is correlated with different stages of metamorphism. Titanite from lowgrade Mn-rich metasediments of Andros, Greece, contains up to 2 wt.q0 V2O, in homogeneous crystals. The valence state of vanadium is uncertain; possible mechanisms of substitution are Ti+O=V3* + (OH,F), Ti=V4+, and 2 Ti = V5+ + (Al,Fe)3+.