COORDINATION CHEMISTRY OF TITANIUM(IV) IN SILICATE-GLASSES AND MELTS .4. XANES STUDIES OF SYNTHETIC AND NATURAL VOLCANIC GLASSES AND TEKTITES AT AMBIENT-TEMPERATURE AND PRESSURE

The coordination environment of Ti(IV) in seven natural and synthetic glasses of basaltic, trachytic, rhyolitic composition as well as four tektites has been studied using high-resolution Ti K-edge x-ray absorp tion near edge structure (XANES) spectroscopy at ambient temperature a nd pressure. Pre-edge features of Ti K-edge XANES spectra for these gl asses suggest that Ti-[15] is the dominant Ti coordination in all volc anic glasses. However, in the less polymerized glasses studied (basalt ic and trachytic), Ti-[6] is also important (30-50% of the total Ti) b ut Ti-[4] was not detected. In contrast, Ti-[4] is important in the mo st polymerized glasses (rhyolites and tektites) (from 30 to 60% of the total Ti depending on NBO/T) with Ti-[6] below the detection level (a pproximate to 10 at%). The local structure around Ti in the natural vo lcanic glasses is similar to that observed in compositionally similar synthetic silicate glasses and also in Ti bearing silicate glass and m elts with simpler compositions. The presence of F, Cl, and H2O does no t appear to affect the coordination of Ti, based on Ti Kedge XANES mea surements of natural glasses bearing these volatile components. In con trast, the presence of nonbridging oxygens (produced by network modifi ers) favors Ti-[5] in these glass/melts. In parallel, Ti-[4] is import ant when nonbridging oxygens are at small concentrations (NBO/T < 0.1) . Ti-[6] is detected (i.e., when present >10% of the total Ti) when al kaline-earths are dominant over alkalis, in agreement with bond-valenc e predictions for Ti-bearing silicate glass/melts below TiO2 saturati on. The abundance of Ti-[5] in these silicate glasses (and presumably their melts) is in sharp contrast with the rarity of this Ti coordinat ion state in common rock-forming minerals. Titanium cannot readily ent er the structure of most rock-forming minerals, because it is present dominantly as titanyl-bearing (Ti-[5]=0) units in most natural magmas. In contrast, Ti-[6] and Ti-[4] (present, respectively, in basic and a cidic magmas) are better able to enter inosilicates, but these coordin ation states represent only a fraction of the Ti in basalts, explainin g the usually moderate level of incompatibility of Ti during magmatic differentiation. Finally, Ti-[5] transforms to Ti-[6] during crystalli zation of Ti-rich minerals (ilmenite, rutile,pyrochlore). Copyright (C ) 1997 Elsevier Science Ltd.