The influence of TiO2 on the structure and derivative properties of silicate melts

Abstract

The influence of TiO, on the structure of silicate melts has been determined at I atm pressure by laser Raman spectroscopy. The following melt compositions were used: CaSiOr, CaMgSirOu, NarSiOr, NaAlSi3Os, CaAl2OE, and NarTiOr. Titanium is in four-fold coordination in all melts studied, as there are no Raman bands below 700 cm-r that can be assigned to Tia* in six-fold coordination. In melts with nonbridging oxygen (NBo), solution of rio, results in a decrease in NBO/T (nonbridging oxyg€ns per tetiahedrally coordinated cation). In metasilicate melts, monomers, chains, and sheets of Sio* and Tio* occur as discrete units. The proportion of the Ti-rich structwal units increas€s relative to the Si-rich units with increasing TiO, content of the system. The overall NBO/T of the melt decreases with increasing TiO, content which results in an increase of chain and sheet units relative to monomers. Titanium substitutes for silicon in quenched Ab and An melts to form tlree-dimensional (3D) aluminotitatrate complexes. The aluminosilicate complexes (3D) in these melts become depleted in Al as a result of this solution mechanism of Ti. Discrete, tfuee-dimensional units of TiO, composition occur together with three-dimensional SiO, units in quenched melts in the system SiO2-TiO2. The position of liquidus boundaries between minerals of different degree of polymerization shifts toward the silica-deficient portions of the appropriate systems with the addition of Tior. This behavior is a result of the decrease in NBO/T in the melts as Tio2 is added. Crystal-liquid partition coefficients for a variety of transition metals and incompatible trace elements are likely to increase with increasing TiO, content of the melt as a result of the decrease of NBO/T of melts with increasing TiO, content. The viscosity of depolymerized silicate melts depends on NBO/T. Inasmuch as this ratio decreases with increasing TiOr, the viscosity of such melts probably increases with increasing titanium content. The viscosity of fully polymerized melts depends on the strength of Al-GSi and Si-O-Si bonds. By substitution of Ti for some of the Si the bonds probably become weaker. The viscosity of such melts will decrease, therefore, with increasing TiO2 content.