RUTILE SOLUBILITY AND TITANIUM COORDINATION IN SILICATE MELTS

Abstract

The solubility of rutile has been determined in a series of compositions in the K2O-Al2O3-SiO2 system ( + Al2O3) = 0.38–0.90), and the CaO-Al2O3-SiO2 system (). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the K∗ ratio. For example, at 1475°C the amount of TiO2 required for rutile saturation varies from 9.5 wt% (K∗ = 0.38) to 11.5 wt% (K∗ = 0.48) to 41.2 wt% (K∗ = 0.90). In the CAS system at 1475°C, rutile solubility is not a strong function of C∗. The amount of TiO2 required for saturation varies from 14 wt% (C∗ = 0.48) to 16.2 wt% (C∗ = 0.59).The solubility changes in KAS melts are interpreted to be due to the formation of strong complexes between Ti and K+ in excess of that needed to charge balance Al3+. The suggested stoichiometry of this complex is K2Ti2O5 or K2Ti3O7. In CAS melts, the data suggest that Ca2+ in excess of A13+ is not as effective at complexing with Ti as is K+. The greater solubility of rutile in CAS melts when C∗ is less than 0.54 compared to KAS melts of equal K∗ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K).TiKβ x-ray emission spectra of KAS glasses (K∗ = 0.43–0.60) with 7 mole% added TiO2, rutile, and Ba2TiO4, demonstrate that the average Ti-O bond length in these glasses is equal to that of rutile rather than Ba2TiO4, implying that Ti in these compositions is 6-fold rather than 4-fold coordinated. Re-examination of published spectroscopic data in light of these results and the solubility data, suggests that the 6-fold coordination polyhedron of Ti is highly distorted, with at least one Ti-O bond grossly undersatisfied in terms of Pauling's rules.