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Rutile solubility and titanium coordination in silicate melts

Authors:	James E Dickinson Paul C Hess

Institution:	Department of Geological Sciences, Brown University, Providence, Rhode Island 02912 USA

Abstract:	The solubility of rutile has been determined in a series of compositions in the K₂O-Al₂O₃-SiO₂ system ( $K^{1} = K_{2} O (K_{2} O$ + Al₂O₃) = 0.38–0.90), and the CaO-Al₂O₃-SiO₂ system ( $C^{1} = CaO (CaO + Al_{2} O_{3}) = 0.47–0.59$ ). Isothermal results in the KAS system at 1325°C, 1400°C, and 1475°C show rutile solubility to be a strong function of the $K^{1}$ ratio. For example, at 1475°C the amount of TiO₂ required for rutile saturation varies from 9.5 wt% ( $K^{1} = 0.38$ ) to 11.5 wt% ( $K^{1} = 0.48$ ) to 41.2 wt% ( $K^{1} = 0.90$ ). In the CAS system at 1475°C, rutile solubility is not a strong function of $C^{1}$ . The amount of TiO₂ required for saturation varies from 14 wt% ( $C^{1} = 0.48$ ) to 16.2 wt% ( $C^{1} = 0.59$ ).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 $C^{1}$ is less than 0.54 compared to KAS melts of equal $K^{1}$ ratio results primarily from competition between Ti and Al for complexing cations (Ca vs. K).TiK_β x-ray emission spectra of KAS glasses ( $K^{1} = 0.43–0.60$ ) 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.

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