The structure of glasses in the K2O-Al2O3-TiO2-SiO2 system was investigated using XANES spectroscopy. Glass samples, synthesized by quenching in air from high temperature fusions, represent the addition of Al2O3 to a base of composition K2TiSi4O11 in amounts corresponding to 0.25, 0.50, 0.75, 1.00, 1.25, and 1.50 mol p.f.u. In the Ti-free system, this range of alkali/aluminum ratios crosses the leucite stoichiometry at 1.0. Si K-edge and Al K-edge spectra indicate tetrahedral environments for these elements, and show no variations related to coordination change as a function of Al content. Changes in the relative intensities of peaks in the Al K-edge, however, suggest variation in the intertetrahedral (T-O-T) angle. We associate the decrease of this angle for the glasses of peraluminous composition with the presence of triclusters of tetrahedra. The pre-edge peak absorption features in the Ti K-edge XANES spectra indicate that the average Ti coordination decreases with the addition of Al2O3. We infer depletion of fivefold-coordinated titanium (possibly as alkali titanyl complexes), which are dominant in the Al-free glass, by the formation of fourfold coordinated Ti and alkali aluminate complexes (up to a concentration of 40% in the most peraluminous glass). Significant amounts of [V]Ti remain present, even at peraluminous compositions, in further support of tricluster formation as a mechanism for Al incorporation.
Claudia Romano1,*,, Eleonora Paris2, Brent T. Poe3, Gabriele Giuli2, Donald B. Dingwell3 and Annibale Mottana1,
1 Dipartimento di Scienze Geologiche, Universitá degli Studi Roma Tre, Largo S. Leonardo Murialdo, I-00146 Rome, Italy
2 Dipartimento di Scienze della Terra, and INFM, Universitá di Camerino, I-62032, Camerino, Italy
3 Bayerisches Geoinstitut Universität Bayreuth, D-95440 Bayreuth, Germany