An X-ray Scattering And Raman Spectroscopy Study Of Iron(3+), Gallium(3+) And Germanium(4+) Substituted Aluminosilicate Glasses

SiO(,2), KFeSi(,3)O(,8) and NaFeSi(,3)O(,8) glass structures were investigated using X-ray radial distribution function (RDF) and quasi-crystalline modelling. Vitreous SiO(,2) exhibits a mean Si-O bond distance of 1.60 (ANGSTROM) and Si (\u27O) Si angle of 151(DEGREES), similar to previous studies. Modelling of the structure against various SiO(,2) polymorphs suggest that vitreous SiO(,2) (v-SiO(,2)) can best be described as a tridymite-like network consisting of six-membered rings of SiO(,4) tetrahedra of which 2/3 are tridymite-like and 1/3 cristobalite-like.;The structures of KFeSi(,3)O(,8) and NaFeSi(,3)O(,8) glasses both have apparent T-O distances of 1.70 (ANGSTROM) but differ in T-T distances and T (\u27O) T angles; which are 3.20 (ANGSTROM)/152(DEGREES) and 3.20 (ANGSTROM)/140(DEGREES) respectively. Individual Fe-O/Si-O bond lengths cannot be resolved. Modelling of these structures suggests that the structure of KFeSi(,3)O(,8) glass is predominantly composed of six-membered rings of SiO(,4) tetrahedra similar to those found in (beta)-Na(,2)Si(,2)O(,5). The NaFeSi(,3)O(,8) RDF resembles that of both nepheline and albite RDF\u27s indicating that the structure is a combination of six and four-membered rings of SiO(,4) tetrahedra. The increased contribution of the four-membered rings to the RDF is reflected in the lower T (\u27O) T angle for NaFeSi(,3)O(,8) (140(DEGREES)) relative to that of KFeSi(,3)O(,8) (152(DEGREES)).;Raman spectroscopy of gallium and germanium substituted sodium aluminosilicate glasses was also used to clarify the effect of different network modifiers (i.e. Al, Ga...) and formers upon the melt structure. The Raman spectra can be interpreted in terms of the vibrational bands observed in v-SiO(,2) and v-GeO(,2). Such an interpretation indicates that the v-GeO(,2) structure is predominantly four-membered rings of GeO(,4) tetrahedra and that germanium does not undergo preferential bonding into specific structures. Germanates can therefore be used as models for silicate glasses. However, gallium substitution indicates ring size reduction upon increasing Ga content and preferential bonding of Ga/Al into the smaller ring structures.;The concept of variations in ring size, ring size reductions and preferential bonding has important petrological consequences in that such effects should decrease viscosity and increase density in silicate melts. In addition, ring size reduction at high pressure appears to be more likely than increase in nearest-neighbour coordination. This can explain the lack of observed high pressure coordination changes