Glasses and melts of more complex compositions

It is generally accepted that aluminum is tetrahedrally coordinated in aluminosilicate glasses and melts, provided that cations such as alkali metals or alkaline earth metals are present in sufficient amounts to charge compensate the replacement of Si by AF+ [i.e., Si + — AP -t- (1/ )M +, where monovalent or divalent M ions occur in nonframework sites associated with the tetrahedral aluminosilicate framework]. Experimental data for aluminosilicate glasses derived from both low-angle x-ray-scattering experiments and EXAFS studies show that as aluminum is substituted for silicon, the average T (tetrahedrally coordinated ion) -O bond 

Source After Navrotsky ct al., 1985, who provide information on original data sources. 

K2FeSi308 (Waychunas et al., 1988). The EXAFS spectra of the melts along with certain model compounds and a quenched glass are shown in Fig. 8.11. There is no doubt that such direct studies of melts by x-ray absorption and related spectroscopic methods are the techniques of the future. 

Volatile components, notably HjO and COj, can dissolve in a molecular form in silicate melts even at 1 atm, and are well known to have a 

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