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In order to describe and quantify the reactivity of silicate melts, the ionic notation provided by the Temkin formalism&#13; has been historically accepted, giving rise to the study of melt chemical equilibria in terms of completely&#13; dissociated ionic species. Indeed, ionic modelling of melts works properly as long as the true extension of the&#13; anionic matrix is known. This information may be attained in the framework of the Toop-Samis (1962a,b) model,&#13; through a parameterisation of the acid-base properties of the dissolved oxides. Moreover, by combining the&#13; polymeric model of Toop and Samis with the «group basicity» concept of Duffy and Ingram (1973, 1974a,b,&#13; 1976) the bulk optical basicity (Duffy and Ingram, 1971; Duffy, 1992) of molten silicates and glasses can be split&#13; into two distinct contributions, i.e. the basicity of the dissolved basic oxides and the basicity of the polymeric&#13; units. Application to practical cases, such as the assessment of the oxidation state of iron, require bridging of the&#13; energetic gap between the standard state of completely dissociated component (Temkin standard state) and the&#13; standard state of pure melt component at P and T of interest. On this basis it is possible to set up a preliminary&#13; model for iron speciation in both anhydrous and hydrous aluminosilicate melts. In the case of hydrous melts, I&#13; introduce both acidic and basic dissociation of the water component, requiring the combined occurrence of H+&#13; cations, OH- free anions and, to a very minor extent, of T-OH groups. The amphoteric behaviour of water revealed&#13; by this study is therefore in line with the earlier prediction of Fraser (1975)

Polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts