583 ANNALS OF GEOPHYSICS, VOL. 48, N. 4/5, August/October 2005 Key words polymerisation – basicity – oxidation state – water speciation – Temkin model 1. Introduction During the last century many models have been proposed dealing with the thermodynamic properties of silicate melts (or slags), especial- ly with the goal of understanding slag-melt par- titioning of elements for industrial reasons rele- vant to steelmaking. The heuristic capability of a model assessing silicate melts energetics be- comes particularly important when dealing with the generalised problem of multicompo- nent, mutliphase equilibria, as shown by the thermochemical treatments presented in Ghior- so et al. (1983), Ghiorso and Sack (1994), Pel- ton (1998), Papale (1999) and Moretti et al. (2003). To model element solubility and speci- ation it is necessary to account fully for the compositional variables of the system. Never- theless, compositional variables cannot be un- derstood without a comprehensive model able to rescale measured concentrations in terms of component activities, which represent the obvi- ous control parameters of chemical reactions taking place in the system. Therefore, the choice of the model for component activities represents a crucial step in silicate melt thermo- dynamics. Here, I will show that ionic models accounting for the variable degree of polymeri- Polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts Roberto Moretti Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Napoli, Italy Abstract In order to describe and quantify the reactivity of silicate melts, the ionic notation provided by the Temkin for- malism has been historically accepted, giving rise to the study of melt chemical equilibria in terms of complete- ly dissociated ionic species. Indeed, ionic modelling of melts works properly as long as the true extension of the anionic matrix is known. This information may be attained in the framework of the Toop-Samis (1962a,b) mod- el, through a parameterisation of the acid-base properties of the dissolved oxides. Moreover, by combining the polymeric model of Toop and Samis with the «group basicity» concept of Duffy and Ingram (1973, 1974a,b, 1976) the bulk optical basicity (Duffy and Ingram, 1971; Duffy, 1992) of molten silicates and glasses can be split into two distinct contributions, i.e. the basicity of the dissolved basic oxides and the basicity of the polymeric units. Application to practical cases, such as the assessment of the oxidation state of iron, require bridging of the energetic gap between the standard state of completely dissociated component (Temkin standard state) and the standard state of pure melt component at P and T of interest. On this basis it is possible to set up a preliminary model for iron speciation in both anhydrous and hydrous aluminosilicate melts. In the case of hydrous melts, I introduce both acidic and basic dissociation of the water component, requiring the combined occurrence of H + cations, OH − free anions and, to a very minor extent, of T-OH groups. The amphoteric behaviour of water re- vealed by this study is therefore in line with the earlier prediction of Fraser (1975). Mailing address: Dr. Roberto Moretti, Istituto Nazio- nale di Geofisica e Vulcanologia, Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy; e-mail: moret- ti@ov.ingv.it