Polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts
Main Article Content
Abstract
In order to describe and quantify the reactivity of silicate melts, the ionic notation provided by the Temkin formalism
has been historically accepted, giving rise to the study of melt chemical equilibria in terms of completely
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) model,
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 revealed
by this study is therefore in line with the earlier prediction of Fraser (1975).
has been historically accepted, giving rise to the study of melt chemical equilibria in terms of completely
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) model,
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 revealed
by this study is therefore in line with the earlier prediction of Fraser (1975).
Article Details
How to Cite
Moretti, R. (2005) “Polymerisation, basicity, oxidation state and their role in ionic modelling of silicate melts”, Annals of Geophysics, 48(4-5). doi: 10.4401/ag-3221.
Issue
Section
OLD
Open-Access License
No Permission Required
Istituto Nazionale di Geofisica e Vulcanologia applies the Creative Commons Attribution License (CCAL) to all works we publish.
Under the CCAL, authors retain ownership of the copyright for their article, but authors allow anyone to download, reuse, reprint, modify, distribute, so long as the original authors and source are cited. No permission is required from the authors or the publishers.
In most cases, appropriate attribution can be provided by simply citing the original article.
If the item you plan to reuse is not part of a published article (e.g., a featured issue image), then please indicate the originator of the work, and the volume, issue, and date of the journal in which the item appeared. For any reuse or redistribution of a work, you must also make clear the license terms under which the work was published.
This broad license was developed to facilitate open access to, and free use of, original works of all types. Applying this standard license to your own work will ensure your right to make your work freely and openly available. For queries about the license, please contact ann.geophys@ingv.it.