Anions In Melts Research Articles

Glass structure, melt structure, and dynamics: Some concepts for petrology

![Figure][1] The thermodynamic and transport properties of the aluminosilicate melts at the heart of most magmatic processes vary in complex ways with composition, temperature, and pressure. Insights into these properties can come from information on the structure of the melts themselves, and more commonly from their glassy, quenched equivalents. Although most such connections remain qualitative or semi-quantitative, they are fundamentally important in interpretation of observations on igneous systems in nature and the laboratory, and in the formulation of physically accurate models. This review presents some of the important concepts of aluminosilicate glass and melt structure and dynamics that are most relevant to furthering our understanding of the igneous processes so central to how our planet has formed and evolved. The relationships among glasses, melts, and crystals are introduced. The structural underpinnings of temperature and pressure effects on melt free energies, densities, and viscosities, constraints on the extent of order/disorder among cations and anions in melts, why silica activity varies so strongly with composition, and how liquid-liquid phase separation can be understood, are discussed. Some simple, but useful, general views are presented on melt disorder and the shapes of liquidus surfaces (key to magmatic phase equilibria), as are links between atomic-scale dynamics and viscous flow and diffusion. [1]: pending:yes

Simulation of molecular mass distributions and evaluation of O2− concentrations in polymerized silicate melts

A new statistical model is proposed for the molecular mass distributions (MMD) of polymerized anions in silicate melts. The model is based on the known distribution of Q n species in the MeO - Me 2 O - SiO 2 system. In this model, chain and ring complexes are regarded as a random series of Q n structons with various concentrations of bridging bonds ( 1 ≤ n ≤ 4, Q 0 corresponds to Si ). This approach makes it possible to esti- mate the probability of formation of various ensembles of polymer species corresponding to the general formula ( Si i O 3 i + 1 - j ) 2( i + 1 - j )- , where i is the size of the ion, and j is the cyclization number of intrachain bonds. The statistical model is utilized in the STRUCTON computer model, which makes use of the Monte Carlo method and is intended for the calculation of the composition and proportions of polyanions at a specified degree of polymerization of silicate melts (STRUCTON, version 1.2; 2007). Using this program, we simulated 1200 MMD for polyanions in the range of 0.52 ≤ p ≤ 98 , where p is the fraction of nonbridging bonds in the silicon- oxygen matrix. The average number of types of anions in this range was determined to increase from three (Si Si 2 and Si 3 ) to 153, and their average size increases from 1 to 7.2. A special option of the STRUCTON program combines MMD reconstructions in silicate melts with the formalism of the Toop-Samis model, which enables the calculation of the mole fraction of the O 2- ion relative to all anions in melts of spec- ified composition. It is demonstrated that, with regard for the distribution and average size of anion complexes, the concentration of the O 2- ion in the MeO - SiO 2 system is characterized by two extrema: a minimum at 40- 45 mol % SiO 2 , which corresponds to the initial stages of the gelenization of the polycondensated silicate matrix, and a maximum, which is predicted for the range of 60-80 mol % SiO 2 .

The acid-base properties of halide anions in melts of the alkali metal nitrates

It has been found from the measurement of the potential of an oxygen electrode immersed in the melt of an equimolar mixture of KNO 3 and NaNO 3 that the Cl − anions do not markedly affect this potential, whereas the F −, Br − and I − anions decrease this potential and thus act as Lux bases toward the nitrate melt. A chemical mechanism was proposed and discussed for this effect on the basis of the character of the dependence of the potential on the halide anion concentration. The equilibrium constants for these chemical reactions have also been estimated.

On the structure of silicate melts

A striking similarity is obtained between the structure of binary silicate melts and the Q n distribution in the corresponding glasses. The former are deduced from voltammetric measurements, the latter from published Raman and NMR data. A closer analysis of Raman data based on the proposed neutralisation and depolymerisation scheme for anions in melts is therefore suggested.