Property and structure data from binary and ternary silicate and aluminosilicate melts and glasses often reveal simple and systematic relationships to composition and temperature. Properties whose dominant structural control is the abundance of fully polymerized Q4-species, exhibit smooth variations with silica content. Such properties include activation energies associated with transport. Properties that depend on the type and abundance of coexisting structural species in depolymerized melts, either show smooth functional relationships to melt polymerization, NBO/T, or exhibit pronounced minimum or maximum values at intermediate NBO/T-values. The former properties include those, which are related to the availability of configurational states. The latter group includes those whose behavior is controlled by the energetics of mixing of the individual structural species. An example is enthalpy of mixing. Crystal-liquid element partition coefficients also exhibit behavior governed by thermodynamics of mixing of the structural species of the melt.
Aluminum in most silicate melts is in tetrahedral coordination at ambient pressure and substitutes for silicon. The Al3+ <=> Si4+ substitution lowers (Si,Al)-O-(Si,Al) bond energy. In principle, the value of properties that depends on bridging oxygen bond strength, decreases, therefore, with increasing Al/(Al+Si). Properties that depend on the proportion of polymerized and depolymerized Qn-species in the melt can show minima or maxima in relationships to Al/(Al+Si) because the abundance of the Qn-species in depolymerized silicate melts depends on the bulk melt Al/(Al+Si). Transport properties fall in this category. It is possible that Al-distribution between Qn-species in depolymerized aluminosilicate melts depends on temperature. It is likely, therefore, at least for high-temperature viscosity, its temperature-dependence is non-Ahrrenian. Other melt transport properties that are functionally related to viscous flow (e.g., diffusivity, electrical, and thermal conductivity) may also show non-Ahrrenian temperature-dependence at high temperature.
Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Rd., NW, Washington DC 20015, USA
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Key-words: silicate melt, structure, spectroscopy, rheology, thermodynamics.