HR: 08:00h
AN: V51D-01 INVITED     [Abstracts]
TI: Melt Structure and Properties: Progress and Prognoses
AU: * Stebbins, J F
EM: stebbins@stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305-2115 United States
AB: Recent advances in quantitative determinations of silicate glass structure are beginning to place important constraints on models of the physical and chemical properties of melts, but much remains to be done before such models can become entirely based on structure. For example, models of free energy and major component activities generally assume (lacking better constraints) that network species (e.g. Al, Si, "Qn" groups) and network modifiers (e.g. Na, Ca, K, Mg) each mix randomly. However, recent spectroscopic studies demonstrate strong ordering in the network (significant if incomplete Al avoidance; preference of Al for "Q4" groups, etc.) and between modifier cations differing greatly in field strength (e.g. K+, Mg2+). Solution models thus may need substantial revision, unless new studies of temperature effects on such ordering indicate approach to randomness at magmatic temperatures. Such studies are ongoing, facilitated by recent developments in hyper-quenching technology. On the other hand, discovery of such low-T ordering provides a likely (and long-suspected) mechanism for at least some of the configurational entropy in multicomponent melts, which in turn is a critical part of models of viscosity: at least in systems with high Al/Si ratios, for example, increase in Al/Si disorder with increasing T will a major part of this term. In many systems, however, the structural origins of Sconf remain mysterious. For both major and minor components, considerable information now exists to constrain models both of activity and of transport processes, but progress awaits development of useful model forms. Examples include new information on the local coordination of anions such as fluoride and chloride, which again show strong chemical ordering. Similarly, we now know a great deal about the concentration of minor "defect" species, that have been considered to be important to viscosity and diffusion (e.g. AlO5, SiO5, "excess" NBO). However, going from speculative mechanism to real prediction is a challenge that probably awaits improved ab-initio based molecular dynamics simulations. Recent results on Al, Si, and O coordination in glasses quenched at high pressure are beginning to provide solid constraints for models of density increase and of P effects on viscosity. These studies are especially promising when they address experimentally issues of thermal and decompression history on properties and structure of recovered glass samples.
DE: 8439 Physics and chemistry of magma bodies
DE: 3640 Igneous petrology
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting