HR: 10:20h
AN: MR32A-01 INVITED    [Abstracts]
TI: Temperature effects on oxide melt structure: spectroscopic constraints and thermodynamic implications
AU: * Stebbins, J F
EM: stebbins@stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences Stanford University, Stanford, CA 94305-2115, United States
AU: Dubinsky, E V
EM: evd@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences Stanford University, Stanford, CA 94305-2115, United States
AU: Kanehashi, K
EM: kanehashi.koji@nsc.co.jp
AF: Nippon Steel Corp., Advanced Technology Research Lab Nippon Steel Corp., Chiba, 293-8511, Japan
AB: Most oxide melts have significant configurational components to their thermodynamic properties (e.g. heat capacities well above the classical vibrational limit), requiring that their structures become increasingly disordered at higher T. In several oxide melt systems, spectroscopic and scattering studies have begun to quantify such structural changes. Comparison of apparently diverse systems can lead to new insights into underlying common features of interactions among structural species. In borate and borosilicate liquids, recent high T spectroscopic studies, as well as results on glasses with different fictive T's prepared by cooling at different rates, have shown that tetrahedral boron ([4]B) converts to trigonal boron ([3]B) at higher T, with a mechanism involving non-bridging oxygens (NBO) often suggested as [4]B = [3]B + NBO. We have recently directly detected the implied effects on oxygen speciation by O-17 NMR in a Ca- aluminoborosilicate. There is now good evidence, also largely from NMR studies, that this reaction occurs reverse for B, Al, and Si during pressure-induced densification, as [n]M + NBO = [n+1]M. However, the interaction of network cation coordination and NBO as a function of temperature in aluminosilicates has remained poorly constrained. In a recent study of Ca aluminosilicates, using in-situ high T Al-27 NMR and MAS and 3QMAS NMR on glasses with varying fictive T, we found clear evidence that the content of [5]Al increases with increasing T. The proportion of "anomalous" NBO (e.g. NBO in CaAl2Si2O8 glass) also increases slightly with T; but in NBO-rich compositions the NBO content seems to be decoupled from [5]Al. Understanding the relationship of this complexity to T and P effects in other systems requires formulations of reactions among species that account more completely for changes in oxygen speciation, particularly the fractions of oxygens with three network cation neighbors. In these systems as well as in boron-rich melts, observed structural changes can account for significant parts of the configurational properties, but other, as yet to be determined, changes much also be important or even predominant.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 8439 Physics and chemistry of magma bodies
SC: Mineral and Rock Physics [MR]
MN: 2007 Fall Meeting