HR: 1340h
AN: MR13A-0049    [Abstracts]
TI: Quench Rate Studies of Framework Ordering in Aluminosilicate Glasses: Implications for Pressure and Temperature Effects on Melt Structure
AU: * Dubinsky, E V
EM: evd@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305-2115 United States
AU: Stebbins, J F
EM: stebbins@pangea.stanford.edu
AF: Stanford University, Dept. of Geological and Environmental Sciences, Stanford, CA 94305-2115 United States
AB: Structural studies of quenched glasses have long served as starting points for investigations of aluminosilicate liquid properties at ambient pressure, and are increasingly being used to explore melt behavior at high pressure. At best, the glass represents the melt structure at the glass transition temperature. The difference between T_g and liquidus temperatures, which can be many hundreds of degrees at 1 bar, generally becomes even larger at higher pressure. Thus, the effects of temperature on melt structure become especially relevant to connecting high-pressure liquid properties to observations of glasses. To begin to investigate temperature effects on aluminosilicate melt structure, we have used ¹⁷O 3QMAS NMR spectroscopy to study framework speciation in ambient-pressure LiAlSiO₄ and NaAlSiO₄ glass samples prepared with different fictive temperatures. Three samples of each composition were synthesized using quench rates separated by up to six orders of magnitude in order to produce fictive temperature differences of approximately 160-220 degrees for each composition. We observe that peak intensities corresponding to Al-O-Al and Si-O-Si bridging oxygen sites increase with increasing sample fictive temperature in both compositions, indicating that the reaction 2 Al-O-Si ↔ Si-O-Si + Al-O-Al shifts to the right with increasing temperature in the liquid. The observed temperature dependence of Al-O-Al species abundance allows us to estimate the ΔH of this reaction as 32±13 kJ/mol for LiAlSiO₄ glasses and 29±11 kJ/mol for NaAlSiO₄ glasses. Extrapolating our results up in temperature to liquidus conditions, we estimate that 13-18% of bridging oxygen occur as Al-O-Al in NaAlSiO₄ and 15-25% in LiAlSiO₄ at 2000 K, which is generally below the statistically-random value of 25% Al-O-Al for compositions with an Al/Si ratio of 1. Using our experimental data to estimate the contribution to configurational heat capacity from Al/Si disordering in NaAlSiO₄ liquid, we find that redistribution of bridging oxygen species with increasing temperature produces a negative dC_p/dT above the glass transition. The observed positive dC_p/dT in this composition may indicate that other temperature-induced structural modifications occur in addition to reorganization of major melt species with increasing temperature. We calculate the contribution from a generalized mechanism for structural rearrangement that is characterized by a small initial species concentration at T_g and a large ΔH, which we find produces a positive dC_p/dT. Among numerous possibilities for these changes is network cation coordination number increase, which may be of particular interest in high-pressure systems.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1099 General or miscellaneous
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005