Effects of temperature on the structures of silicate liquids: 29 Si NMR results

Details Effects of temperature on the structures of silicate liquids: 29 Si NMR results Effects of temperature on the structures of silicate liquids: 29 Si NMR results

Nov 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988gecoa..52.2659b&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 52, Issue 11, pp.2659-2669

Computer Science

17

Scientific paper

The effect of temperature on the structures of three silicate liquids has been studied by 29 Si NMR spectroscopy on glass samples prepared with different glass transition temperatures. The compositions studied are (Na 2 O) .34 (SiO 2 ) .66 and (CaO) .10 (Na 2 O) .15 (SiO 2 ) .75 and (CaO) .25 (MgO) .25 (SiO 2 ) .50 . In the first two, the abundance of SiO 4 tetrahedra with four bridging oxygens ( Q 4 ) increases significantly with temperature. This change results from the disproportionation reaction 2 Q 3 á i Q 2 + Q 4 ( Q n is a SiO 4 tetrahedron with n bridging oxygens), with the observed increase in disproportionation at higher temperatures implying increased randomization of the anionic structure. In the (Na 2 O) .34 (SiO 2 ) .66 composition, the temperature dependence of Q 4 abundance enables us to estimate H 0 for this reaction to be 30 ± 15 kJ per mole of Si. Although this reaction thus makes only a minor contribution to the conngurational heat capacity of each liquid, it probably has a major influence on the thermodynamic activity of SiO 2 . Widening of the (CaO) .25 (MgO) .25 (SiO 2 ) .50 spectral peak with increasing temperature likewise implies increasing randomization of the anionic structure, although specific structural contributions to this widening are not identified. Models of melt structures at liquidus temperatures will be limited in accuracy if they assume that the speciation observed in glasses is unaffected by temperature change.

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