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

Journal Article · Tue Nov 01 00:00:00 EST 1988 · Geochimica et Cosmochimica Acta; (USA)

DOI:https://doi.org/10.1016/0016-7037(88)90034-8· OSTI ID:7164124

Brandriss, M E; Stebbins, J F ^[1]

---

1. Stanford Univ., CA (USA)

The effect of temperature on the structures of three silicate liquids has been studied by {sup 29}Si NMR spectroscopy on glass samples prepared with different glass transition temperatures. The compositions studied are (Na{sub 2}O){sub .34}(SiO{sub 2}){sub .66} and (CaO){sub .10}(Na{sub 2}O){sub .15}(SiO{sub 2}){sub .75} and (CaO){sub .25}(MgO){sub .25}(SiO{sub 2}){sub .50}. In the first two, the abundance of SiO{sub 4} tetrahedra with four bridging oxygens (Q{sup 4}) increases significantly with temperature. This change results from the disproportionation reaction 2Q{sup 3} {r equilibrium} Q{sup 2} + Q{sup 4} (Q{sup n} is a SiO{sub 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{sub 2}O){sub .34}(SiO{sub 2}){sub .66} composition, the temperature dependence of Q{sup 4} abundance enables them to estimate {Delta}H{sup 0} for this reaction to be 30 {plus minus} 15 kJ per mole of Si. Although this reaction thus makes only a minor contribution to the configurational heat capacity of each liquid, it probably has a major influence on the thermodynamic activity of SiO{sub 2}. Widening of the (CaO){sub .25}(MgO){sub .25}(SiO{sub 2}){sub .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.

• OSTI ID: 7164124
• Journal Information: Geochimica et Cosmochimica Acta; (USA), Vol. 52:11; ISSN 0016-7037
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
CRYSTAL STRUCTURE
TEMPERATURE EFFECTS
MOLECULAR STRUCTURE
SILICATE MINERALS
PHASE STUDIES
STRUCTURAL CHEMICAL ANALYSIS
AMORPHOUS STATE
CHEMICAL COMPOSITION
CRYSTAL MODELS
CRYSTALLIZATION
CRYSTALLOGRAPHY
GEOCHEMISTRY
LIQUIDS
QUANTITATIVE CHEMICAL ANALYSIS
SPECTROSCOPY
THERMODYNAMIC ACTIVITY
THERMODYNAMIC MOLECULAR MODEL
THERMODYNAMICS
CHEMICAL ANALYSIS
CHEMISTRY
FLUIDS
MATHEMATICAL MODELS
MINERALS
MOLECULAR MODELS
PHASE TRANSFORMATIONS
580000* - Geosciences