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Published February 1990 | public
Journal Article

The influence of bulk composition on the speciation of water in silicate glasses


Infrared spectroscopy was used to determine the concentrations of molecular water and hydroxyl groups in hydrous rhyolitic, orthoclasic, jadeitic, and Ca-Al-silicate glasses synthesized by quenching of melts from elevated presure and temperature. The rhyolitic glasses and some of the Ca-Al-silicate glasses were quenched from water-vapor- saturated melts and used to determine the solubility of water in melts of these compositions. For all compositions studied, hydroxyl groups are the dominant hydrous species at low total water contents, whereas molecular water dominates at elevated water contents. Although the trends in species concentrations in all these compositions are similar, the proportions of the two hydrous species are influenced by silicate chemistry: increasing silica content and K relative to Na both favor molecular water over hydroxyl. Results on rhyolitic glass demonstrate that molecular water is also favored by decreasing temperature at T< 850° C. For rhyolitic glasses quenched from vapor-saturated melts, the mole fraction of molecular water is proportional to water fugacity for P(H_2O) ≤ 1500 bars, demonstrating that the behavior of molecular water is approximately Henrian at total water contents up to at least several weight percent. Data on water solubility for albitic, orthoclasic, and Ca-Al-silicate melts to higher pressures can also be fit by assuming Henrian behavior for molecular water and can be used to set constraints on the partial molar volume of water in these melts. The demonstration of Henry's law for molecular water in these liquids provides a link between spectroscopic measurements of microscopic species concentrations and macroscopic thermodynamic properties.

Additional Information

© 1990 Springer-Verlag. Received August 30, 1988; Accepted July 24, 1989. Editorial responsibility: I.S.E. Carmichael. We thank Dr. H. Eckert for the NMR spectroscopic analyses; Drs. P. Dobson and S. Newman for assistance with the hydrogen extraction procedure; Dr. D.L. Hamilton for unpublished data on the solubility of water in orthoclasic melts; Dr. G. Lofgren and Profs. S. Epstein, A.L. Montana, G. Rossman, and P.J. Wyllie for opening their laboratories to us; Mr. T. Stanton, Profs. W. Johannes and P. McMillan, and Drs. B. Mysen, H.R. Shaw, and D. Stewart for providing us with samples; and Dr. K. Burke of the Lunar and Planetary Institute for financial assistance to support work done at the Johnson Space Center by L.A.S. We also acknowledge suggestions from an anonymous reviewer. This work was supported by NSF Grants EAR-8212765, EAR-8417434, and EAR-8618229 and by the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant 17737-AC2). Caltech Division of Geological and Planetary Sciences Contribution 4540.

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