The temperature dependence of the speciation of water in NaAlSi3O8-KAlSi3O8 melts: an application of fictive temperatures derived from synthetic fluid-inclusions |
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| Authors: | C Romano D B Dingwell H Behrens |
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| Institution: | Bayerisches Geoinstitut, Universit?t Bayreuth, D-95440 Bayreuth, Germany, DE
Institut für Mineralogie, Universit?t Hannover, Welfengarten 1, D-30167 Hannover, Germany, DE
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| Abstract: | The speciation of water dissolved in glasses along the join NaAlSi3O8-KAlSi3O8 has been investigated using infrared spectroscopy. Hydrous melts have been hydrothermally synthesized by chemical equilibration
of cylinders of bubble-free anhydrous start glasses with water at 1040° C and 2 kbar. These melts have been isobarically and rapidly (200° C/s) “drop”-quenched to room temperature and then subsequently depressurized. The speciation of water in the quenched glasses
reflects the state of water speciation at a temperature (the so-called fictive temperature) where the quenched-in structure
of the glasses closely corresponds to the melt structure at equilibrium. This fictive temperature is detectable as the macroscopically
measureable glass transition temperature of these melt compositions. A separate set of experiments using vesicular samples
of the same chemistry has precisely defined the glass transition temperature of these melts (±5° C) on the basis of homogenization temperatures for water-filled fluid inclusions (Romano et al. 1994). The spectroscopic
data on the speciation of water in these quenched glasses has been quantified using experimentally determined absorptivities
for OH and H2O for each individual melt composition. The knowledge of glass transition temperatures, together with quantitative speciation
data permits an analysis of the temperature dependence of the water speciation over the 113° C range of fictive temperatures obtained for these water-saturated melts.
The variation of water speciation, cast as the equilibrium constant K where
K = H2O] O
m
]/OH]2
is plotted versus the fictive temperature of the melt to obtain the temperature dependence of speciation. Such a plot describes
a single linear trend of the logarithm of the equilibrium constant versus reciprocal temperature, implying that the exchange
of K for Na has little influence on melt speciation of water. The enthalpy derived from temperature dependence is 36.5(±5) kJ/mol.
The results indicate a large variation in speciation with temperature and an insensitivity of the speciation to the K–Na exchange.
Received: 8 March 1995/Accepted: 6 June 1995 |
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