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Water speciation measurements of volcanic glasses usually rely on the assumption that the glass retains the structure of the original melt, frozen in at the glass transition temperature. However, over the last few years, work performed mostly at the Geoinstitut has shown that this may be an invalid assumption and that there can be substantial changes in the water speciation below the glass transition in the structure. This can easily be studied using spectroscopic techniques, as the near infrared region of the spectrum of a hydrous glass has been shown to contain peaks reflecting the water species contained in the structure. This project is designed to increase our knowledge of such changes, by examining a series of rhyolitic glasses spectroscopically, and determining how the water speciation changes with both temperature and pressure.

A series of hydrous rhyolite glasses with water contents ranging from almost 0 to 5 wt% was synthesized using rapid-quench TZM-autoclaves. The total water content of each of the hydrous glasses was determined by Karl-Fischer titration and was found to be in good agreement with the amount of water originally weighed into the sample capsule. The homogeneity of the glasses was checked by infrared spectroscopy on various parts of the charge.

The water speciation can be determined using the peak found around 4500 cm^-1, which is due to the combination stretching and bending modes of X - OH groups (where X = Si, Al), and the peak around 5200 cm^-1, due to the combination stretching and bending modes of molecular H₂O. Spectra obtained from samples chosen from across the range of water contents reveal that, in agreement with previous reported studies, all the glasses containing >0.5 wt% H₂O_total contain detectable amounts of molecular H₂O. The spectra were used to determine the integral extinction coefficients for the two peaks, revealing  and  of 215.92 L mol^-1cm^-2 and 266.50 L mol^-1 cm^-2, respectively.

To investigate the effects of temperature on the water speciation, samples of the glasses were placed on a heating stage, and spectra obtained at temperature. In order to check that no water was lost during data collection at elevated temperatures, the sample was allowed to cool to ambient conditions after each heating experiment, and a room temperature spectrum was obtained. This was checked against the original spectrum recorded at 25°C. Figure 3.5-7 shows
 

Fig. 3.5-7: Near infrared spectra obtained at high temperature from a rhyolite glass containing 4.94 wt% water. The variation in the intensities of the peaks due to the presence of H2O and OH can be clearly seen.

the evolution of the spectra obtained with increasing temperature for a glass containing 4.94 wt% water. Apparently the absorbance A_H2O of the molecular water peak at 5200 cm^-1 decreases with temperature, while A_OH at 4500 cm^-1 increases. These effects could either be due to real speciation changes or changes in extinction coefficients. In order to test for a possible temperature dependence of extinction coefficients, Figure 3.5-8 shows a plot of the ratio of c_H2O / A_OH versus the ratio A_H2O / A_OH , where c_H2O is the total water concentration. A linear regression in this plot yields the extinction coefficients of OH and H₂O from the intercept at the y-axis and the slope of the curve. Inspection of Fig. 3.5-8 shows that within experimental uncertainty, there is no significant change of extinction coefficients with temperature. Accordingly, the changes observed in the spectra must be due to real speciation changes. Using the room temperature extinction coefficients, the concentrations of OH and H₂O in the samples can be calculated as a function of temperature (Fig. 3.5-9). Clearly, there are massive changes above the glass transformation around 400°C and possibly small changes already at lower temperature. Experiments using externally heated diamond cells are currently under way to study water speciation in rhyolite over a wide range of pressures and temperatures. These experiments will also be extended to basaltic, andesitic and phonolitic compositions.
 
 

Fig. 3.5-8: Plot used to derive infrared extinction coefficients of OH and H2O in rhyolite glass. Shown are data at room temperature and at higher temperatures up to 500°C. AOH and AH2O are the measured absorbances of OH and H2O, respectively. cH2O is the total water content, MH2O the molar mass of water, rho the density of the glass and d the thickness of the sample.

Fig. 3.5-9: Variation of water speciation with temperature for a glass containing 3.93 wt% water. The curves are a guide for the eye only.