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3.5 c. Solubility of water in coesite at high pressure and temperature (J.L. Mosenfelder)

Water solubility in coesite, one of the high-pressure forms of SiO2, has been investigated in five multianvil experiments performed at 1200°C over the pressure range of 5 to 9 GPa. The starting material for each experiment was a cylinder of pure silica glass plus talc, which dehydrates at high P and T to provide a source of water (plus enstatite and excess SiO2). A layer of Ni/NiO powder was also added to one end of the capsule in order to buffer oxygen fugacity. The recovered samples contain large coesite crystals up to several hundred microns in size, fine-grained enstatite crystals, and excess H2O. FTIR spectra of coesite show four sharp bands at 3574, 3523, 3459, and 3299 cm-1, indicative of structurally bonded hydrogen (Fig. 3.5-6). The concentration of hydrogen increases with pressure from 45 H/106 Si (at 5 GPa) to 132 H/106 Si (at 9 GPa). These results document a significantly lower water solubility than that suggested by earlier studies.

Fig. 3.5-6: Representative, unpolarized FTIR spectra of coesite showing increase in water solubility with pressure. All spectra normalized for 1 mm thickness.

Previous work (see Annual Report 1997) suggested that the preservation of coesite in regionally metamorphosed, ultra-high pressure rocks is crucially dependent on limited fluid infiltration during exhumation. The kinetics of reversion to quartz are also likely enhanced by the presence of hydrogen incorporated into coesite. However, the present results indicate that coesite is unlikely to incorporate significant amounts of hydrogen, consistent with FTIR measurements on natural coesite crystals that failed to detect the presence of hydrogen.

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