Physics
Scientific paper
Jun 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...55..326r&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 55, Issue 3-4, p. 326-334.
Physics
2
Scientific paper
Treatment of natural quartz under hydrogen fugacities (ƒ;H2) buffered by iron-wüstite-fluid (OH) or nickel-nickel oxide-fluid(OH) at 1.5 GPa and 900-1050°C introduces two types of hydroxyl defects into the mineral lattice: (1) interstitial protons screening Al in Si sites, giving rise to sharp IR peaks near 3400 cm-1, and (2) hydroxyl defects characteristic of synthetic quartz and amethyst, giving rise to sharp IR peaks near 3600 cm-1 and broad-band absorbance. The latter type of IR absorbance has been assigned to SiOH and H2O defects believed to be responsible for the hydrolytic weakening of quartz single crystals. Quartz treated in H2-buffered experiments at 900°C, 1.5 GPa, and ƒ;H2 ~ 15 MPa incorporated 102-103 OH per 106 Si in uncracked regions of the sample after 20 h of treatment. Unbuffered experiments performed by other investigators at 900°C, 1.5 GPa, and ƒ;H2 < 0.05 MPa incorporated < 100 per OH 106 Si after treatment for 43 days and showed no spectroscopic evidence for the presence of SiOH or H2O defects. A thermodynamic model is proposed for the formation of hydroxyl defects in quartz that can account for ƒ;H2 dependence. Hydroxyl defects form by the diffusion of hydrogen into the quartz lattice and the subsequent reaction of hydrogen interstitials with lattice oxygen. No diffusion of oxygen is required; therefore, this mechanism can produce hydroxyl defects in quartz crystals without the additional assumption of H2O transport through microfractures. Equilibrium concentrations of three model hydroxyl defects in quartz, [Si O-OH]t, [(Al)'SiO-OH]t, and [HOH]O, are calculated as functions of ƒ;H2 and temperature at a total pressure of 1.5 GPa. Calculated XOH = ƒ;(T, ƒ;H2) surfaces fit experimental data from three laboratories and it is possible to attribute much interlaboratory variation to differences in experimental ƒ;H2. Comparing the model with an empirical Al-quartz geothermometer shows that quartz from hydrothermal veins, contact metamorphic zones, and high-grade gneisses equilibrated under hydrogen fugacities near nickel-nickel oxide-fluid (OH). Therefore, mineral properties strongly influenced by the concentration of hydroxyl defects, such as plasticity and self-diffusion, need to be examined in experiments buffered to relatively high ƒ;H2 in order to reproduce crustal conditions.
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