Effect of Water on the Compressional Behavior of SiO2 Stishovite up to 30 GPa
Abstract
Stishovite, a rutile-structured high-pressure silica phase with six-fold coordinated silicon atoms, has been observed naturally in meteorites and is believed to occur in silica-rich parts of subducted oceanic slabs and crustal fragments in the Earth's mantle. Although stishovite may contain small amounts of hydroxyl in conjunction with Al incorporation, Spektor et al. (2011, PNAS) showed that significant amounts of water (1.3 wt%) can be incorporated into pure SiO2 stishovite even without Al. However, it has been unknown how the large amount of water can alter the physical properties of stishovite at mantle-related pressures. We synthesized hydrous stishovite at 723 K and 10 GPa in the multi anvil press at Arizona State University. The measured unit-cell parameters for the sample combined with the water content versus unit-cell parameter calibration by Spektor et al. (2011) indicates that the sample contains 2.7 wt% water. We also synthesized anhydrous stishovite at 1473 K and 10 GPa in the multi anvil press. Separate cold pressed foils were made from both samples and were loaded in the sample chamber of a diamond-anvil cell together with a helium pressure medium and ruby pressure standard. We measured in situ synchrotron X-ray diffraction of both hydrous and anhydrous stishovite samples at room temperature and up to 30 GPa at the GSECARS sector of Advanced Photon Source. Le Bail refinements were conducted to obtain unit-cell parameters from both hydrous and anhydrous stishovites. We found that hydrous stishovite is much more compressible than anhydrous stishovite: while the unit-cell volume of hydrous stishovite is 1.3% higher than that of anhydrous stishovite at 1 bar, the volume difference decreases to 0.41% at 30 GPa. Fitting of the pressure-volume data to Vinet equation yields K0 = 295 × 6 GPa and K'0 = 3.9 × 0.7 for anhydrous stishovite, which is consistent with previous studies, while K0 = 223 × 5 GPa and K'0 = 8.0 × 0.6 for hydrous stishovite. We found systematic deviation of data points above 15 GPa in the equation of state fitting for hydrous stishovite, suggesting a change in compressional mechanism or a phase transition at the pressure range. The a axis of hydrous stishovite is greater than that of anhydrous stishovite throughout the pressure range. The c axis is similar between hydrous and anhydrous stishovites at 1 bar, but the c axis of hydrous stishovite becomes increasingly smaller than that of anhydrous stishovite with compression. The small difference in unit-cell volume between hydrous and anhydrous stishovites above 15 GPa may indicate further stabilization of hydrous stishovite at the pressure range and even greater water capacity of pure SiO2 stishovite at deeper mantle pressures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMMR21A2316N
- Keywords:
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- 3900 MINERAL PHYSICS;
- 3919 MINERAL PHYSICS Equations of state;
- 3924 MINERAL PHYSICS High-pressure behavior