HR: 0830h
AN: V31D-0970    [PDF]
TI: High-Temperature Phase Transitions and Elasticity of Low-Pressure Silica Polymorphs.
AU: * Lakshtanov, D L
EM: lakshtan@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 1301 W.Green st., Urbana, IL 61801 United States
AU: Sinogeikin, S V
EM: sinogeik@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 1301 W.Green st., Urbana, IL 61801 United States
AU: Bass, J D
EM: jaybass@uiuc.edu
AF: University of Illinois at Urbana-Champaign, 1301 W.Green st., Urbana, IL 61801 United States
AB: Low-pressure silica polymorphs are common rock-forming mineral phases in igneous, sedimentary and metamorphic rocks, play a vital role in crustal processes and are important industrially. Therefore, knowledge of the high-temperature properties of silica polymorphs and the high temperature phase relations of pure silica are of great importance. Silica has three major low-pressure crystalline modifications: quartz, cristobalite, and tridymite. All of these are related to each other through reconstructive phase transitions. The transition of $\beta$-quartz to HP-tridymite is thought to occur at $867\deg$C but this transition has never been observed in pure silica without using a flux (e.g. NaWO$_{4}$). $\beta$-Quartz can exist metastably up to the $\beta$-cristobalite stability field at $1470\deg$, however $\beta$-quartz - $\beta$-cristobalite transition usually takes place at lower temperatures ($>$$1150\deg$C) as a metastable inversion. The temperature of the $\beta$-quartz to $\beta$-cristobalite metastable inversion is still unclear due to the reconstructive nature of this transition and sluggish kinetics. We measured the single-crystal acoustic velocities of $\alpha$- and $\beta$-quartz by Brillouin spectroscopy to a maximum temperature $>$$1500\deg$C at room pressure. From these data the single-crystal elastic moduli were calculated up to $1050\deg$C. This exceeds the temperature range of previous studies by $350\deg$C for elastic moduli and by $710\deg$C for acoustic velocities. The ordinary refractive index (n$_{o}$) of $\alpha$- and $\beta$-quartz was also measured from room temperature to $800\deg$C. Brillouin scattering is a technique for observation of phase transitions, as the change in acoustic velocities across the phase transitions can be a sensitive indicator of the appearance of small quantities of new phases. At a temperature of $1238\deg$($\pm$$5\deg$C)Ÿn in the [100] direction we observe the appearance of new non-quartz Brillouin peaks. The peaks persist on temperature decrease. The room-temperature velocities corresponding to the new non-quartz peaks are in good agreement with those of $\alpha$-cristobalite in $\sim$[100] direction. In the temperature interval from $\sim$$950\deg$C to $1000\deg$C a subtle but reproducible change in temperature derivative of the longitudinal acoustic velocity was observed in platelet geometry for all measured directions. The high-temperature acoustic velocity data are suggestive of a second phase (possibly $\beta$-cristobalite) below $1000\deg$C.
DE: 3630 Experimental mineralogy and petrology
DE: 3699 General or miscellaneous
DE: 3909 Elasticity and anelasticity
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3999 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting