Dynamics of the ?-? phase transitions in quartz and cristobalite as observed by in-situ high temperature 29Si and 17O NMR

Abstract

Abstract. Relaxation times (T1) and lineshapes were examined as a function of temperature through the c~-// transition for z9si in a single crystal of amethyst, and for 29Si and 170 in cristobalite powders. For single crystal quartz, the three z9Si peaks observed at room temperature, representing each of the three differently oriented SiO, tetrahedra in the unit cell, coalesce with increasing temperature such that at the c~-//transition only one peak is observed. 29Si Tl's decrease with increasing temperature up to the transition, above which they remain constant. Although these results are not uniquely interpretable, hopping between the Dauphin6 twin related configurations, el and ez, may be the fluctuations responsible for both effects. This exchange becomes observable up to 150~ below the transition, and persists above the transition, resulting in//-quartz being a time and space average of el and e2. 298i Tl's for isotopically enriched powdered cristobalite show much the same behavior as observed for quartz. In addition, 170 Tl's decrease slowly up to the e-// transition at which point there is an abrupt 1.5 order of magnitude drop. Fitting of static powder ~70 spectra for cristobalite gives an asymmetry parameter (t/) of 0.125 at room T, which decreases to <0.040 at the transition temperature. The electric field gradient (EFG) and chemical shift anisotropy (CSA), however, remain the same, suggesting that the decrease in t/is caused by a dynamical rotation of the tetrahedra below the transition. Thus, the mechanisms of the e-// phase transitions in quartz and cristobalite are similar: there appears to be some fluctuation of the tetrahedra between twin-related orientations below the transition temperature, and the//-phase is characterized by a dynamical average of the twin domains on a unit cell scale.

The α–β phase transition in volcanic cristobalite

“…This observation agrees with studies on synthetic cristobalite (e.g. Leadbetter & Wright, 1976;Swainson et al, 2003;Schmahl et al, 1992;Spearing et al, 1992), whereby a smooth change in the intensity of these modes is observed as the proportion of cristobalite existing as either or varies with temperature (x1.2). The coexistence interval is larger in volcanic samples than has been observed for synthetic samples; two potentially complementary factors could contribute to the observed coexistence interval (x1.2): cation substitution and crystallite size.…”

“…Furthermore, a proportion of grains may persist metastably after they have passed their transition temperature. Consequently, at a given temperature within the transitional range, and forms coexist both stably and metastably (Leadbetter & Wright, 1976;Swainson et al, 2003;Schmahl et al, 1992;Spearing et al, 1992). The -transition has been shown to depend on temperature, but not on time, demonstrating that the finite width of the transition range is not simply a kinetic effect (Schmahl, 1993;Leadbetter & Wright, 1976).…”

29Si and 17O (Q)CPMG-MAS solid-state NMR experiments as an optimum approach for half-integer nuclei having long T1 relaxation times

“…Some very long longitudinal relaxation times have been reported for 29 Si [10,13,15] and the CPMG-MAS technique may provide a valuable tool in this context as well because sensitivity enhancement by an order of magnitude may turn into a reduction of experiment time of several days. Previously the QCPMG-MAS approach has been applied to lineshapes being several kHz broad but in case of spin-1/2 nuclei the isotropic line width is in the range of 100's of Hz and therefore the separation of the spin-echo sidebands have to be adjusted such that 1=s a is smaller than the linewidth.…”

Semi-empirical Gibbs free energy formulations for minerals and fluids for use in thermodynamic databases of petrological interest

“…Castex and Madon 1995; Fig. 4a-c Comparison of calculated isobaric heat capacity (a) and molar volume (b,c) of brucite with measured data Spearing et al 1992). 298.15 K in Eq.…”