High-pressure Raman spectroscopy on low albite

The pressure dependence of the Raman spectrum of low albite, NaAlSi3O8, has been investigated from 0.0001 to 10.4 GPa, at room temperature, on a single crystal compressed hydrostatically in a diamond anvil cell. The Raman vibrational features move to higher wavenumbers \u3c5i with increasing pressure, due to the decrease in the unit-cell volume corresponding to a drastic shrinkage of the framework. The slopes \u394\u3c5i/\u394P of the four investigated bending modes (i.e. at 478, 507, 578 and 815 cm 121, at 0.0001 GPa) show evident changes at ~6.5 and ~8.5 GPa. This behaviour may be ascribed, in the absence of phase transitions, to the evolution of the compressional mechanisms at the atomic scale found in previous high-pressure studies on albite (mainly by X-ray diffraction), through a model based on tilts of rigid tetrahedra. The Raman data of this study allowed also to bracket the pressure range in which the occurrence of the first change in the compressional behaviour was found by X-ray diffraction

High-pressure Raman spectroscopy of Ca(Mg,Co)Si2O6 and Ca(Mg,Co)Ge2O6 clinopyroxenes

In situ high-pressure Raman spectra were collected on four pyroxenes, with composition CaCoSi2O6, CaMgSi2O6, CaCoGe2O6 and CaMgGe2O6, up to P = 7.6 and 8.3 GPa for silicates and germanates, respectively. The peak wavenumbers \ucf\u85i increase almost linearly with pressure; the slope d\ucf\u85i/dP is more pronounced for the modes at higher wavenumbers, and higher in germanates than in silicates. No phase transition or change in the compressional behaviour was observed within the investigated P-range. The strong dependence of the peak position with pressure of the high energy stretching modes is due to the high sensitivity of the vibrational frequencies probed by Raman spectroscopy to subtle changes in the tetrahedral deformation, which are overlooked by single-crystal X-ray diffraction

Al-Si ordering in albite : a combined single-crystal X-ray diffraction and Raman spectroscopy study

Raman spectra of a low albite single crystal were collected during annealing at 1076 degrees C for up to 46 days. At seven annealing stages, the single crystal structure was refined by X-ray intensity data to determine the degree of Al-Si order using the tetrahedral bond distances. Single-crystal X-ray diffraction showed that residuals in the difference-Fourier map of the electron density and atomic displacement parameters of Si, O, and, most, Na, increase with Al-Si disorder. The Raman spectra show a significant broadening with disorder, and some slight peak shift. Three strong peaks, at 290 cm(-1) (nu(c)), 478 cm(-1) (nu(b)), and 507 cm(-1) (nu(a)) in ordered albite, were examined in further detail. nu(c) and nu(b) show a red-shift with broadening and Al-Si disorder; nu(a) blue-shifts with disorder and shows only a minor broadening. The broadening and shifts in Raman spectra are caused by structural deformation associated with Al-Si disorder. The nu(a) peak at 507 cm(-1) is the least affected by Al-Si disorder and is suitable to assess compositional changes in plagioclase. The Al-Si order can be determined in albite by the wavenumber difference Delta(ab) between the two main peaks nu(a) and nu(b) as follows: Q(od)(2) = 9.50(75) - 0.307(25)Delta(ab), R-2 = 0.94, where Q(od) is an order parameter derived from average tetrahedral Al-O and Si-O bond distances

Raman spectroscopy of CaCoSi2O6-Co2Si2O6 clinopyroxenes

Raman spectra were collected on a set of synthetic clinopyroxenes along the series CaCoSi2O6–Co2Si2O6. Changes in peak position and peak width show: (1) evidence of a phase transition from C2/c to P21/c, at Ca0.4Co1.6Si2O6, in agreement with previous X-ray observations; (2) peak broadening for intermediate compositions, with sharper peaks close to the end members. The phase transition is revealed by a decrease or inversion in the slope of the peak position versus composition and by peak splitting of the peaks at 660 and 1,000 cm−1, related to Si–O bending and stretching modes within the tetrahedral chains, respectively. The observed changes with composition depend more on variation in bond lengths due to structural rearrangement with cation substitution, rather than by changes of the M2 cation mass. A comparison with the structurally analogous CaMgSi2O6–Mg2Si2O6 (Diopside-Estatite, Ca-Mg) series shows that one of the two splitted peaks is fainter than the Ca–Co pyroxenes. Therefore the frequency of the peak at about 1,000 cm−1 does not change for Ca–Mg substitution, whereas it shifts by as much as 20 cm−1 between CaCoSi2O6 and Co2Si2O6. Despite the mechanism of cation substitution is qualitatively similar in the two series, the effect of structural changes and polyhedral deformation on the Raman spectra appeared different. Peak broadening in samples with intermediate compositions could be interpreted as arising by compositional disorder, due to coexistence of local Ca-rich and Co-rich configurations which affect the short range interactions and therefore the Raman frequencies