10:45 AM - 11:00 AM
[SMP43-07] Determination of channel CO2 contents in random cordierite crystals using Raman spectroscopy
Keywords:Cordierite, Raman spectroscopy, Crystal orientation, CO2 determination
For the identification of crystal orientation of cordierite, euhedral cordierite crystals, from the volcanic ash deposit in the Takiga swamp, Gunma Prefecture, Japan were used to prepare crystallographically oriented thin sections, and examined in detail using micro-Raman spectroscopy. In addition, to examine the effect of crystal orientation to the intensity of CO2 for its determination, two cordierite samples were analyzed. One is cordierite crystal from a pelitic cordierite-bearing from gneisses in the Kerala Khondalite Belt (KKB), southern India, and the other is a standard cordierite with known CO2 contents (SH). Since Raman spectral intensity also depends on polarization of the incident laser, Raman spectra were obtained by rotating the sample at an interval of 10°. The crystal orientation of cordierite was cross-checked by using 5-axis universal stage and conoscopic figures.
Raman spectral patterns obtained for (001), (100) and (010) crystallographic planes change cyclically with the polarization of incident laser. We selected six peaks of cordierite (1: 554 cm-1, 2: 575 cm-1, 3: 670 cm-1, 4: 970 cm-1, 5: 1010 cm-1, 6: 1180 cm-1) for a detailed analysis. The intensity of peak-5 and peak-6 changed systematically when compared with other peaks, and so these peaks were used for the identification of crystal orientation. The intensity of peak-3 did not change and we used it as a normalizing peak for instrumental intensity variations. The intensity ratio of peak-5/ peak-3 versus intensity ratio of peak-6/ peak-3 (I5/ I3 vs. I6/ I3) in (001), (100) and (010) plane showed a linear relation. The value of other oriented cordierite crystals and random ones fell within this range. Therefore, it is possible to identify the crystal orientation of cordierite using the relation of I5/ I3 vs. I6/ I3. The cyclic patterns can be expressed mathematically using a combination of sine curves, where it is possible to determine the crystal orientation. Furthermore, the peak intensity of CO2 for SH cordierite with known CO2 contents also showed cyclic variations, similar to the periodicity of the peak-6 in the crystallographically oriented crystals. Accordingly, using the mathematical expression we could retrieve the maximum peak intensity of CO2 at 1383cm-1 from a random crystal, which was then used for determining the CO2 contents of unknown cordierite crystals.
Kaindl, R., Tropper, P. and Deibl, I (2006) A semi-quantitative technique for determination of CO2 in cordierite by Raman spectroscopy in thin sections. European Journal of Mineralogy, 18, 331-335