Raman spectroscopy of CO2, H2O and N2 in cordierite

The Fe-Mg silicate cordierite has a broad field of interest in modern research. Because of its low thermal expansion coefficients and dielectric constant its synthetic form is used for numerous technical applications like ceramics in microelectronics or diesel particulate filters. Large, natural, bluish and inclusion-poor single crystals named "Ijolite" are used as gemstones. In Earth Science cordierite is used amongst others for the reconstruction of pressure and temperature conditions of metamorphic rocks and the composition of the fluid phase, which is involved in almost all geologic processes. CO 2 , H2 O, N2 and large cations like sodium and potassium can be incorporated in the channel-like structure of cordierite. This influences its stability conditions and results of thermobarometric calculations. The exact knowledge of the chemical composition therefore is very important. Micro-Raman spectroscopy allows fast and non-destructive in-situ measurements of both single crystals and intergrown crystals in rock thin sections. The reachable volume resolution is lower than 5m3 . Aim of this project is the improvement, development, respectively, of a semi-quantitative method based on micro- Raman spectroscopy for the in-situ determination of CO 2 , H2 O and N2 contents of cordierite single crystals and crystals in rock thin sections. For this purpose relative intensities of characteristic vibrational modes of CO 2 , H2 O and N2 will be used. Furthermore, by investigating the linear shift of certain vibrational modes fast and non- destructive determination of the iron-magnesium ratio at the micrometer scale is possible. Synthetic cordierite solid solutions will be used for calibration purposes. CO 2 , H2 O and N2 contents of the synthetic cordierite will be determined by independent measurements using Infrared Spectroscopy, Thermogravimetry - Differential Thermoanalysis and Coulometric Titration. The developed method will be applied to cordierite from several high-grade metamorphic areas in order to determine its CO 2 and H2 O content. Variations of the main, trace element and fluid content of the cordierite will be determined by electron probe micro analysis at a high spatial resolution. Automated Micro-Raman spectroscopic measurements on selected samples shall deliver indications of volatile zoning of single grains. A better understanding of volatile distribution in cordierite from high-grade metamorphic crustal rocks will contribute to a better reconstruction of their evolution history.

Cordierite, an important mineral in the Earths crust but also in numerous technical applications, was synthesized and a method for the quantitative determination of carbon dioxide, water or nitrogen contents by Micro-Raman spectroscopy was developed. This technique, which is based on the interaction of light with mater, allows analysis of crystals down to a thousandth of a millimetre. Furthermore, changes of the iron-magnesium ratio at this microscopic scale are detectable. The method was applied to natural and synthetic cordierite crystals and contributed to a better understanding of its growth, crystal structure, physical properties, phase transitions and reconstruction of the geologic history of host rocks.Because of its low expansion at changing temperatures and electrically insulating properties the synthetic form of cordierite is used for ceramics in microelectronics, diesel particulate filters or catalytic converters. The findings of the project provide the basics for the development of possible new methods of manufacturing and applications. In Earth Science cordierite enables calculations of pressure and temperature of rocks, its temporal evolution and of the chemical composition of the almost always involved fluid phase. In the channel-like structure the large incorporated molecules carbon dioxide, water or nitrogen influence the results of such calculations. Exact knowledge of their quantity and distribution therefore is very important. Fast and non-destructive measurement of high-quality, inclusion-free and transparent cordierite named Ijolite and the closely related beryl minerals emerald and aquamarine, are of interest for provenience analysis in the gemstone industry and archaeology.