Micro-Raman Spectroscopy in Thin Section Analysis of Rock Mineralogy

Importance of the Topic

Precise identification of minerals in rock thin sections is fundamental for petrographic and petrologic research and teaching. Optical microscopy is widely used but struggles with sub-100 micron grains and chemically continuous solid solutions, limiting detailed mineral phase discrimination.

Objectives and Overview

This study demonstrates how micro-Raman spectroscopy integrated with a standard petrographic microscope can enhance mineral phase identification in polished rock thin sections. It compares performance and costs with established high-resolution techniques such as SEM-EDS and electron microprobe analysis.

Methodology and Instrumentation

Micro-Raman analyses were performed using a portable B&W Tek i-Raman Plus system with 785 nm excitation interfaced to an Olympus BX-40 microscope. Objectives of 10x 50x and 150x produced spot sizes around 16 microns. Thin sections were mounted on glass slides with epoxy resin. Laser power settings and integration times were adjusted to minimize heating and maximize signal. BWSpec and BWID software were used for spectral acquisition and phase identification.

Key Results and Discussion

Raman spectra acquired from opaque grains clearly matched reference hematite peaks confirming reliable identification of iron oxides. Analysis of garnet grains in the spessartine almandine series showed A1g band shifts indicating 25 to 50 percent spessartine component. Plagioclase feldspar spectra displayed characteristic peaks near 481 and 510 inverse centimeters diagnostic of andesine composition. The strong luminescence background from glass slide and epoxy was recognized and excluded during phase assignment.

Benefits and Practical Applications

• Spatial resolution down to single-digit microns enables in situ analysis of fine accessory and alteration minerals
• Chemical and polymorph discrimination within solid solution series
• Seamless transition between optical and Raman modes on the same microscope platform
• Modest acquisition costs tens of thousands of dollars and minimal maintenance
• No requirement for dedicated climate controlled lab or specialized technicians

Future Trends and Potential Applications

Advances may include confocal Raman mapping of thin sections, automated stage scanning for rapid mineral distribution imaging, integration with chemometric libraries for automated phase recognition, and combined Raman fluid inclusion studies to reconstruct geochemical histories.

Conclusion

Integrating micro-Raman spectroscopy into standard petrographic workflows overcomes key limitations of optical microscopy enabling high spatial resolution and chemical phase identification in rock thin sections. Given low operational overhead and cost micro-Raman systems represent a valuable enhancement for academic research facilities agencies and consulting laboratories.

References

• Freeman JJ Wang A Kuebler KS Joliff BL Haskin LA 2008 Characterization of natural feldspars by Raman spectroscopy for future planetary exploration The Canadian Mineralogist v46 p1477 1500
• Kolesov BA Geiger CA 1998 Raman spectra of silicate garnets Physics and Chemistry of Minerals v25 p142 151
• Nasdala L Smith DC Kaindl R Ziemann MA 2004 Raman spectroscopy analytical perspectives in mineralogical research European Mineralogical Notes in Mineralogy v6 p281 343