Micro-Raman Spectroscopy in Thin Section Analysis of Rock Mineralogy

Significance of the Topic

Micro-Raman spectroscopy in thin section analysis addresses the limitations of conventional optical microscopy for identification of rock mineral phases at sub-100 µm scale and for distinguishing members of solid-solution series. It enhances petrographic and petrologic investigations by providing both morphological and chemical information without extensive infrastructural requirements.

Objectives and Study Overview

This application note demonstrates the use of a portable Micro-Raman system to identify minerals in polished rock thin sections. The focus is on evaluating opaque phases, fine-grained accessory minerals, and solid-solution series such as garnet and plagioclase by comparing acquired spectra to reference libraries.

Methodology

Analyses were performed using a 785 nm excitation laser generating spot sizes from single-digit to several tens of micrometers. Spectra were recorded at full laser power with integration times of 2–10 s. Polished thin sections were mounted on glass slides with epoxy resin and examined under transmitted light. Laser power density was optimized to minimize thermal alteration artifacts.

Used Instrumentation

• i-Raman Plus portable Raman spectrometer (785 nm laser) with BWSpec and BWID software
• Olympus BX-40 polarizing microscope with X–Y mechanical stage
• Metallurgical objectives: 10×, 50×, 150×
• Standard glass microscope slides bonded with epoxy resin

Main Results and Discussion

• Opaque Minerals: Hematite was unambiguously identified by matching Raman peak positions and intensities to reference spectra, overcoming challenges in reflected-light microscopy.
• Garnet Solid Solutions: Shifts in the primary A1g band around 900 cm⁻¹ enabled estimation of spessartine content (25–50 mol %) within the almandine–spessartine series.
• Plagioclase Series: Diagnostic Si–O stretch peaks at ~481 and 510 cm⁻¹ confirmed andesine composition. Luminescence from the glass slide and resin appeared at higher shifts (1100–1900 cm⁻¹) but did not impede mineral identification.

Benefits and Practical Applications

• High spatial resolution mineral identification down to a few micrometers.
• Capability to distinguish polymorphs and quantify solid-solution compositions.
• Lower acquisition and operating costs compared to SEM–EDS or electron microprobe.
• No need for dedicated climate-controlled laboratories or specialized technicians.
• Seamless integration with existing petrographic microscopes for combined optical and chemical analyses.

Future Trends and Opportunities

• Automated Raman mapping and hyperspectral imaging for detailed mineral distribution mapping.
• Adoption of confocal Raman techniques for three-dimensional microanalysis.
• Advancements in laser sources and detectors to reduce fluorescence background and enhance sensitivity.
• Expansion of comprehensive Raman spectral libraries and application of machine-learning for rapid phase classification.
• Development of fully portable systems for in-field thin section analysis.

Conclusion

Micro-Raman spectroscopy provides a cost-effective, high-resolution approach for mineral phase identification in rock thin sections. It overcomes challenges inherent to optical microscopy for fine-grained and solid-solution minerals and offers an attractive enhancement for academic, governmental, and industrial geoscience laboratories.

References

• Freeman J.J., Wang A., Kuebler K.S., Joliff B.L., Haskin L.A., 2008, Characterization of Natural Feldspars by Raman Spectroscopy for Future Planetary Exploration: The Canadian Mineralogist, v.46, p.1477–1500.
• Kolesov B.A., Geiger C.A., 1998, Raman spectra of silicate garnets: Physics and Chemistry of Minerals, v.25, p.142–151.
• Nasdala L., Smith D.C., Kaindl R., Ziemann M.A., 2004, Raman Spectroscopy: Analytical Perspectives in Mineralogical Research: European Mineralogical Notes in Mineralogy, v.6, p.281–343.