Micro-Raman Spectroscopy in Thin Section Analysis of Rock Mineralogy

Significance of the Topic

Accurate identification of mineral phases in rock thin sections underpins petrographic and petrologic studies but faces challenges when grain sizes fall below 100 microns or when minerals form solid solutions. Standard optical microscopy often cannot resolve fine-grained or compositionally variable phases, creating a need for complementary analytical tools that deliver both optical and chemical information at micron scales.

Objectives and Study Overview

This study evaluates the use of micro-Raman spectroscopy for in situ mineral identification in polished and covered rock thin sections. The goal is to demonstrate that a portable micro-Raman system interfaced with a conventional petrographic microscope can overcome limitations of traditional optical techniques and provide semi-quantitative compositional data for solid-solution series and polymorphs.

Methods

Mineral spectra were acquired using a 785 nm laser excitation focused to spot diameters of about 16 microns. Polished thin sections mounted on glass slides with epoxy were analyzed under transmitted light conditions. Laser power and integration times were optimized to minimize heating and fluorescence artifacts. Reference spectra from the RRUFF database were used for phase identification and compositional interpretation.

Instrumentation

• B&W Tek i-Raman Plus portable Raman system with 785 nm laser and BWSpec / BWID software
• Olympus BX-40 petrographic microscope with X-Y mechanical stage
• Metallurgical objectives: 10×, 50×, and 150×
• RRUFF spectral library for reference matching

Key Results and Discussion

• Opaque phases such as hematite were unambiguously identified by matching acquired spectra with reference patterns, overcoming limitations of reflected-light microscopy.
• Garnet solid-solution composition was estimated from shifts in the A1g Raman band, enabling semi-quantitative determination of spessartine-almandine ratios.
• Plagioclase members were distinguished by diagnostic peaks near 481 and 510 cm−1, allowing identification of andesine despite strong luminescence background from the glass slide and epoxy.
• Fine-grained accessory and alteration minerals were resolved in their textural context, demonstrating in situ applicability.

Benefits and Practical Applications

Micro-Raman spectroscopy integrated with existing petrographic microscopes offers:

• Sub-100 µm spatial resolution for chemical and structural analysis
• No need for specialized lab space or full-time technicians
• Modest acquisition and sample preparation costs
• Seamless switching between optical imaging and Raman measurement

Future Trends and Applications

Advances may include confocal Raman mapping, automated mineral classification, integration with hyperspectral imaging, and expansion of spectral libraries for improved quantification. Portable and field-deployable systems could enable rapid on-site petrographic analysis in exploration and environmental studies.

Conclusion

Micro-Raman spectroscopy represents a cost-effective, high-resolution enhancement to traditional petrographic methods. It enables reliable identification and semi-quantitative compositional analysis of fine-grained and solid-solution minerals directly in thin sections, making it a valuable tool for academic, governmental, and commercial geoscience 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, v 46, p 1477–1500.
• Kolesov BA, Geiger CA, 1998, Raman spectra of silicate garnets, Physics and Chemistry of Minerals, v 25, p 142–151.
• Nasdala L, Smith DC, Kaindl R, Ziemann MA, 2004, Raman spectroscopy: analytical perspectives in mineralogical research, European Mineralogical Notes in Mineralogy, v 6, p 281–343.