A wealth of information can be obtained from geological samples. Identifying which elements and minerals are present is just part of a much more detailed story that these samples can tell. Analysis of rocks and minerals reveals the conditions and environments under which they were formed, such as the temperatures, pressures, and chemical reactions that occurred during each phase of the rock formation. This information provides pieces of the puzzle that help geologists understand the history of the Earth and how this affects our current world. These studies are not only important for things like mineral and hydrocarbon exploration and extraction, but can also give insights into the prediction of natural disasters (earthquakes, floods, landslides, and volcanic activity), climatic changes and how this relates to the evolution of life, and locating and managing water resources. Understanding the past helps shape the present and foresee the future. A wide range of analytical techniques are used to determine the conditions under which rocks and minerals are formed. Geological samples can be difficult to analyze because some sample preparation methods are time consuming and costly, can disturb textures, introduce contamination, and cause damage to delicate features such as inclusions. One feasible method, however, is Raman spectroscopy. Raman spectroscopy is an established analytical technique for the analysis a variety of different types of geological samples because it offers the following advantages:
- Raman spectroscopy is fast, non-destructive, and requires little or no sample preparation.
- Raman spectroscopy identifies both the chemical composition and molecular structure of specific materials. Ever expanding databases of Raman spectra aid in the positive identification of minerals, while Raman spectroscopy can be used to evaluate various phases of the same or very similar chemical composition.
- Confocal Raman micro-spectroscopy can be used to characterize inclusions within samples without the need to expose the inclusions. Different solid, liquid, or gaseous phases within the inclusion can be studied this way. This type of analysis can also be useful for analyzing archived samples that have been mounted under glass cover slips.
- Raman spectroscopy can even be used to evaluate things like residual pressure and stress within a sample by looking at shifts in the Raman peaks.
Raman is a form of vibrational spectroscopy that is known to exhibit excellent selectivity for the purposes of material identification. In Raman spectroscopy, an unknown sample of material is illuminated with monochromatic (single wavelength or single frequency) laser light, which can be absorbed, transmitted, reflected, or scattered by the sample. Light scattered from the sample is due to either elastic collisions of the light with the sample’s molecules (Rayleigh scatter) or inelastic collisions (Raman scatter). Whereas Rayleigh scattered light has the same frequency (wavelength) of the incident laser light, Raman scattered light returns from the sample at different frequencies corresponding to the vibrational frequencies of the bonds of the molecules in the sample. Since the bonds for every molecule are different, the Raman scattering for every molecule is also different. Thus, a Raman spectral “fingerprint” can be generated by recording the intensity of light as a function of the frequency difference between the laser and Raman scattered light. Raman micro-spectroscopy analyzes very small samples or sample domains. Raman imaging, however, provides a powerful alternative where large areas of the sample can be analyzed and still preserve the spatial information. The vast amounts of data generated by Raman imaging gives a more statistically relevant analysis of the sample. See detailed data of just a few of the many geological samples that can be analyzed using Raman imaging.