Natural rocks are very complex and contain an aggregate of one or more minerals. Defined by its crystalline structure and chemical composition, each mineral can sometimes contain fluid inclusions. In order to obtain in-depth information on the history of rock formation, a powerful characterization technique is required. Raman spectroscopy is one such technique that can be used for geological studies, since it has high spatial resolution and non-destructive properties.
Specificities of Raman Spectroscopy
An important aspect of Raman effect is that it is extremely sensitive to even small amount of differences in chemical structures. Even if all vibrations are not observed with Raman spectroscopy, adequate amount of data can be acquired to differentiate varied structural phases or groups associated with the same mineral class, such as carbonates or silicates.
A high spectral resolution is sometimes required to resolve peaks that are spaced closely. This is the case when particular temperature and pressure conditions are applied to a mineral using, for instance, a diamond anvil cell (DAC) to simulate what can happen deep in the earth.
Multiple laser excitations provided by specific systems can effectively offset the potential fluorescence of geological samples. Raman spectroscopy provides data with high spatial resolution of < 1μm when coupled to confocal microscopy. Moreover, on-site analysis can be performed using mobile systems and remote sampling accessories
Silicates: An Important Class of Rocks
Silicate minerals, also known as silicates, are an important class of rocks that form the Earth’s crust and mantle. A silicate is a compound that basically consists of silicon and oxygen tetrahedral, along with other elements such as potassium, magnesium, aluminum (aluminosilicates), calcium, iron, etc. The Raman spectra of the different classes of silicates provide instant identification of the specific class. Figure 1 shows some of the different arrangements of fused tetrahedral.
Figure 1. Different tetrahedral arrangements
The most common minerals of the igneous rocks type is olivine. The Raman spectrum of this mineral reveals high bending modes from 300 to 650cm-1 and stretching modes between 800 and 1000cm-1.
Figure 2. Raman spectrum of olivine
The term inosilicates is referred to chain silicates and these are categorized into two groups, such as amphiboles and pyroxenes. Similar to jadeite, the Raman spectra of inosilicates present a high Raman band from 650 and 700cm-1. Pyroxenes include single chains of SiO4 groups, while amphiboles are composed of double chains of tetrahedra.
Zeolites and feldspars are part of the tectosilicates groups. Zeolites are rarely pure because of their porous properties and occur in different types of frameworks. Feldspars can include three end-membered elements, such as calcium (anorthite), sodium (albite) and potassium (orthoclase).
Carbonates are similar to silicates and occur in large amounts on Earth’s surface. They are differentiated by the presence of a carbonate ion CO3. Depending on the divalent cation (Ca2+, Fe2+, etc) coordinated to the carbonate ion, the Raman spectra of this carbonate are slightly different.
Figure 3. a- Raman imaging of different types of carbonates crystals using the Duo-San option and their associated spectra b- High spectral resolution map of two dolomite crystals based on the small spectral shift existing between the two crystals spectra.
A test sample comprising different types of carbonates was imaged using the DuoScan option in macro-mapping mode. The low frequency region was examined and six different species were easily detected. Their related spectra and distribution are shown in figure 3a.
Another map in standard point-by-point mode was performed on a small area comprising just two dolomite crystals. The LabRAM HR with its high spectral resolution helped in detecting the small frequency shift present between the two carbonates, the green one has traces of Fe, as shown in figure 3b. Both carbonate and silicate include various species, each of them having characteristic Raman bands. This shows that Raman spectroscopy is a suitable method for rapid identification of these minerals.
Minerals are usually composed of solids or fluids, which can offer information about the conditions existing during the mineraliziation. Gas or liquid bubbles are dubbed as fluid inclusions. A Raman system combined to a confocal microscope is needed to achieve high spatial resolution and also to study inclusions located under the surface sans destruction.
Figure 4. Raman spectra obtained in different areas of the fluid inclusion
Due to their size and location in minerals, it is often difficult to analyze fluid inclusions. However, Raman spectroscopy makes it easy to determine the composition of the trapped fluids. In fact, fluid inclusions can also be imaged, and with the help of corrected objective higher signals can be obtained.
Raman spectroscopy is a suitable technique for analyzing geological materials. Information on different types of chemicals can be achieved without any sample preparation or extraction process. Moreover, Raman imaging is also effective for analyzing heterogeneous samples delivered by different fields of application of Earth Science.
This information has been sourced, reviewed and adapted from materials provided by HORIBA Scientific.
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