Part 1 of this post introduced Raman spectroscopy as a technique for rapid identification of carbonate minerals. Here we’ll review the results of an experiment demonstrating that Raman spectroscopy is a powerful technique for analyzing many varieties of rock and mineral samples. Read the complete study in the application note, Raman Spectroscopy, Unveiling the Secrets of Limestone.
The samples were prepared by standard petrological techniques (sawing, grinding with SiC and diamond suspension). A 532 nm laser, providing between 5 and 9 mW at the sample surface, was used to obtain all spectra. Reflected and transmitted light optics were used to select and photograph analysis regions. The spectral acquisition times and distance between analysis locations were 2 s and 20 μm, 2 s and 10 μm and 12.5 s and 1.5 μm for the Strimbes Limestone, the oolitic limestone and the coral samples, respectively. The oolitic limestone sample fluoresced strongly and a photo bleaching period of 5 s was used prior to the acquisition of each spectrum.
Dolomite and Calcite in the Strimbes Limestone (Jurassic, Greece)
The eroded surface of this limestone has striking concentric circular textures. In order to understand how these textures formed, a Raman map of a thin section was collected to assess the correlation between the visually-observed textures and the mineralogy. As Figure 2a shows, the sample has alternating areas of coarse and fine grains. Typical Raman spectra of these two regions are shown in Figure 2b. Comparison to a spectral library identifies the coarse-grained material as calcite and the fine-grained material as dolomite. By processing the map to show the ratio of dolomite to calcite, as identified by their characteristic peaks at 1098 and 1086 cm-1 respectively, it is clear that the mineralogical difference between the layers is consistent and not limited to one or two spots, as illustrated by Figure 2c.
Cementation and Organic Material in an Oolitic Limestone (Carboniferous, Wales)
This sample below is a Visean (Carboniferous) limestone from south Wales (UK). This limestone is principally composed of ooids, carbonate spheres that usually form in tropical beach environments, illustrated by the cross section shown in Figure 3a. The mineralogy of ooids is usually either calcite or aragonite, which correspond to climatic greenhouse and icehouse conditions: being able to distinguish the two is important in interpreting the environmental significance of a particular rock formation. Raman spectroscopy is the only practical non-destructive way to distinguish the two minerals. A Raman map of the polished surface shows calcite to be the only carbonate mineral present, however, there are two interesting features of the map that provide further information. First, peaks at ~1350 and 1600 cm-1, shown in the spectrum in Figure 3b, indicate the presence of amorphous kerogen-like carbon. This is likely to derive from the breakdown of biological material.
Raman spectroscopy is the only technique that could have detected the amorphous carbon and distinguished it from graphite, which could be sedimentary in origin. The organic carbon is absent from the core of one ooid and from the cement between the grains, as shown in Figure 3c, indicating changes in the availability of biological material during the formation of the rock. The second interesting insight Raman spectroscopy provides is that the fluorescence of the sample varies within the ooids and between regions of the cement, and allows multiple phases of ooid growth and cementation to be identified, which is depicted in Figure 3d, e. Although the exact cause of the fluorescence is not known, the ability to distinguish multiple cementation events is still useful in determining the history of the rock. This works on the same principle as cathodoluminescence imaging.
Aragonite and calcite are both present in a coral sample, which is being used to determine the seawater chemistry at the time the coral was alive. Screening the sample to exclude later unrelated carbonates was essential. Many hard components of organisms are formed from either calcite or aragonite (or both), and Raman spectroscopy is the easiest and most convenient method of distinguishing the two minerals. An additional challenge that precluded the use of alternate techniques was that the sample was covered with a cover-slip. The confocal capabilities of the Raman microscope allowed the sample to be analyzed with minimal contribution from the surrounding glass and mounting glue.
Click here to read Part 1