Raman spectroscopy is a powerful technique that can be used to analyze many varieties of rock and mineral samples. With the information obtained from Raman spectroscopy, geologists and mineralogists can learn about material composition and infer information about the environment local to the sample at the time of its formation. Raman spectroscopy is advantageous because it is relatively easy to obtain spectra from just about any sample conformation, even under a glass coverslip and mounting glue.
In the next two posts, we’ll discuss Raman spectroscopy as a technique for rapid identification of carbonate minerals and for determining relevant information that facilitates interpretation of these rocks. Raman spectroscopy is especially beneficial for this application because it can be performed on unpolished sawn surfaces of rocks, so minimal sample preparation is required to perform an analysis.
Carbonates provide a wealth of information about sedimentary environments and ancient climates, and they are important in the formation of oil reservoirs. While interpreting the textures in a rock sample can be challenging, these interpretations are important for a number of reasons. The rock record allows us to investigate the relationship between ocean currents, seawater chemistry and global warming and cooling, thus informing predictions about what the climate might do in the future. Similarly, many limestones are a snapshot of ancient ecosystems and allow us to investigate how the remarkable array of modern life may have arisen. The high solubility of carbonate minerals in water means that these rocks often undergo complex alteration, known as diagenesis, after their initial formation. Interpreting the extent of diagenesis can be crucial for distinguishing information about the time of sediment deposition from secondary processes, and the chronology of diagenesis controls whether the rock could contain oil. For example, a rock in which the pores between grains are sealed at an early stage by a carbonate cement may be less effective as a reservoir than a highly porous limestone. However, a non-porous limestone could act as the seal on a reservoir, but only if the cement sealing the pore space grew early on, before the oil escaped.
Carbonate minerals such as calcite, dolomite, and aragonite have similar optical properties and distinguishing them is often difficult. Optical techniques also require the preparation of thin sections, which is time-consuming and consequently expensive. Methods such as X-ray diffraction lack spatial resolution (in most cases being restricted to examination of crushed rock or single crystals extracted from a sample) and electron microscopy techniques require polished and carbon-coated surfaces that may not be possible on the material provided (e.g. traditional covered thin sections). Moreover, calcite and aragonite cannot be distinguished by compositional data.
Stay tuned for Part 2 of this post which describes an experiment in which Raman instruments are used to identify calcite, dolomite, aragonite, and organic carbon in various samples, or read the application note, Raman Spectroscopy, Unveiling the Secrets of Limestone.
Note: Part 2 will be published October 18, 2016