Raman Imaging Easily Distinguishes Carbonate Minerals

Carbonates provide a wealth of information about sedimentary environments and ancient climates, as well as being important in the formation of oil reservoirs; however, interpreting the textures in a mined 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 (XRD) 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. Raman spectroscopy rapidly identifies carbonate minerals Raman spectroscopy is a non-destructive alternative that provides rapid identification of carbonate minerals. Raman spectroscopy is a versatile, highly sensitive vibrational technique that excels at identifying both organic and inorganic compounds in solids and liquids. Raman spectroscopy analyses the chemical composition and molecular structure of materials based on their specific Raman spectrum, which is unique for every mineral. With the information obtained from Raman spectroscopy, geologists and mineralogists can learn about material composition as well as 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 with little or no sample preparation. See data and analysis of calcite, dolomite, aragonite and organic carbon identified in various samples using a Raman imaging microscope.