Information on spatial relationships, mineral content, polymorphism, and chemical details can be generated by correlating SEM-EDS images with Raman microscopy images. The EDS shows the presence of titanium, aluminum, and silicon. The Raman image provides additional information and shows the location of quartz (blue), orthoclase (green), amphibole (yellow), and albite (red). Raman has identified the silica polymorph present in this sample to be quartz. It also has identified the K-feldspar polymorph to be orthoclase.
Scanning electron microscopy (SEM) offers exquisite, high-resolution imaging of samples in order to study their morphology, grain size, grain shape, and microstructural features. Despite these valuable insights, SEM alone cannot provide compositional data for a sample. For this reason, a technique such as energy dispersive x-ray spectroscopy (EDS) is often paired with SEM in the study of geological samples in order to provide “color” to greyscale SEM images. EDS is a microanalysis tool that uses the electron beam to inspire the emission of characteristic x-rays that can be used to identify the elemental composition of the sample. By scanning the electron beam across a region, compositional maps can be correlated with the electron microscopy image of the sample region. The correlation of the SEM and EDS data affords the ability to correlate microstructure to elemental composition.
Unfortunately, SEM and EDS have difficulty in sample speciation and morphology characterization. Polymorphic minerals with identical elemental compositions, but differing crystal structures, are critical to understand because their formation is dependent on temperature and pressure, and understanding these parameters helps to elucidate a geological sample’s origins and formation conditions.
Raman spectroscopy is a non-destructive, chemically-selective, optical technique that is used to characterize chemical backbone, polymorph structure, and degree of crystallinity. Additionally, Raman polarization measurements enable the elucidation of crystal orientation, making Raman spectral analysis perfect for understanding a material’s structure-function relationship.