Mineral Inclusion Analysis Made Easy with Raman Spectroscopy – Part 1

Correct mineral identification is essential to determining how rock formed. Mineral inclusions are important in petrology because they reveal important information about the early stages of an igneous or metamorphic event. In metamorphic rocks, mineral inclusions can preserve early fabrics and minerals that have subsequently been removed from the matrix of the rock by metamorphic reactions. In lavas, mineral inclusions can be extremely useful in determining the conditions of equilibration of the minerals. In ore bodies, sulphide melts can be trapped along fractures in quartz; the cracks subsequently “heal,” leaving numerous minute mineral inclusions distributed along a plane within a single quartz crystal. The small size of these inclusions can make it difficult to expose them for analysis, and their dispersed nature means only a few can be exposed at a given time. Diamond often contains mineral inclusions of geological interest but because diamond is the hardest known material, polishing down and exposing these inclusions is time-consuming and often costly. It’s best to identify mineral inclusions before they are exposed at the surface, but this is usually challenging because their small size precludes optical identification, and routine analytical techniques such as scanning electron microscope (SEM) based X-ray spectroscopy are limited to surface analysis. For some materials this isn’t practical, either because of the time required, the need to avoid chemical contamination of the inclusion that could occur during polishing, or because the material is rare and cannot undergo destructive preparation procedures. Infrared spectroscopy could in some cases be used to identify mineral inclusions, but would require double polished wafers that are often impractical to prepare. Raman spectroscopy is a relatively easy technique that can provide a wealth of information about the composition of microscopic mineral and fluid inclusions within minutes. Very little sample preparation is required and the Raman signal from the mineral matrix can often be excluded from the signal of the inclusion. Raman spectroscopy is also advantageous because of the high spatial resolution that is possible, especially when compared with X-ray diffraction and other analytical methods. This is a non-destructive process that only requires normal petrological preparation procedures. The information obtained in this way can constrain the origins of the material being studied, as well as indicating targets for future analysis by alternate methods (e.g. identifying accessory mineral phases that are important for trace element or isotopic studies). In my next article, we will have a look at three examples that demonstrate how a Raman imaging microscope can provide a powerful alternative method to analyze mineral inclusions.

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