Confocal Raman Microscopy
Confocal Raman Microscopy is a relatively new technique that allows chemical imaging without specific sample preparation. By integrating a sensitive Raman spectrometer within a state-of-the-art microscope, Raman microscopy with a spatial resolution down t
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Raman Spectroscopy and Confocal Raman Imaging in Mineralogy and Petrography Marc Fries and Andrew Steele
Abstract Raman spectroscopy has long been used in geosciences and a wealth of data and publications are available. The majority of this information originates from point measurements using micro-Raman setups. With the application of confocal Raman imaging, additional analytical possibilities arise with respect to analyzing the three-dimensional spatial distribution of inorganic as well as organic phases on the centimeter to sub-micrometer scale. This chapter will highlight some of the key aspects experimenters should take into consideration when performing confocal Raman measurements as well as experimental results showing the insight gained into geological samples by the use of confocal Raman imaging.
6.1 Introduction Raman spectroscopy is especially well suited for use in mineralogy and petrographic studies, as it provides non-destructive mineral identification very quickly and with excellent specificity. Additionally, it is sensitive to the presence and structure of carbonaceous phases that are difficult to characterize by optical microscopy or electron beam methods. Raman spectroscopy has the advantage over electron beam methods that Raman spectra can reveal the presence of a wide variety of mineral phases in a single spectrum of sub-micrometer resolution, as opposed to an electron beam that performs an elemental analysis that can include any or all phases within the excitation volume. The practical result of this is that a Raman spectrum can identify individual mineral phases in fine-grained clusters, while electron probe analysis only produces the bulk composition of the intermixed phases. With the relatively recent development of confocal Raman imaging, definitive petrography (the study of rocks in thin section) is now possible in fields of view ranging from tens of micrometers to centimeters across. Data gleaned in this way are used in turn for petrology studies (the study of the formation and alteration of rocks primarily through petrographic analysis). An additional benefit of the use of a visible light excitation laser is the interrogation of features that are buried in the interior of mineral grains, such as opaque and fluid inclusions. All of this can be done on either prepared thin sections or unprepared mineral surfaces, which places Raman spectroscopic techniques very prominently in a mineralogist’s “tool kit” of analytical techniques. 111
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M. Fries and A. Steele
6.2 Raman Spectroscopy and Imaging as a Mineralogy/Petrography Tool Mineralogy and petrography techniques have traditionally relied heavily on optical microscopy, utilizing reflected, transmitted, and cross-polarized microscopy to identify minerals in thin section and describe their layout. The identities and textures of mineral phases are used to interpret the formation and subsequent alteration history of those minerals and their parent rock formation (Fig. 6.1). From there the investigator can draw conclusions about the histo
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