Raman spectra and images were collected using a Witec α-Scanning Near-Field Optical Microscope that has been customized to incorporate confocal Raman spectroscopic imaging. The excitation source was a frequency-doubled solid-state yttrium-aluminum-garnet laser (532 nm) operating between 0.3- and 1-mW output power (dependent on objective), as measured at the sample using a laser power meter. Objective lenses used included a ×100 Long Working Distance (LWD) and a ×20 LWD, with a 50-μm optical fiber acting as the confocal pinhole. Spectra were collected on a Peltier-cooled Andor electron-multiplied charge-coupled device chip, after passing through an f/4 300-mm focal length imaging spectrometer typically using a grating of 600 lines/mm. The lateral resolution of the instrument was as small as 360 nm in air when using the ×100 LWD objective, with a focal plane depth of ~800 nm.

This instrument is capable of operating in several modes. Typically, 2D imaging and single spectra modes were used during this study. Single spectra mode allows the acquisition of a spectrum from a single spot on the target. Average spectra were produced typically using integration times of 30 s per accumulation and 10 accumulations to allow verification of weak spectral features.

Dependent on whether the sample was a thin section or fresh fracture surface, we used either transmitted or reflected light microscopy to locate the field of interest. Target areas were identified on the thin section in transmitted light. The microscope was then switched to reflected light and refocused to the surface, at which point X, Y, and Z piezos of the stage were reset. Switching back to transmitted light then allowed an accurate measurement of the depth of the feature of interest. The height and width of the field of interest within the light microscopy image were then measured and divided by the lateral resolution of the lens being used, to give the number of pixels per line. The instrument then acquired a Raman spectrum (0 to 3600 cm−1 using a grating of 600 lines/mm) at each pixel using an integration time of between 1 and 6 s per pixel. For depth profiles, stack scan mode was used with a depth spacing of 1 μm per scan for a maximum of 16 scans (16 μm) into the surface.

A cosmic ray reduction routine was used to reduce the effects of stray radiation on Raman images, as was image thresholding to reject isolated bright pixels. Fluorescence effects were inhibited by the use of specific peak fitting in place of spectral area sums and by the confocal optics used in this instrument. The effects of interfering peaks were removed by phase masking routines based on multiple peak fits, as compared with standardized mineral spectra. This produced an average spectrum over the number of pixels chosen in the area of interest. In a typical scan at ×100 of a 30-μm2 area, 100 pixels by 100 pixels were scanned, amounting to 10,000 separate and spatially aligned Raman spectra, from which average spectra from areas of interest were generated to ascertain the presence or absence of a certain peak.

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