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Spectral reflectance in the visible and near infrared (0.35 to 2.5 µ) offers a rapid, inexpensive, nondestructive technique for determining the mineralogy and minor element chemistry of the hard-to-identify carbonate minerals, and can, in one step, provide information previously obtainable only by the combined application of two or more techniques.
When light interacts with a mineral, certain wavelengths are preferentially absorbed. The positions, intensities, and widths of these absorption bands are diagnostic of the sample's mineralogy and chemical composition. Absorption bands in the 1.5 to 2.5 µ region are due to vibrations of the carbonate radical, and their
precise positions are used for mineral identification. At shorter wavelengths, absorption features are due to electronic processes within the partly filled d-shells of transition metal ions such as Fe+2 and Mn+2. Positions, intensities, and widths of these bands allow determination of the chemical composition of the mineral. Studies indicate detection limits for these cations are less than 0.5 mole %. The figure shows a typical carbonate spectrum and the sources of the absorption bands.
Absorption bands caused by water also occur in reflectance spectra. The exact shapes and positions of these bands indicate the form in which the water occurs (i.e., as bound water in clay minerals, or as liquid water in fluid inclusions), and relative band intensities indicate the amount of water present. Fluid inclusions appear to be nearly ubiquitous in carbonate rocks and are particularly abundant in skeletal material. Diagenesis of skeletal material results in loss of a large percentage of these inclusions, and spectral studies can be used to monitor these changes.
Reflectance spectra may be obtained from powders, sands, rock surfaces, and thin sections.
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