Abstract

Reflectance spectroscopy in the visible and near-infrared (0.35 to 2.55 µm) offers a rapid, inexpensive, nondestructive tool for determining the mineralogy and for investigating the minor-element chemistry of the hard-to-discriminate carbonate minerals, and can, in one step, provide information previously obtainable only by the combined application of two or more analytical techniques. When light interacts with a mineral, certain wavelengths are preferentially absorbed. The positions and widths of absorptions due to vibrations of the carbonate radical, and the positions and shapes of absorptions due to transition metal cations, if present, can be used to determine mineralogy. Relative intensities of absorptions due to transition metal cations can be used to determine concentration of these cations. Because absorptions due to Fe2+ and Mn2+ occur in different regions of the spectrum, the presence of one cation does not interfere with analysis for the other. Detection limits for Mn2+ are about 0.1 wt% Mn and for Fe2+ are about 0.01 wt% Fe.

This technique is also very sensitive to the presence of water and indicates that aqueous fluid inclusions are nearly ubiquitous in carbonate minerals and rocks. Spectra indicate that inclusions are particularly abundant in skeletal carbonates and that the quantity of inclusions in skeletal material varies from one type of organism to another. Spectra indicate that inclusions are lost during diagenesis of skeletal material, providing a new criterion for diagenetic studies.

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