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Optica Publishing Group
  • Applied Spectroscopy
  • Vol. 68,
  • Issue 7,
  • pp. 740-748
  • (2014)

Darkfield Reflection Visible Microspectroscopy Equipped with a Color Mapping System of a Brown Altered Granite

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Abstract

Visible darkfield reflectance spectroscopy equipped with a color mapping system has been developed and applied to a brown-colored Rokko granite sample. Sample reflectance spectra converted to Kubelka-Munk (KM) spectra show similar features to goethite and lepidocrocite. Raman microspectroscopy on the granite sample surface confirms the presence of these minerals. Here, <i>L*a*b*</i> color values (second Commission Internationale d'Eclairage [CIELab] 1976 color space) were determined from the sample reflection spectra. Grey, yellow, and brown zones of the granite show different <i>L*</i>, <i>a*</i>, and <i>b*</i> values. In the <i>a*</i>-<i>b*</i> diagram, <i>a*</i> and <i>b*</i> values in the grey and brown zones are on the lepidocrocite/ferrihydrite trends, but their values in the brown zone are larger than those in the grey zone. The yellow zone shows data points close to the goethite trend. Iron (hydr)oxide-rich areas can be visualized by means of large <i>a*</i> and <i>b*</i> values in the <i>L*</i>, <i>a*</i>, and <i>b*</i> maps. Although the present method has some problems and limitations, the visible darkfield reflectance spectroscopy can be a useful method for colored-material characterization.

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