Quantitative analysis of rock and mineral surface color for ornamental and semi-ornamental rocks using image scanner

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  • MOTOKI Akihisa
    Departamento de Mineralogia e Petrologia Ignea, Universidade do Estado do Rio de Janeiro (DMPI/FGEL/CTC/UERJ)
  • VARGAS Thais
    Departamento de Mineralogia e Petrologia Ignea, Universidade do Estado do Rio de Janeiro (DMPI/FGEL/CTC/UERJ)
  • NEVES José Luíz Peixoto
    Departamento de Mineralogia e Petrologia Ignea, Universidade do Estado do Rio de Janeiro (DMPI/FGEL/CTC/UERJ)
  • ZUCCO Lóris Lodir
    Colégio Técnico da Universidade Federal Rual do Rio de Janeiro (CTUR/UFRRJ)

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  • イメージスキャナーによる装飾石材，準装飾石材の全岩及び鉱物表面反射色の定量測定法
  • イメージスキャナー ニ ヨル ソウショク セキザイ ジュンソウショク セキザイ ノ ゼン ガン オヨビ コウブツ ヒョウメン ハンシャショク ノ テイリョウ ソクテイホウ

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Abstract

This paper describes a method of quantitative reflection color analysis for minerals and whole rock using an image scanner and a general use personal computer. The scanner obtains true color (24 bits) digital images of polished rock surfaces. The software, Wilber, that has been developed for the present project, analyzes the color bitmap information of the area indicated manually by the operator and calculates mean value and standard deviation of RGB, HSB, XYZ, and x-y parameters.<br> Mineral and whole rock colors are generally weak (low saturation) and highly heterogeneous (high hue dispersion) in comparison with artificial objects, animals, and plants. This makes rock color measurement technically difficult, especially in the case of coarse-grained plutonic and metamorphic rocks. To solve the problem, hundreds of trial-and-error experiments have been performed, arriving at the following recommendations.<br> The best optical resolution of the scanner for mineral color and whole rock color analyses is, respectively, 150 dpi (dot per inch) and 75 dpi. The x-y chromaticity diagram, H-S circular diagram, H parameter, and S parameter are convenient to express the measured mineral colors. The RGB color system is not recommended for this use because of strong random reflection effects.<br> By means of S parameter (saturation) of whole rock color, ornamental and semi-ornamental rocks are classified into high-color (S>10.0), middle-color (8.0<S<10.0), low-color (6.0<S<8.0), slight-color (4.0<S<6.0), and grayscale (S<4.0) groups. B parameter (brightness) is efficient for quantitative color evaluation of the grayscale rocks. Using this parameter, they can be subdivided into black (B<25), dark gray (25<B<55), light gray (55<B<85), and white (B>85) groups. The standard deviation of B parameter may represent color hardness for gray granitic rocks. For evaluation of the high-, middle-, and low-color rocks, the H parameter (hue) is recommended. They can be classified into red (0°<H<20°), orange (20°<H<40°), and yellow groups (40°<H<60°). Sodalite syenite and charnockite are classified to be grayscale rocks due to their low color saturation. However, it is recommended that they are exceptionally evaluated by S parameter, or color intensity, because the market price is highly related to their color.

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