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[SCG58-04] Study of the mechanism of color change due to weathering of granite: Example of Mt. Gagara, Hiroshima
Keywords:Granite, Weathering, Color, Iron hydroxide
It is known that when granite is weathered, iron (hydr)oxides are formed and the overall color changes to brown or yellow. In this study, granite cores drilled to a depth of 20 m below the surface at Mt. Gagara, Hiroshima were studied in order to understand the characteristics and mechanisms of color change associated with weathering of granite.
A spectrophotometer was used to measure the color at each depth (approximately every 2-10 cm) of the borehole core and the color of standard samples of four minerals (goethite, ferrihydrite, lepidocrocite, and hematite) as iron (hydr)oxides that can form during weathering, respectively. The measured colors were quantified according to the L*a*b* color space, where the higher the L* the brighter, the higher the a* the more reddish for a*>0 , and the higher the b* the more yellowish for b*>0. The standard sample of iron (hydr)oxides is a white powder of SiO2 mixed with various mineral contents (0-100%). Goethite is the most yellowish and hematite is the most reddish. The color of the borehole cores is white in the unweathered part at depths below ∼12 m. The a* and b* values increase stepwise with the progress of weathering from ∼12 m to ∼4 m depth. The color distribution of the granite at various weathering degrees plotted on the a*-b* diagram is consistent with the trend of color change with increasing goethite content. Thus, the overall trend appears to be an increase in goethite content with progressive weathering. In addition to this overall trend, yellow to brown coloration extends around the fractures in various parts of the core, starting from the fractures, and the area near the front edge of the colored area is often dark brown in color. The color of the dark brown area is closer to that of ferrihydrite than to goethite, and ferrihydrite is likely to be present in the dark brown area. In aqueous solution, ferrihydrite is known to transform to goethite and hematite over time. There is a possibility that ferrihydrite is formed in the initial stage of the reaction, in which water infiltrates the rock matrix through fractures, subsequently ferrihydrite is converted to goethite over time. Since in most cases the dark brown areas are limited to near the front edge of the coloration, it is possible that the concentrated areas of ferrihydrite moved deeper into the rock matrix as water infiltrated the matrix, and the goethite formed along the way remained in place. We speculate that this process will continue as weathering progresses, resulting in a change in the overall color of the rock in the direction of increased goethite content, but these details are a subject for future study.
A spectrophotometer was used to measure the color at each depth (approximately every 2-10 cm) of the borehole core and the color of standard samples of four minerals (goethite, ferrihydrite, lepidocrocite, and hematite) as iron (hydr)oxides that can form during weathering, respectively. The measured colors were quantified according to the L*a*b* color space, where the higher the L* the brighter, the higher the a* the more reddish for a*>0 , and the higher the b* the more yellowish for b*>0. The standard sample of iron (hydr)oxides is a white powder of SiO2 mixed with various mineral contents (0-100%). Goethite is the most yellowish and hematite is the most reddish. The color of the borehole cores is white in the unweathered part at depths below ∼12 m. The a* and b* values increase stepwise with the progress of weathering from ∼12 m to ∼4 m depth. The color distribution of the granite at various weathering degrees plotted on the a*-b* diagram is consistent with the trend of color change with increasing goethite content. Thus, the overall trend appears to be an increase in goethite content with progressive weathering. In addition to this overall trend, yellow to brown coloration extends around the fractures in various parts of the core, starting from the fractures, and the area near the front edge of the colored area is often dark brown in color. The color of the dark brown area is closer to that of ferrihydrite than to goethite, and ferrihydrite is likely to be present in the dark brown area. In aqueous solution, ferrihydrite is known to transform to goethite and hematite over time. There is a possibility that ferrihydrite is formed in the initial stage of the reaction, in which water infiltrates the rock matrix through fractures, subsequently ferrihydrite is converted to goethite over time. Since in most cases the dark brown areas are limited to near the front edge of the coloration, it is possible that the concentrated areas of ferrihydrite moved deeper into the rock matrix as water infiltrated the matrix, and the goethite formed along the way remained in place. We speculate that this process will continue as weathering progresses, resulting in a change in the overall color of the rock in the direction of increased goethite content, but these details are a subject for future study.