R4: Mineral sciences of the Earth surface

[R4-02] Color change process of Hiroshima granite due to weathering

*Tadashi YOKOYAMA¹, Yuka Inkyo, Masahiro Kaibori¹ (1. Hiroshima University)

Keywords:Granite, Weathering, Color

The characteristics and mechanisms of color change associated with weathering of Hiroshima granite were investigated, using a core drilled to a depth of 20 m. Color (L* a* b* values) were measured at approximately every 2-10 cm at each depth using a spectrophotometer. The color is: brighter for larger L* values, more reddish for larger a* values (for a* > 0), and more yellowish for larger b* values (for b* > 0). For core color, whitish unweathered areas at depths greater than 12 m have smaller a* and b* values and larger L* values. From about 12 m to 4 m depth, both a* and b* values increase and L* values decrease closer to the surface, although there is some variation depending on the location. The color at each depth was compared with that of a reference material made from four typical iron-bearing secondary minerals (goethite (yellow), ferrihydrite (dark brown), lepidocrocite (light brown), and hematite (dark red)), where each mineral mixed with SiO2 powder in various proportions from 0-100%. In the early stage of weathering, a dark brown band close to the color of ferrihydrite is often seen around the fractures. For more weathered areas, the entire rock matrix is often yellowish, the color close to that of goethite. In general, ferrihydrite is known to transform to more stable goethite and hematite with time. From the color measurement results of the drilled cores, it is inferred that ferrihydrite was first formed in the early stage of weathering, and then changed from ferrihydrite to goethite with the progress of weathering. Secondary iron minerals were dissolved and quantified by the selective iron dissolution method at several locations with different degrees of weathering. The amount of secondary iron minerals (all assumed to be goethite) at each location was estimated by comparing the color of the core with that of the reference material and plotted against the values obtained by the selective iron dissolution method, showing a generally proportional relationship. Although the results may change after correction for the effect of ferrihydrite, the results indicate that it may be possible to roughly estimate the amount of secondary iron minerals from a quick and easy core color measurement alone.