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[O02-03] The rainbow color of scoria by a structural color
★Invited Papers
Keywords:volcanic eruption, structural color, scoria, micotexture
Structural color refers to the color produced by the physical effect of light on the microtexture of a material, rather than the color of the material itself. We recognized some brilliant rainbow-colored scoriae in the 1986 deposits of the Izu-Oshima eruption. We hypothesized that the rainbow color is a structural color caused by the microstructure of the scoria surface, as its color is unlikely to be the material color of the scoria. Based on the microscopic observations and elemental analysis of the scoria, we report the mechanism of rainbow brilliance on the Izu-Oshima scoria and discuss the relationship between colors in nature and art.
The color of the scoria brilliance changed from blue, yellow to red from the outside to the inside of the scoria, changing its transparency from transparent to opaque (metallic luster). Optical and electron microscopic observations showed that the dense aggregation of composite minerals (spherulites) with sizes slightly smaller than the wavelength of visible light was distributed on the surface of the scoria. The size of the spherulites was increased from the blue, yellow to red regions. The difference in transparency was probably sourced from the covered ratio of a mineral with a high reflective index. Based on these observations, we concluded that the rainbow brilliance of the scoria is a structural color due to the size of or the gap between the spherulites.
The rainbow-colored brilliance attracts human eyes. The structural colors well known in nature are the wings of some insects (e.g., jewel beetles and Morphos) and the feathers of some birds (e.g., peacock and mallard duck), which are acquired characters during their evolution. In the case of the scoria, on the other hand, the color is not an evolutionary character. When the rainbow color attracts human eyes, it must be due to human factors. Various colors from a single object and a regular order of colors may often be attractive to humans, as their information largely relies on visual sights. In other words, the brilliant rainbow scoria itself is not an art, but the visual insight through humans makes the scoria an art. The colors produced by the interaction between materials and light are transformed into an “art” when they are perceived as beautiful through human eyes.
The color of the scoria brilliance changed from blue, yellow to red from the outside to the inside of the scoria, changing its transparency from transparent to opaque (metallic luster). Optical and electron microscopic observations showed that the dense aggregation of composite minerals (spherulites) with sizes slightly smaller than the wavelength of visible light was distributed on the surface of the scoria. The size of the spherulites was increased from the blue, yellow to red regions. The difference in transparency was probably sourced from the covered ratio of a mineral with a high reflective index. Based on these observations, we concluded that the rainbow brilliance of the scoria is a structural color due to the size of or the gap between the spherulites.
The rainbow-colored brilliance attracts human eyes. The structural colors well known in nature are the wings of some insects (e.g., jewel beetles and Morphos) and the feathers of some birds (e.g., peacock and mallard duck), which are acquired characters during their evolution. In the case of the scoria, on the other hand, the color is not an evolutionary character. When the rainbow color attracts human eyes, it must be due to human factors. Various colors from a single object and a regular order of colors may often be attractive to humans, as their information largely relies on visual sights. In other words, the brilliant rainbow scoria itself is not an art, but the visual insight through humans makes the scoria an art. The colors produced by the interaction between materials and light are transformed into an “art” when they are perceived as beautiful through human eyes.