Structural colors of rainbow scoria: formation process of the microtexture during the 1986 Izu-Oshima volcano eruption

Brilliant rainbow-colored scoriae have been recognized in deposits ejected from the B-fissure vents during the 1986 Izu-Oshima eruption. We hypothesize that the microtexture on the surface of the scoriae forms a structure that causes the colors. Based on microscopic observations, we found precipitates of approximately ∼ 1 µm thickness on the surface of the rainbow-colored scoriae. The precipitates consist of aggregates of spherulites, which are a composite mainly of polygonal Fe-oxide(s), dendritic Fe-silicate(s), and Mg-Ca-bearing mineral(s). The sizes of the spherulites range from 0.07-0.29 µm, 0.07-0.55 µm, and 0.16-0.81 µm for blue, yellow, and red areas, respectively. The areas showing metallic brilliance have larger Fe-oxide(s) areas, leading to a high reflectivity. The colors did not change for respective areas with different observation angles, indicating non-iridescent structural color from a randomly arranged component. Beneath the crystallized precipitates, Na-rich silicate glass (∼ 2 µm) was found to exist above the original silicate glass. These textural and compositional characteristics can be explained by the oxidation of basaltic glass at a high temperature in an SO2-bearing gas and the subsequent rapid cooling of the glass. These results indicate that the structural colors are produced by randomly arranged spherulites with the size of visible wavelengths that crystallized near the glass surface. Thus, rainbow-colored scoriae record the high temperatures oxidation and rapid cooling, which can be attained inside an eruption column. The colors of scoriae can be an indicator for magma-air interactions during an eruption.