Color-change processes of a plinian pumice and experimental constraints of color-change kinetics in air of an obsidian |
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| Authors: | Mihoko Moriizumi Satoru Nakashima Satoshi Okumura Yuta Yamanoi |
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| Institution: | (1) Research Team for Soil Management on Paddy-Upland Rotation Systems, National Agricultural Research Center, 3-1-1 Kannondai, Tsukuba Ibaraki, 305-8666, Japan;(2) Department of Earth and Space Science, Graduate School of Science, Osaka University, Machikaneyama-cho 1-1, Toyonaka 560-0043, Osaka, Japan;(3) Institute of Mineralogy, Petrology and Economic Geology, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan |
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| Abstract: | Colors of plinian pumices were measured by spectrocolorimetry, and their quantitative color parameters in the L*a*b* color
space were determined. A series of heating experiments of obsidian was conducted to simulate the color-change processes of
rhyolitic glasses. In these experiments, following three stages of color-change processes were observed. Stage I showed a
rapid b* (yellowishness) increase associated with fast dehydration controlled by water diffusivity (D
water). In stage II, a* (reddishness) increase was accompanied by Fe2+ decrease. Both a* increase and Fe2+ decrease can be simulated by a diffusion model. Obtained diffusivity D
oxidation were about two orders of magnitude smaller than D
water
. The a*-value increase after the oxidation in stage III appeared to be quasi-linear with time, indicating the zeroth order
reaction corresponding to the formation of hematite-like structures in rhyolitic glasses. The diffusion-limited a* increase
model in stage II was applied to a natural plinian pumice fall unit to evaluate time periods of color-change processes through
oxidation by air of fragmented rhyolitic materials. |
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| Keywords: | Spectrocolorimetry Oxidation Pumice Plinian eruption Iron Obsidian Color-change process |
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