Colors of plinian pumices were measured by spectrocolorimetry, and their quantitative color parameters in the L*ab* 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 (Dwater). 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 Doxidation were about two orders of magnitude smaller than Dwater. 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.
ASJC Scopus subject areas
- Geochemistry and Petrology