Color-change processes of a plinian pumice and experimental constraints of color-change kinetics in air of an obsidian

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.

Appendix

Color is caused by the reflectance of light in the visible range of the electromagnetic spectrum, i.e., approximately between 400 and 700 nm. The spectral reflectance data obtained by these instruments are converted to three parameters (“tristimulus values” X, Y, and Z) that define the color perceived by the human eye. The tristimulus values can, then, be mathematically converted to color parameters in uniform color spaces, one of them being the CIE-L*a*b* system with the Cartesian coordinates L* (lightness), a*(reddishness–greenishness), and b*(yellowishness–bluishness). Figure 10 represents this L*a*b* color space with their color attributes.

The colors of materials can be measured in the laboratory or on the field by using spectrocolorimeters. Modern commercially available spectrocolorimeters (e.g., Minolta CM2600-d; Fig. 11a) allow a quick measurement of visible reflectance spectra and quantitative color values such as L*, a* and b* of materials. The spectrocolorimeter uses a diffuse illumination from a pulsed xenon light source by using an integrating sphere whose internal surfaces are coated with a white material such as barium sulfate so that the light is uniformly diffused (Fig. 11b). Another light source is also used to measure the reflected light with the same incident angle as the reflected angle to the detector for the specular component. The technical details are given in (http://www.konicaminolta.com/instruments/knowledge/color/).

Fig. 10

Color attributes in the L*a*b* color space (CIE 1976)

Fig. 11

KONICA MINOLTA Spectrocolorimeter CM-2600d (a) and its integrating sphere head for diffuse illumination and collection of specular component excluded (SCE) and included (SCI) lights reflected from the sample surface (b)