[Abstract]

In order to study quantitatively color changes of basaltic scoria erupted at Takatsukayama, North-east Izu, Japan, the scoria colors were measured by spectro-colorimetry and described in the international standard color space (CIE L* a* b*). The color values [a* (red) and b* (yellow) values] of Takatsukayama scoria cone increase from margin to center, indicating their color change from black to red. The bulk chemical compositions of scoria, including total Fe content, remain almost unchanged despite the color variations. On the other hand, the FeO (Fe2+) contents determined by the phenanthroline method have a good linear relation with a* values (red) of scoria. Under the optical microscope, the red parts are not found in the black scoria, whereas the red scoria have red parts in the groundmass and the olivine phenocryst. For dull red scoria, the red parts are observed only in the groundmass. These indicate that red parts appear first in the groundmass and then in the olivine phenocryst. Visible and Raman microspectroscopy indicated the presence of hematite in these red parts.
A series of heating experiments of the black scoria under an atmospheric condition at 500-1150C were conducted to simulate the color changes of Takatsukayama scoria. The heated scoria showed the increases in a* and b* values (i.e. red coloring). The FeO (Fe2+) contents of the heated scoria had similar linear correlation with a* values (red) as for natural ones. The presence of hematite was also found by visible and Raman microspectroscopy.
These results imply that the increases in a* and b* values (red coloring) of Takatsukayama scoria can be explained by the high temperature oxidation of Fe2+, resulting in the formation of hematite first in the groundmass and then in the olivine phenocryst. Since the direction of the red coloring is oblique to the depositional sequences of the cone, the high temperature oxidation process might have occurred after the scoria deposition from the center of a heat source to the outer parts.

Key words:

color, scoria, spectro-colorimetry, oxidation, hematite