1:45 PM - 3:15 PM
[O06-P12] Consideration of the elements that make up the black color of obsidian
Keywords:obideian
Some people believe that the black color of obsidian is due to Fe. However, when comparing the Fe content of milky obsidian and black obsidian by X-ray fluorescence analysis, no significant difference was found. Therefore, we focused on the structure that can be seen under the microscope and discussed the elements that make up the color of obsidian.
In this study, we made thin sections of obsidian from different localities and of different colors, and observed them under a polarized light microscope. Samples from Shirataki, Hokkaido, Japan (black), Wadatoge, Nagano, Japan (also black), and Himejima, Oita, Japan (milky white) were used. The presence of crystals, microcrystals, or bubbles inside the obsidian was observed, and their occupied area was determined by modal measurement. Light transmittance of the obsidian was determined by making slabs of uniform thickness, fixing a light in a dark place, and comparing the illuminance measured through the slab and without any light through the slab using an illuminometer.
The characteristic crystalline morphology of the obsidian slabs from each locality was observed in the Shirataki and Wadatoge samples. The Shirataki sample had "black crystals on spots," while the Wadatoge sample had "black needle crystals" and "spherical siliciclastic material. The grid used for mode measurement was reused and their occupied areas are shown in the figure. Propylitic alteration was observed in the Himejima product, and structures growing within the bubbles were also identified. Measurements showed that the larger the area of bubbles within the flake, the lower the light transmittance.
Mode measurements showed that the Himejima and Wadatoge flakes had approximately the same aspect, but the Shirataki flake had a higher plagioclase value.
We thought that the occupancy of bubbles in the flakes and the black crystals inside the obsidian might be related to the coloration of the obsidian, so we measured them. The results suggest that the bubble occupancy affects the light transmittance of obsidian. In addition, the presence of black crystals within the black obsidian and the absence of such crystals in the milky-white obsidian suggests that the black crystals inside the obsidian affect the color tone. It is also thought that the internal crystals affect the slight difference in color from one locality to another.
In this study, we made thin sections of obsidian from different localities and of different colors, and observed them under a polarized light microscope. Samples from Shirataki, Hokkaido, Japan (black), Wadatoge, Nagano, Japan (also black), and Himejima, Oita, Japan (milky white) were used. The presence of crystals, microcrystals, or bubbles inside the obsidian was observed, and their occupied area was determined by modal measurement. Light transmittance of the obsidian was determined by making slabs of uniform thickness, fixing a light in a dark place, and comparing the illuminance measured through the slab and without any light through the slab using an illuminometer.
The characteristic crystalline morphology of the obsidian slabs from each locality was observed in the Shirataki and Wadatoge samples. The Shirataki sample had "black crystals on spots," while the Wadatoge sample had "black needle crystals" and "spherical siliciclastic material. The grid used for mode measurement was reused and their occupied areas are shown in the figure. Propylitic alteration was observed in the Himejima product, and structures growing within the bubbles were also identified. Measurements showed that the larger the area of bubbles within the flake, the lower the light transmittance.
Mode measurements showed that the Himejima and Wadatoge flakes had approximately the same aspect, but the Shirataki flake had a higher plagioclase value.
We thought that the occupancy of bubbles in the flakes and the black crystals inside the obsidian might be related to the coloration of the obsidian, so we measured them. The results suggest that the bubble occupancy affects the light transmittance of obsidian. In addition, the presence of black crystals within the black obsidian and the absence of such crystals in the milky-white obsidian suggests that the black crystals inside the obsidian affect the color tone. It is also thought that the internal crystals affect the slight difference in color from one locality to another.