MICRO-ANALYTICAL STUDY OF THE OPTICAL PROPERTIES OF RAINBOW AND SHEEN OBSIDIANS

Samples of Mexican obsidian that exhibit either “sheen ” or “rainbow ” optical properties were examined with a combination of EMP, SEM, TEM, as well as visible and IR spectroscopy. Electron-microprobe analyses of the sheen matrix give (in wt.%): 76.2 SiO2, 0.2 TiO2, 11.6 Al2O3, 2.2 FeOTOT, 0.07 MgO, 0.1 CaO, 4.8 Na2O and 4.4 K2O. The sheen is attributed to the presence of aligned flow-stretched hollow vesicles in the gemological literature. SEM images show that many of the flow-aligned lenticu-lar areas are a second rhyolite glass with, on average, 74.6 SiO2, 0.2 TiO2, 12.7 Al2O3, 2.1 FeOTOT, 0.1 MgO, 0.9 CaO, 5.6 Na2O and 4.6 K2O. These two compositions do not overlap at the 2 level. Their inferred indices of refraction differ by as much as 0.04, leading to optical interference along the elliptical interfaces of the two glasses. Thus we postulate that the sheen reflects differ-ences in indices of refraction () between the matrix obsidian and the lower of either gas-filled or glass-filled vesicles. In our sample, the presence of the second glass probably correlates with incorporation (and remelting?) of fragments of an earlier rhyolitic ash or tuff. Two different types of Mexican rainbow obsidian were studied. The first has layers of numerous trachyticall