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 SiO_2, 0.2 TiO_2, 11.6 Al_2O_3, 2.2 FeO_(TOT), 0.07 MgO, 0.1 CaO, 4.8 Na_2O and 4.4 K_2O. 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 lenticular areas are a second rhyolite glass with, on average, 74.6 SiO_2, 0.2 TiO_2, 12.7 Al_2O_3, 2.1 FeO_(TOT), 0.1 MgO, 0.9 CaO, 5.6 Na_2O and 4.6 K_2O. These two compositions do not overlap at the 2σ 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 differences in indices of refraction (η) between the matrix obsidian and the lower η 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 trachytically oriented rods (0.2–2 by 10–20 μm) of hedenbergite (Ca_(0.88)Mg_(0.07)Fe_(0.98)Mn_(0.06)Si_(2.01)O_6). The composition of the matrix is: 76.3 SiO_2, 12.5 Al_2O_3, 1.7 FeO_(TOT), 0.01 MgO, 0.16 CaO, 4.4 Na_2O and 4.6 K_2O. The second type has trachytically aligned plagioclase (~An_(20)), also rod-shaped (as small as 0.5 × 2.0 μm). The composition of the matrix is: 76.1 SiO_2, 13.5 Al_2O_3, 0.7 FeO_(TOT), 0.09 MgO, 0.7 CaO, 3.75 Na_2O and 4.85 K_2O. Multiple hypotheses are considered for the possible cause of the rainbow effects: gas or fluid inclusions, a small component of scattering centers, differential indices of refraction, Bragg diffraction of visible light, and thin-film interference. Our data support the last hypothesis