Micro-analytical study of the optical properties of rainbow and sheen obsidians
Abstract
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.
Additional Information
© 2001 Mineralogical Association of Canada. Received May 23, 2000, revised manuscript accepted December 28, 2000. The authors are grateful to the following dealers for their assistance in the procurement of samples and in their attempts to help determine the provenances of the samples: Mr. H. Siegel of Aurora Mineral Corp., Freeport, New York; Mr. M. Belski of Howards Minerals, Brooklyn, New York; Mr. and Mrs. Bey of Beys Minerals, Bartow, Pennsylvania; Mr. L. Castle of Carved Opal and Obsidian, Madison, Wisconsin; Rock H. Currier of Jewel Tunnel Imports, California, and Mike New of Top Stones, Tucson, Arizona. Some of the pieces photographed by the authors were provided through the kindness Bob Gilman, Sr. and Jr. of Lost River Caverns Rock Shop of Hellertown, Pennsylvania. We acknowledge the Princeton Center for Complex Materials grant DMR 94–00362 (EPV) and the White Rose Foundation (GRR) for their support. For his computer manipulation and management of the more than 50 megabytes of images and data in the text file, the authors thank Mr. Mark Manna of Temple University. We thank John B. Brady and an anonymous reviewer for valuable comments.Additional details
- Eprint ID
- 38204
- DOI
- 10.2113/gscanmin.39.1.57
- Resolver ID
- CaltechAUTHORS:20130501-101711056
- DMR 94–00362
- Princeton Center for Complex Materials grant
- White Rose Foundation
- Created
-
2013-05-01Created from EPrint's datestamp field
- Updated
-
2021-11-09Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences