Micro-analytical Study of the Optical Properties of

Mexican Rainbow and Sheen Obsidians


Chi Ma1, Jennifer Gresh2*, George R. Rossman1, Gene C. Ulmer2, and Edward P. Vicenzi3**

1) Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125
2) Geology Department, Temple University, Philadelphia, PA 19122
3) Princeton Materials Institute, Princeton University, Princeton, NJ 08540
**Now at Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington DC 20560

Canadian Mineralogist 39, 57-71


Rainbow obsidian

ABSTRACT

     Mexican obsidians that exhibit either "sheen" or "rainbow" optical properties were examined with a combination of electron microprobe analysis, scanning electron microsocpy, transmission electron microscopy, as well as visible and near-infrared spectroscopy. Electron microprobe analyses of the sheen matrix have an average composition (wt %): 76.2 SiO2, 0.2 TiO2, 11.6 Al2O3, 2.2 FeOTOT, 0.07 MgO, 0.1 CaO, 4.8 Na2O, 4.4 K2O.  Gemological literature attributes the sheen phenomenon to the presence of aligned flow-stretched hollow vesicles. In these samples, SEM images show instead flow-aligned lenticular areas of a second rhyolite glass with an average composition: 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. There is no compositional overlap within 2s for the matrix and glass-filled lenticular areas.  The indices of refraction of these two glasses differ by as much as 0.04, so that optical interference is provided along the elliptical interfaces of the two glasses.

    Thus, in these interesting Mexican sheen obsidians we postulate that the optical sheen property correlates  with the differences in indices of refraction (nD) between the matrix obsidian and the lower nD of either gas-filled or glass-filled vesicles.  In our studied sample, the presence of the second glass in lenticular shapes probably correlates with an earlier rhyolitic ash or tuff fragments becoming incorporated (and remelted?) by the superposition of a newer obsidian flow.

     Two different types of Mexican rainbow obsidians were studied.  The first has layers of numerous trachytically-oriented rods (0.2-2 mm by 10-20 mm) of hedenbergite of composition (Ca0.88 Mg0.07 Fe0.98 Mn0.06 Si2.01O6).  The composition of the obsidian matrix is: 76.3 SiO2, 12.5 Al2O3, 1.7 FeOTOT, 0.01 MgO, 0.16 CaO, 4.4 Na2O, and 4.6 K2O. The second type of obsidian has  trachytically-aligned feldspar (~ An20) also rod-shaped (as small as 0.5 by 2.0 mm). The composition of its obsidian matrix is: 76.1 SiO2, 13.5 Al2O3, 0.7 FeOTOT, 0.09 MgO, 0.7 CaO, 3.75 Na2O, and 4.85 K2O.   Multiple hypotheses are considered for the possible cause of the rainbow effects: gas/fluid inclusions, small component scattering centers, differential indices of refraction, Bragg diffraction of visible light and thin film interference. Our data support the last hypothesis, i.e., thin film interference is the reason for color band optic effects in rainbow obsidian.


Additional high-resolution figures

in tif format

Infrared spectrum in the water region taken through a slab of rainbow obsidian.

Reflectance spectrum of colored zones in rainbow obsidian

Parallel rods of hedenbergite in rainbow obsidian (2.9 MB) 
 

A single rod of hedenbergite in rainbow obsidian