Structural analysis of an obsidian by X-ray diffraction method
Okuno, Masayuki; Iwatsuki, Hidefumi; Matsumoto, Takeο
European Journal of Mineralogy Volume 8 Number 6 (1997), p. 1257 - 1264
published: Jan 8, 1997
manuscript accepted: May 23, 1996
manuscript received: Jul 6, 1995
ArtNo. ESP147050806004, Price: 29.00 €
Abstract The average local structure of an obsidian-has been analyzed using X-ray radial distribution function and compared with the structure of a fused obsidian glass. The T-O distances of these samples are close to the average T-O distance of SiO4 and AlO4 tetrahedra. The T-T distances and calculated T-O-T angles are similar to those of SiO2, albite and orthoclase glasses. The T-O-T angles are larger than that of anorthite glass. Therefore, the basic structure of the obsidian can be well explained by a stuffed framework structure model with six-membered rings of TO4 (T = Si and Al) tetrahedra. The peaks of the T-O and T-T pairs on the RDF(r) curve of the obsidian are broader than those of the fused obsidian. These results suggest that there are large deformations of TO4 tetrahedra and their linkages during the crystallization process below the melting point. The RDF(r) curve of the obsidian shows longer range ordering than the fused obsidian. Therefore, it is supposed that the repolymerization of supercooled liquid was detected at the beginning of its crystallization process. The structure difference between dry volcanic glasses such as obsidian and wet volcanic glasses such as perlite is also discussed.