Why are blue zhamanshinites blue? Liquid immiscibility in an impact melt

We report here a study of the cause of the coloration of blue zhamanshinites, which are glassy impact melt rocks from the Zhamanshin crater in the USSR. We find that the blue color results from Rayleigh scattering from spherical, 100 nm-diameter inclusions of a separate Ca-Fe-Mg-P-rich silicate glass. These observations can best be explained by the operation of liquid immiscibility in the zhamanshinite melt, and suggest that liquid immiscibility may have a more general role in impactite evolution. IMPACT-RELATED GLASSES FROM the Zhamanshin impact crater, located at 48”24’N, 60”58’E in the Kazakhstan Republic of the USSR, have been the subject of recent studies (FLORENSKIJ, 1975; FLORENSKIJ and DABIZHA, 1980; KOEBERL and FREDRIKSSON, 1986; KOEBERL and STORZER, 1987; KOEBERL, 1988). The Zhamanshin crater, situated in a semiarid environment, has a minimum diameter of approximately 6.5 km and is the source of four different types of impact glasses. The tektite-like, ropy, droplet-shaped black glasses of small size are called irghizites and are chemically the most homogeneous of the Zhamanshin glasses. The other glass group, the zhamanshinites, have been divided into Si-rich and Si-poor varieties. The Si-rich zhamanshinit~, which have a chunky, biocky appearance and a layered structure, have been subdivided into normal and blue varieties (KOEBERL and FREDRIKSSON, 1986; KOEBERL, 1988). The color of the blue zhamanshinites ranges from turquoise to very dark blue, and opacity also varies. Many specimens appear to show a gradation to brown, transparent, so-called “Si-rich” zhamanshinite. The blue-to-brown color, layered appearance, opacity changes, and vesicularity characteristics of blue zhamanshinites render them very similar in appearance to some man-made glass slags. The blue color is apparent only in reflected light, the color being brownish in transmitted light. KOEBERL (1988) found that blue zhamanshinites had higher Ca and rare earth element (REE) contents than Sirich zhamanshinites, but did not identify the blue coloring agent. We report here the results of a study which has revealed the cause of the coloration of blue zhamanshinites. We suggest that liquid immiscibility may have a previously little-recognized role in impactite melt evolution (see also DENCE et al., 1974). EXPERIMENTAL PROCEDURE Portions of blue zhamanshinite samples Zh 3 l/6 and BZ860 1 (KOEBERL, 1988) were broken to reveal fresh, unweathered portions. Grains of the unweathered material were then set into a block of low-viscosity epoxy (EMBED8 12) and ultmmicrotom~ by standard techniques. Serial sections were produced with an approximate thickness of 100 nm. These sections were badly shattered but were still adequate for examination. Analytical work was performed using a JEOL 2000FX Scanning Transmission Electron Microscope (STEM), operated at 200 kV equipped with a LINK eXL energy dispersive X-ray (EDX) analysis system. Quantitative EDX analyses were made using standard LINK software, natural mineral and tektite standards, and inte~ally derived K-factors. All EDX analyses are accurate to within 8% relative. Electron microprobe analyses were performed using the ARL-SEMQ instrument at the Naturhistorisches Museum, Vienna.