New Mexico Mineral Symposium — Abstracts

One of the geologically interesting features of Rockhound State Park, in southern New Mexico, are the many spherulites within the rhyolite lava. The spherulites range in size from less than 1 nun to greater than 30 cm, and many are solid, concentrically zoned, dark-gray to pinkish colored material with a distinct, nodular, reddish core. Others consist of the same material, but are partly hollow. A third population consists of two distinct parts: a dark-gray to pinkish outer part that appears similar to the material that forms the solid spherulites, with a white, blue, or gray inner part, or core, which is recognizable as agate, chalcedony, and quartz crystals. These two parts appear to be a shell and a filling. This type of filled spherulitic form is commonly called a "thunderegg." In order to gain some insight into the process by which these spherulites form, samples were examined using back-scattered electron imaging and qualitative X-ray analysis and imaging using a Cameca SX-100 electron microprobe. Microprobe examination of the "shell" part of Rockhound spherulites show that they are composed of intergrown crystals of quartz (SiO₂), alkali feldspar (K,Na)[AlSi₃O₈], plagioclase feldspar Na[AlSi₃O₈]-Ca[Al₂Si₂O₈], and magnetite (Fe₃O₄). The images from the microprobe show that the spherulites are formed either of intimately intergrown quartz, feldspar, and magnetite, or of bands of quartz systematically interspersed with bands of intergrown feldspar and quartz. The bands range in width from approximately 100-200 microns, and it is this banding that produces the concentric structure that is apparent in some parts of the spherulites. The reddish, nodular core of the spherulites are composed mainly of intergrown quartz and plagioclase, whereas the outer part of the spherulites contain quartz and alkali feldspar.

The observed patterns in the spherulites suggest that they may have formed during the cooling of the rhyolite lava. Similar spherulitic forms, with similar internal growth features were observed in an artificial melt that was rapidly cooled (Jacobs et a1.,1992; Dunbar et al., 1995). The temperature of the artificial melt was monitored during the cooling process, and exothermic crystallization was observed at high temperatures (1,100°C). Spherulitic growth occurred during this crystallization process, and the internal structure of the spherulites was indistinguishable from the internal structure of Rockhound spherulites. This similarity suggests that the Rockhound spherulites may have formed by a similar high-temperature, rather than sub-solidus process. Furthermore, the feathery and non-equant crystal shapes observed in the Rockhound spherulites are very similar to crystal forms described by Lofgren (1970, 1971) for crystals that grew rapidly at high temperatures (700°C), probably very soon after the rhyolitic lava was erupted onto the Earth's surface.

The agate, chalcedony, and 'quartz veins and open-space fillings within voids in the spherulites formed later by multiple cycles of hydrothermal fluids. Hydrothermal fluids are a mixture of late-stage fluids escaping the magma, mixed with local ground water. Some of these fluids move into hollow spherulites through fractures and precipitate crystals along the walls of the inner cavity, forming geodes and geode-like-filled spherulites. The Rockhound spherulites show evidence of multiple hydrothermal events, expressed by multiple colors and textures of fills, which could be accounted for by different temperatures and fluid compositions. By carefully studying the crystal fill and textures in spherulites and geodes, we hope to piece together the different processes through time that formed them. The banding found within some spherulites and geodes consists of multiple layers of different colored agate, chalcedony, and local quartz and may have been formed by fluids supersaturated in silica (Fournier, 1985). The different colors of the bands are a result of trace amounts of impurities, such as iron (red), manganese (black, pink), cobalt (blue, violet-red), copper (green, blue), chromium (orange-red), nickel (green), etc.

Faceted quartz crystals indicate that the fluids were somewhat supersaturated with silica, and precipitation occurred under relatively slow-changing conditions (Fournier, 1985). Tilted thundereggs, locally found at Rockbound State Park, are filled with horizontal layers of agate and chalcedony that are overlain by concentric-banded agate and chalcedony; the contact between the layered and banded agate resembles an angular unconformity. These thundereggs record either small local landslides or tilting of local fault blocks within the Little Florida Mountains as the crystals were precipitating from the fluid (Shaub, 1979; Colburn, 1999).
 

References:

1. Colburn, R., 1999, The formation of thundereggs (lithophysae): Robert Colburn, CD-ROM, 385 pp.
2. Dunbar, N. W., Jacobs, G. K., and Naney, M. T., 1995, Crystallization processes in an artificial magma: Variations in crystal shape, growth rate, and composition with melt cooling history: Contributions to Mineralogy and Petrology, v. 120, pp. 412-425.
3. Fournier, R. 0., 1985, The behavior of silver in hydrothermal solutions; in Berger, B. R., and Bethke, P. M. (eds.), Geology and geochemistry of epithermal systems: Reviews in Economic Geology, v. 2, pp. 45-62.
4. Jacobs, G. K., Dunbar, N. W., Naney, M. T., and Williams, R. T., 1992, In-situ vitrification: Observations of petrological processes in a man-made magmatic system: EOS, Transactions of the American Geophysical Union, v. 73, pp. 401-411.
5. Lofgren, G., 1970, Experimental devitrification rate of rhyolitic glass: Geological Society of America, Bulletin, v. 81, pp. 553-560.
6. Lofgren, G., 1971, Spherulitic textures in glassy and crystalline rocks: Journal of Geophysical Research, v. 76, no. 23, pp. 5635-5648.
7. Shaub, B. M., 1979, Genesis of thundereggs, geodes, and agates of igneous origin: Lapidary Journal, v. 32, pp. 2340-2354, 2548-2566.