THE ORIGIN OF RHYOLITIC SPHERULITES AT ROCKHOUND STATE PARK

Nelia W. Dunbar, Virginia T. McLemore
New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, Socorro, NM, 87801

One of the geologically-interesting features of the Rockhound State Park, in southern New Mexico, is the presence of numerous spherulites within the rhyolite lava. The spherulites range in size from less than 1 mm 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" portion 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 around 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 al., 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 (1100oC). 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 (~700oC), probably very soon after the rhyolitic lava was erupted onto the earth's surface.

The presence of siliceous veinlets crosscutting the crystalline, outer portion of the spherulites indicates that the siliceous filling was emplaced after the outer part of the spherulite was brittle. The physical conditions of the filling process and under investigation.

References: 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.

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.

Lofgren, G., 1970, Experimental devitrification rate of rhyolitic glass: G.S.A. Bulletin, v. 81, pp. 553-560.

Lofgren, G., 1971, Spherulitic textures in glassy and crystalline rocks: Journal of Geophysical Research, v. 76, no. 23, pp. 5635-5648.