New Mexico Geological Society Annual Spring Meeting — Abstracts

Rhyolitic lava flows at Rockhound State Park contain numerous spherulites (thundereggs) ranging in size from less than 1 mm to greater than 30 cm. Many spherulites are solid, concentrically-zoned dark gray to pinkish colored material with a nodular, reddish core. In order to gain some insight into the process by which thses 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 solid portion of the spherulites show that they are composed of intergrown crystals of quartz (SiO₂), alkali feldspar (K,Na)[AISi₃O₈], plagioclase feldspar Na[AISi₃O₈]-Ca[Al₂Si₂O₈] and magnetite (Fe₃O₄). 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 is composed mainly of intergrown quartz and plagioclase, whereas the outer part of the spherulites contains quartz and alkali feldspar.

The physical and chemical characteristics of 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). Spherulitic growth occurred in this melt due to high temperature crystallization, and the internal structure of the spherulites was indistinguishable from that of the 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 quench crystal forms (e.g. Lofgren, 1970), suggesting that the crystals that grew rapidly at high temperatures (~700°C), probably very soon after the rhyolitic lava was erupted onto the eath's surface.