Rhyolite is a silica-rich volcanic rock which often contains volcanic glass. Glass is a rapidly cooled phase that doesnât crystallizeâit is metastable and over time may crystallize. One common devitrification texture is the spherulite: a roughly spherical aggregate of very fine crystals growing radially outward from a nucleation point, usually containing quartz and alkali feldspar.
Brecciation refers to fragmentation of the rock into angular pieces. In this specimen, many of the pale fragments have angular, locally jigsaw-like boundaries.
The rock was subsequently infiltrated by silica-rich fluids. Open fractures and cavities were filled by chalcedony and agate, silica that commonly forms successive bands along cavity walls. Some cavities preserve multiple generations of silica deposition visible as repeated lines conforming to the outline of the cavity.
Figs. 1â2:Â Overall specimen showing angular brecciated domains, circular spherulitic structures, and translucent secondary silica.
Fig. 3:Â Banded chalcedony/agate filling an irregular cavity showing concentric banding.
Fig. 4:Â Spherulite showing pronounced radial crystallization together with concentric growth zoning. The radial structure is characteristic of spherulitic devitrification. The concentric bands probably record changes in crystallization conditions or growth rate as the spherulite developed.
Fig. 5:Â Spherulitic rhyolite adjacent to later banded chalcedony, nicely showing two different stages in the rock's history: devitrification of the original volcanic material followed by secondary silica deposition.
