This spectacular piece of volcanic breccia displays many features common in felsic volcanic rocks. In addition to the brecciation, it is also significantly altered by fluids. There appears to be no relict igneous mineralogy other than some possible feldspars within the lithic fragments that are present. The pervasive alteration has also produced the green hue through the whole rock, which is likely to be secondary chlorite.
The textures here are more informative than the mineralogy is. Much of the specimen is made up of spherical structures ranging from 1 to about 3 centimetres in diameter. Most of these have a central ‘thunderegg’, largely occupied by banded chalcedony (agate), with a few exhibiting hollow centres partly filled with drusy quartz, some of which is amethyst. The structures likely began as spherulites, formed during devitrification of volcanic glass - so the original rock was likely a vitric tuff or ignimbrite. A scalebar is on the reverse side of the specimen.
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