Rhyolite: Fine-Grained Felsic Rock
Most rhyolites are light gray to pinkish in color, but red or even black rhyolites are not rare. Most rhyolites are porphyritic and are identified by the quartz phenocrysts and white to salmon K-feldspar phenocrysts. The groundmass is aphanitic or even glassy. In the absence of phenocrysts, a light color suggests, but does not prove, a rhyolite.
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This hand sample of rhyolite shows a pinkish color, abundant white phenocrysts of K-feldspar, and some quartz phenocrysts. These are better seen in the close up below.
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Several quartz phenocrysts are clearly seen as dark gray, translucent crystals in this close-up of the above sample. The abundance of phenocrysts can fool careless observers into thinking that this is coarse-grained, intrusive rock. However, close examination shows that these crystals are set in a very fine-grained groundmass.
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