Rhyolite  

This crystal-rich rhyolite contains phenocrysts of quartz, K-feldspar (sanidine), plagioclase, and biotite in a fine-grained groundmass. Note the interesting shape of the (partially resorbed? skeletal?) quartz grain in the center of the photograph.

UNC sample

R-15

("rhyolite" card"

Rock type

rhyolite

Locality

unknown