Spherulites in rhyolite  

Spherulites are radiating masses of fibrous crystals in a glassy matrix. These spherulites are probably composed of alkali feldspars and some polymorph of SiO2. Note the large phenocryst which forms the nucleus of one of the spherulites at center-left.

UNC sample

WYO-14

Rock type

rhyolite

Locality

Wyoming