Spherulitic and perlitic rhyolite, Kaiser Spring Volcanic Field, west-central Arizona. Spherulites are named for the Greek σφαίρα (sphaira)= ball or orb and λίθος (lithos) = rock or stone. Spherulites are round or ellipsoidal balls, composed of radiating, fibrous crystals (usually quartz or feldspar) and form when glassy, silica-rich rhyolites devitrify. The spherulites are surrounded by gray perlite, or onion-skin fractured, hydrated obsidian, which forms when from rapid cooling contraction and hydration of the lava. If the lava is only partially hydrated, fresh obsidian cores, or Apache tears, remain at the center of the concentric perlitic fractures. Invited post by L.A.M. Thompson)
