igneous
Porphyritic Rhyolite
Porphyritic Rhyolite
Hardness: 6-7 (Mohs), Color: Reddish-brown to purple with white or light gray phenocrysts, Luster: Dull or earthy matrix with vitreous crystals, Crystal Structure: Aphanitic matrix with visible phenocrysts, Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.6
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Physical properties
Hardness: 6-7 (Mohs), Color: Reddish-brown to purple with white or light gray phenocrysts, Luster: Dull or earthy matrix with vitreous crystals, Crystal Structure: Aphanitic matrix with visible phenocrysts, Cleavage: None (conchoidal fracture), Specific Gravity: 2.4-2.6
Formation & geological history
Formed from the rapid cooling of silica-rich lava at or near the Earth's surface. The larger crystals (phenocrysts) grew slowly at depth before the final eruption. They are typically associated with continental volcanic arcs.
Uses & applications
Mainly used as decorative stone, crushed stone for construction, road ballast, and sometimes for stone tools (arrowheads) by ancient peoples due to its conchoidal fracture.
Geological facts
The name 'Rhyolite' comes from the Greek word 'rhyax' meaning stream of lava. It has the same chemical composition as granite but cooled much faster.
Field identification & locations
Identify in the field by the light-colored, glassy matrix containing distinct larger crystals (phenocrysts) that 'twinkle' in the light. Commonly found in volcanic regions like the American Southwest or the Lake Superior region.
