igneous
Porphyritic Rhyolite
Porphyritic Rhyolite (Felsic volcanic rock)
Hardness: 6-7 (Mohs scale); Color: reddish-brown or brick red groundmass with light-colored phenocrysts; Luster: dull to earthy matrix with vitreous crystals; Structure: porphyritic with visible phenocrysts of quartz or feldspar; Cleavage: none (conchoidal fracture).
- Hardness
- 6-7 (Mohs scale)
- Luster
- dull to earthy matrix with vitreous crystals
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Physical properties
Hardness: 6-7 (Mohs scale); Color: reddish-brown or brick red groundmass with light-colored phenocrysts; Luster: dull to earthy matrix with vitreous crystals; Structure: porphyritic with visible phenocrysts of quartz or feldspar; Cleavage: none (conchoidal fracture).
Formation & geological history
Forms from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. The large crystals (phenocrysts) formed slowly underground before being erupted and encased in a fine-grained matrix. Often found in continental volcanic arcs.
Uses & applications
Used as crushed stone for road construction, decorative gravel in landscaping, and occasionally as a building facade material. High-quality specimens are popular with amateur rock collectors.
Geological facts
The term 'porphyritic' refers to the distinct difference in crystal sizes within the rock. Red rhyolites often get their color from minute inclusions of hematite (iron oxide). It is the volcanic equivalent of granite.
Field identification & locations
Identified by large, light-colored crystals (usually feldspar or quartz) set in a very fine-grained, reddish groundmass. Commonly found in areas with historical volcanic activity such as the American Southwest or the Lake Superior region.
