igneous
Porphyritic Rhyolite
Porphyritic Rhyolite (SiO2)
Hardness: 6-7 (Mohs scale); Color: light gray to pinkish-tan with dark phenocrysts; Luster: earthy to dull groundmass with glassy crystals; Structure: fine-grained (aphanitic) matrix with visible crystals (phenocrysts); Cleavage: none.
- Hardness
- 6-7 (Mohs scale)
- Color
- light gray to pinkish-tan with dark phenocrysts
- Luster
- earthy to dull groundmass with glassy crystals
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Physical properties
Hardness: 6-7 (Mohs scale); Color: light gray to pinkish-tan with dark phenocrysts; Luster: earthy to dull groundmass with glassy crystals; Structure: fine-grained (aphanitic) matrix with visible crystals (phenocrysts); Cleavage: none.
Formation & geological history
Formed from the rapid cooling of silica-rich lava on or near the Earth's surface. The larger crystals (phenocrysts) formed at depth before the final eruption. Typically occurs in continental volcanic arcs.
Uses & applications
Used as decorative stone, crushed stone for construction, and as a specimen for geological study. Occasionally used for small lapidary items.
Geological facts
The term 'porphyritic' refers specifically to the texture where large crystals are embedded in a fine-grained groundmass. Rhyolite is the volcanic equivalent of granite.
Field identification & locations
Identified by the distinct contrast between the fine-grained 'background' and the larger, visible mineral crystals. Found in volcanic regions like the American West or ancient volcanic belts.
