igneous
Porphyritic Rhyolite
Porphyritic Rhyolite (Extrusive Igneous Rock)
Porphyritic texture with large phenocrysts in a fine-grained matrix. Hardness: 6-7 on Mohs scale. Color: Pink to reddish-brown with white/black speckles. Luster: Dull to sub-glassy. Specific gravity: 2.4-2.6.
- Hardness
- 6-7 on Mohs scale
- Color
- Pink to reddish-brown with white/black speckles
- Luster
- Dull to sub-glassy
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Physical properties
Porphyritic texture with large phenocrysts in a fine-grained matrix. Hardness: 6-7 on Mohs scale. Color: Pink to reddish-brown with white/black speckles. Luster: Dull to sub-glassy. Specific gravity: 2.4-2.6.
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. The larger crystals (phenocrysts) grew slowly at depth before the final eruption. Found in volcanic arcs and continental rifting zones.
Uses & applications
Used as crushed stone for road construction and ballast, architectural stone, and occasionally as a decorative semi-precious stone for cabochons if the pattern is unique.
Geological facts
Rhyolite has the same chemical composition as granite but cooled much faster. Its high silica content makes the lava extremely viscous, often resulting in explosive volcanic eruptions.
Field identification & locations
Identify by looking for distinct larger crystals (likely quartz or feldspar) embedded in a very fine-grained, light-colored or pinkish groundmass. Often found in riverbeds near volcanic mountain ranges.
