igneous
Porphyritic Rhyolite
Porphyritic Rhyolite (compositionally similar to granite)
Hardness: 6-7 (Mohs); Color: Light gray to tan with white crystals; Luster: Dull/earthy groundmass with vitreous crystals; Crystal structure: Aphanitic groundmass with rectangular phenocrysts (orthoclase feldspar); Specific gravity: 2.4-2.6
- Hardness
- 6-7 (Mohs)
- Color
- Light gray to tan with white crystals
- Luster
- Dull/earthy groundmass with vitreous crystals
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Physical properties
Hardness: 6-7 (Mohs); Color: Light gray to tan with white crystals; Luster: Dull/earthy groundmass with vitreous crystals; Crystal structure: Aphanitic groundmass with rectangular phenocrysts (orthoclase feldspar); Specific gravity: 2.4-2.6
Formation & geological history
Formed via rapid cooling of silica-rich volcanic lava at or near the Earth's surface, where larger crystals (phenocrysts) grew slowly at depth before the final eruption. They often date from the Proterozoic to the Cenozoic eras.
Uses & applications
Commonly used as crushed stone for road construction and aggregate; some fine-grained varieties are used as decorative stone or for ancient stone tools (lithics).
Geological facts
Rhyolite is the volcanic equivalent of granite. The larger white crystals visible in this specimen are likely feldspar, which solidified while the magma was still deep underground before it erupted.
Field identification & locations
Identified by light color (silica-rich), fine-grained base, and the presence of visible rectangular crystals. Found in volcanic regions like the American West (Yellowstone), Andes, and Iceland.
