igneous
Porphyritic Rhyolite
Rhyolite (Porphyritic)
Hardness: 6-7 (Mohs scale); Color: Light gray to tan with brown or reddish phenocrysts; Luster: Dull/Earthy groundmass with vitreous crystals; Crystal structure: Aphanitic matrix with embedded phenocrysts; Cleavage: None (conchoidal fracture)
- Hardness
- 6-7 (Mohs scale)
- Color
- Light gray to tan with brown or reddish phenocrysts
- Luster
- Dull/Earthy groundmass with vitreous crystals
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Light gray to tan with brown or reddish phenocrysts; Luster: Dull/Earthy groundmass with vitreous crystals; Crystal structure: Aphanitic matrix with embedded phenocrysts; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the rapid cooling of silica-rich (felsic) lava at or near the Earth's surface. The larger phenocrysts formed slowly at depth before the final eruption. Found in continental volcanic arcs.
Uses & applications
Used as decorative stone, crushed stone for road construction, and occasionally in stone tools due to its toughness and fracture patterns.
Geological facts
Rhyolite is the volcanic equivalent of granite. The term 'porphyritic' refers to the distinct difference in crystal sizes, representing two stages of cooling.
Field identification & locations
Identify in the field by its light color, high silica content, and characteristic 'spotted' appearance caused by phenocrysts. Often found in volcanic mountain ranges like the Rockies or Andes.
