igneous
Porphyry (likely Porphyritic Andesite or Basalt)
Porphyritic Igneous Rock
Hardness: 6-7 (Mohs scale); Color: Tan/brown groundmass with dark phenocrysts; Luster: Dull/earthy (weathered); Crystal structure: Aphanitic groundmass with larger visible phenocrysts; Specific gravity: 2.5-3.0
- Hardness
- 6-7 (Mohs scale)
- Color
- Tan/brown groundmass with dark phenocrysts
- Luster
- Dull/earthy (weathered)
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Tan/brown groundmass with dark phenocrysts; Luster: Dull/earthy (weathered); Crystal structure: Aphanitic groundmass with larger visible phenocrysts; Specific gravity: 2.5-3.0
Formation & geological history
Formed through two-stage cooling: slow cooling at depth forming large crystals (phenocrysts), followed by rapid cooling during eruption/ascent forming the fine-grained matrix. Common in volcanic arcs and continental crust.
Uses & applications
Used primarily as crushed stone for construction, road ballast, and sometimes as decorative dimension stone if colors are striking.
Geological facts
The word 'porphyry' comes from Ancient Greek for 'purple', as the most famous variety was a royal purple Egyptian stone. This specimen shows a classic texture where minerals crystallize at different temperatures.
Field identification & locations
Look for larger, distinct mineral grains (dark flecks) embedded in a much finer-grained, uniform background material. Often found as rounded river pebbles or near volcanic mountain ranges.
