igneous
Porphyry
Porphyritic Basalt or Andesite
AI-generated identification · may be incorrectHardness: 6-7 (Mohs scale); Color: Dark gray to black groundmass with prominent pink or cream phenocrysts; Luster: Dull (groundmass) to vitreous (crystals); Crystal structure: Porphyritic texture (large crystals in fine-grained matrix); Specific gravity: 2.7–3.3
- Hardness
- 6-7 (Mohs scale)
- Luster
- Dull (groundmass) to vitreous (crystals)
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Dark gray to black groundmass with prominent pink or cream phenocrysts; Luster: Dull (groundmass) to vitreous (crystals); Crystal structure: Porphyritic texture (large crystals in fine-grained matrix); Specific gravity: 2.7–3.3
Formation & geological history
Formed when magma cools in two stages: first, slowly underground to form large crystals (phenocrysts), then rapidly at or near the surface during an eruption to form the fine-grained matrix (groundmass).
Uses & applications
Commonly used as crushed stone in construction, road base, and decorative masonry. Historically, 'Imperial Porphyry' was highly prized for statues and sarcophagi in Ancient Rome.
Geological facts
The word 'porphyry' comes from the Greek word for purple. The specific texture is a time-capsule of a volcano's cooling history, showing exactly when the environment changed from slow subterranean cooling to rapid surface cooling.
Field identification & locations
Identify in the field by looking for distinct, isolated crystals (often feldspar) 'floating' in a tight, solid background color. Found globally in volcanic regions like the Cascades, Andes, or ancient volcanic sites in the UK and Egypt.
