igneous
Porphyry
Porphyritic Basalt/Andesite
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with white to off-white phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with large visible crystals (phenocrysts).
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with white to off-white phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic matrix with large visible crystals (phenocrysts).
Formation & geological history
Formed through a two-stage cooling process of magma: slow cooling at depth creates large crystals, followed by rapid eruption and cooling of the remaining melt. Often associated with volcanic arcs and tectonic plate boundaries.
Uses & applications
Used primarily in construction as crushed stone or road ballast. Historically used as a decorative building stone and for monuments in Ancient Rome.
Geological facts
The word 'porphyry' comes from the Greek word for 'purple.' While many are grey/black, the most famous historical porphyry was Imperial Purple Porphyry from Egypt, reserved exclusively for Emperors.
Field identification & locations
Identify by looking for 'spots' or 'stars' (large distinct crystals) embedded in a much finer, uniform background grain. This texture signifies a complex cooling history.
