igneous
Porphyritic Basalt
Porphyritic Mafic Volcanic Rock
Hardness: 6-7 (Mohs); Color: Dark grey to black matrix with white to light-grey phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Aphanitic matrix with visible macro-crystals (porphyritic); Specific gravity: 2.8-3.0
- Hardness
- 6-7 (Mohs)
- Luster
- Dull/earthy matrix with vitreous crystals
Identified More igneous →
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Physical properties
Hardness: 6-7 (Mohs); Color: Dark grey to black matrix with white to light-grey phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Aphanitic matrix with visible macro-crystals (porphyritic); Specific gravity: 2.8-3.0
Formation & geological history
Formed from the cooling of magnesium-rich and iron-rich lava. The large crystals (phenocrysts) grew slowly underground before the remaining lava erupted and cooled quickly on the surface, creating the fine-grained matrix.
Uses & applications
Used primarily as crushed stone for road construction, railroad ballast, and high-strength concrete. Occasionally used as decorative garden stone or for architectural facing.
Geological facts
The term 'porphyry' comes from the Ancient Greek word for purple. While this specimen is black, porphyritic textures are key to understanding two-stage cooling histories in volcanic activity.
Field identification & locations
Identify by the 'chocolate chip cookie' appearance of light crystals embedded in a dark, fine-grained mass. Commonly found in volcanic fields, oceanic crust, and tectonic divergent plate boundaries.
