igneous
Porphyritic Basalt
Porphyritic Fine-grained Mafic Volcanic Rock
Hardness: 6.0 (Mohs scale); Color: Dark grey to black matrix with white or creamy phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Aphanitic matrix with larger visible crystals (phenocrysts); Cleavage: Poor; Specific gravity: 2.8-3.0
- Hardness
- 6
- Color
- Dark grey to black matrix with white or creamy phenocrysts
- Luster
- Dull/earthy matrix with vitreous crystals
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Physical properties
Hardness: 6.0 (Mohs scale); Color: Dark grey to black matrix with white or creamy phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Aphanitic matrix with larger visible crystals (phenocrysts); Cleavage: Poor; Specific gravity: 2.8-3.0
Formation & geological history
Formed through a two-stage cooling process of mafic lava. The larger crystals form slowly underground before the magma erupts, while the fine-grained matrix forms quickly upon reaching the Earth's surface.
Uses & applications
Commonly used as crushed stone for road construction, railroad ballast, and concrete aggregate. Select specimens are used as decorative garden stones or for architectural cladding.
Geological facts
The term 'porphyritic' comes from the Greek word for purple, originally referring to a specific purple Egyptian stone. This texture provides a visual timeline of a volcano's cooling history.
Field identification & locations
Identify in the field by looking for a dark, dense matrix containing distinct, larger crystals (often plagioclase feldspar). It is commonly found in oceanic crust and large volcanic provinces (flood basalts).
