igneous
Porphyritic Basalt
Porphyritic Basalt (with plagioclase phenocrysts)
Hardness: 6.0 (Mohs); Color: Dark gray to black (mafic) groundmass with white/clear rectangular crystals; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Porphyritic texture (aphanitic groundmass with visible phenocrysts); Cleavage: Distinct in phenocrysts; Specific gravity: 2.8–3.0.
- Hardness
- 6
- Luster
- Dull/earthy matrix with vitreous crystals
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Physical properties
Hardness: 6.0 (Mohs); Color: Dark gray to black (mafic) groundmass with white/clear rectangular crystals; Luster: Dull/earthy matrix with vitreous crystals; Crystal structure: Porphyritic texture (aphanitic groundmass with visible phenocrysts); Cleavage: Distinct in phenocrysts; Specific gravity: 2.8–3.0.
Formation & geological history
Formed via two-stage cooling: slow underground cooling allows large phenocrysts (plagioclase) to form, followed by rapid eruption and cooling of the surrounding lava (groundmass). Commonly found in volcanic arcs and hotspots from any geological era.
Uses & applications
Primarily used as construction aggregate, road base, and ballast. High-quality specimens with dramatic contrast are used as decorative garden stone or lapidary slabs for bookends.
Geological facts
This texture is effectively a 'geological time capsule,' recording two distinct stages of a volcano's thermal history. It is a common component of oceanic crust and lunar maria.
Field identification & locations
Identify by looking for white, often lath-shaped or rectangular crystals (phenocrysts) embedded in a very fine-grained, dark-colored rock. Common in the Pacific Northwest, Hawaii, and Iceland. Collectors should look for sharp, unbroken crystals for best value.
