igneous
Porphyritic Basalt (with Granite inclusion)
Porphyritic Mafic Volcanic Rock
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with pink and white inclusions; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals or clasts); Specific Gravity: 2.8 - 3.0.
- Hardness
- 6-7 on Mohs scale
- Color
- Dark grey to black groundmass with pink and white inclusions
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with pink and white inclusions; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals or clasts); Specific Gravity: 2.8 - 3.0.
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava at or near the Earth's surface. The larger pink inclusion is likely a piece of granite (xenolith) or feldspar phenocryst caught in the flow. Common in volcanic arcs and mid-ocean ridges.
Uses & applications
Primarily used as crushed stone for aggregate in construction, road base, and railroad ballast. Smooth river-worn specimens are popular in landscaping and as pocket stones for collectors.
Geological facts
The presence of larger crystals in a fine-grained matrix indicates two stages of cooling: a slow cooling period deeper underground followed by a rapid eruption and cooling at the surface.
Field identification & locations
Identify by the dark, dense, fine-grained matrix contrasted with visible pinkish-orange feldspar crystals. Commonly found in glacial till, riverbeds, and volcanic regions. Collectors look for high contrast between the matrix and inclusions.
