igneous
Basalt (Porphyritic)
Porphyritic Basalt
Hardness: 6 (Mohs); Color: Dark grey to greenish-grey with white phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic matrix with visible crystals; Cleavage: None (conchoidal fracture); Specific gravity: 2.8–3.0
- Hardness
- 6 (Mohs)
- Color
- Dark grey to greenish-grey with white phenocrysts
- Luster
- Dull/Earthy
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Physical properties
Hardness: 6 (Mohs); Color: Dark grey to greenish-grey with white phenocrysts; Luster: Dull/Earthy; Crystal structure: Aphanitic matrix with visible crystals; Cleavage: None (conchoidal fracture); 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 white spots (phenocrysts) formed slowly underground before the final eruption.
Uses & applications
Crushed stone for road base, concrete aggregate, railroad ballast, and occasionally used as decorative garden stones.
Geological facts
Basalt is the most common rock type in the Earth's crust and makes up most of the ocean floor. Porphyritic texture indicates a two-stage cooling history.
Field identification & locations
Identify by its heavy weight (density), dark fine-grained groundmass, and distinct larger crystals of plagioclase feldspar. Commonly found in volcanic regions or glacial till.
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Dark Igneous Rock (Likely Basalt or Diabase)
Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.
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