igneous
Basalt with Plagioclase Phenocrysts (Porphyritic Basalt)
Porphyritic Basalt (primarily Plagioclase Feldspar, Pyroxene, and Olivine)
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal matrix with white crystalline laths; Luster: Dull/Sub-metallic; Structure: Aphanitic matrix with visible phenocrysts; Cleavage: None (conchoidal fracture).
- Hardness
- 6 (Mohs scale)
- Color
- Dark grey to charcoal matrix with white crystalline laths
- Luster
- Dull/Sub-metallic
Identified More igneous →
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Physical properties
Hardness: 6 (Mohs scale); Color: Dark grey to charcoal matrix with white crystalline laths; Luster: Dull/Sub-metallic; Structure: Aphanitic matrix with visible phenocrysts; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed from the rapid cooling of magnesium-rich and iron-rich lava on or near the Earth's surface. The white 'spots' (phenocrysts) formed deeper in the magma chamber before the eruption. Age varies by location but ubiquitous throughout Earth's history.
Uses & applications
Used primarily as crushed stone for construction, road base, concrete aggregate, and railroad ballast. Exceptionally durable for high-traffic paving stone.
Geological facts
Basalt is the most common rock type in Earth's crust and makes up most of the ocean floor. Porphyritic varieties are often called 'Chinese Writing Stone' when the feldspar crystals form patterns resembling calligraphy.
Field identification & locations
Identify by its heavy weight (high density), dark fine-grained groundmass, and distinct rectangular white crystals. Commonly found in volcanic regions, riverbeds near volcanic flows, and coastal areas.
