Igneous
Porphyritic Basalt or Andesite
Porphyritic Basalt / Andesite
Hardness: 5-6 (Mohs); Color: Dark gray to black matrix with light-colored (white/cream) phenocrysts; Luster: Dull to earthy; Crystal structure: Fine-grained matrix with larger visible crystals (phenocrysts).
- Hardness
- 5-6 (Mohs)
- Luster
- Dull to earthy
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Physical properties
Hardness: 5-6 (Mohs); Color: Dark gray to black matrix with light-colored (white/cream) phenocrysts; Luster: Dull to earthy; Crystal structure: Fine-grained matrix with larger visible crystals (phenocrysts).
Formation & geological history
Formed from the cooling of magma or lava. The larger light-colored crystals (phenocrysts, likely plagioclase feldspar) cooled slowly deep underground, while the darker, fine-grained matrix cooled rapidly upon or near the surface.
Uses & applications
Commonly used as crushed stone for construction, road base, and railroad ballast. Porphyritic varieties can sometimes be used as decorative stone.
Geological facts
The term 'porphyritic' refers to the distinct difference in crystal size, indicating a two-stage cooling history of the magma.
Field identification & locations
Identifiable by the stark contrast between the large, light-colored crystals and the dark, fine-grained background. Found in areas with volcanic or prior volcanic activity.
