igneous
Porphyritic Basalt (or Andesite) containing Augite
Porphyritic Mafic Volcanic Rock
Hardness: 6 (matrix) to 7 (phenocrysts); Color: Grey to reddish-purple matrix with dark black phenocrysts; Luster: Dull/earthy matrix with sub-vitreous crystals; Structure: Porphyritic (large crystals in a fine-grained groundmass).
- Hardness
- 6 (matrix) to 7 (phenocrysts)
- Color
- Grey to reddish-purple matrix with dark black phenocrysts
- Luster
- Dull/earthy matrix with sub-vitreous crystals
Identified More igneous →
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Physical properties
Hardness: 6 (matrix) to 7 (phenocrysts); Color: Grey to reddish-purple matrix with dark black phenocrysts; Luster: Dull/earthy matrix with sub-vitreous crystals; Structure: Porphyritic (large crystals in a fine-grained groundmass).
Formation & geological history
Formed through a two-stage cooling process of volcanic lava: first cooling slowly underground to form large crystals (phenocrysts), then erupting and cooling rapidly on the surface to form the fine-grained matrix.
Uses & applications
Crushed stone for road construction, railroad ballast, and building materials; also a popular specimen for educational use and amateur geological collecting due to its distinct texture.
Geological facts
The large dark spots are likely Augite or Hornblende crystals. The reddish hue of the matrix suggests oxidation (weathering) of iron-bearing minerals within the rock after it formed.
Field identification & locations
Identify by looking for 'spots' (crystals) that are distinct from the surrounding fine-grained rock. Commonly found in volcanic regions like the Pacific Northwest, Hawaii, or historical volcanic fields in the Midwest.
