igneous
Porphyritic Basalt or Porphyry
Porphyritic Extrusive Igneous Rock
Hardness: 6 on the Mohs scale. Color: Brownish-grey to dark grey with lighter lath-shaped phenocrysts (likely plagioclase feldspar). Luster: Dull to sub-vitreous. Crystal structure: Porphyritic texture featuring distinct large crystals (phenocrysts) embedded in a finer-grained groundmass.…
- Hardness
- 6 on the Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6 on the Mohs scale. Color: Brownish-grey to dark grey with lighter lath-shaped phenocrysts (likely plagioclase feldspar). Luster: Dull to sub-vitreous. Crystal structure: Porphyritic texture featuring distinct large crystals (phenocrysts) embedded in a finer-grained groundmass. Specific gravity: Around 2.7 to 3.0.
Formation & geological history
Formed by a two-stage cooling process in volcanic or subvolcanic environments. Magma first cools slowly deep underground, forming large crystals, then erupts onto the surface or into shallow dikes, cooling rapidly to form the fine-grained matrix.
Uses & applications
Used in crushed stone for construction, road aggregate, and sometimes as decorative dimension stone or architectural facing when polished.
Geological facts
Porphyritic textures are classic examples of complex cooling histories in magmatic systems. The term 'porphyry' historically comes from the purple imperial porphyry used by the ancient Romans, derived from Egypt.
Field identification & locations
Easily identified in the field by the distinct 'chocolate-chip' or needle-like interlocking crystals scattered throughout a darker, finer matrix. Commonly found in volcanic regions, ancient lava flows, and intrusive dikes worldwide.
