igneous
Porphyritic Basalt or Diabase
Mafic igneous rock with porphyritic texture
Hardness: 6-7 on Mohs scale, Color: Dark grey to black with distinct lighter plagioclase feldspar phenocrysts, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic groundmass with phaneritic phenocrysts, Specific gravity: 2.8-3.0
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Physical properties
Hardness: 6-7 on Mohs scale, Color: Dark grey to black with distinct lighter plagioclase feldspar phenocrysts, Luster: Dull to sub-vitreous, Crystal structure: Aphanitic groundmass with phaneritic phenocrysts, Specific gravity: 2.8-3.0
Formation & geological history
Formed through two-stage cooling of mafic magma: slow cooling at depth allowing larger crystals (phenocrysts) to form, followed by rapid cooling at or near the surface creating the fine-grained groundmass.
Uses & applications
Used in construction, as crushed stone for road aggregates, and as decorative stone.
Geological facts
Porphyritic textures are classic indicators of complex thermal histories in magma chambers before volcanic eruption or shallow intrusion.
Field identification & locations
Identify by contrasting light-colored mineral grains embedded in a dark, fine-grained matrix. Found worldwide in volcanic and sub-volcanic settings.
