igneous
Porphyritic Basalt or Andesite
Igneous rock with porphyritic texture
Hardness: 6-7 on Mohs scale. Color: Dark reddish-brown to purple matrix with lighter plagioclase feldspar phenocrysts. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained) groundmass with visible phenocrysts. Specific gravity: 2.7-3.0.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark reddish-brown to purple matrix with lighter plagioclase feldspar phenocrysts. Luster: Dull to sub-vitreous. Crystal structure: Aphanitic (fine-grained) groundmass with visible phenocrysts. Specific gravity: 2.7-3.0.
Formation & geological history
Formed through a two-stage cooling process in volcanic environments. Initially, magma cools slowly beneath the surface allowing larger crystals (phenocrysts) to form, followed by rapid eruption and cooling at the surface, creating the fine-grained matrix.
Uses & applications
Used primarily as crushed stone for construction, road aggregate, and occasionally in landscaping or as a decorative stone.
Geological facts
The reddish-brown color in this specific specimen is often due to the oxidation (rusting) of iron-bearing minerals like olivine or pyroxene exposed to weathering over time.
Field identification & locations
Commonly found in volcanic regions, ancient lava flows, and alluvial gravel beds. Identify by its distinct fine-grained background contrasting with embedded lighter crystals.
