igneous
Porphyritic Basalt or Porphyritic Andesite
Porphyritic Extrusive Igneous Rock
Hardness: 6-7 on Mohs scale; Color: Dark grey to greenish groundmass with lighter plagioclase feldspar phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with larger distinct crystals (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 grey to greenish groundmass with lighter plagioclase feldspar phenocrysts; Luster: Dull to sub-vitreous; Crystal structure: Aphanitic (fine-grained) groundmass with larger distinct crystals (phenocrysts); Specific gravity: 2.7-3.0.
Formation & geological history
Formed through a two-stage cooling process: slow cooling deep underground allowing large crystals (phenocrysts) to form, followed by rapid eruption and cooling at or near the Earth's surface creating the fine-grained groundmass.
Uses & applications
Used in construction, road aggregate, and sometimes as ornamental stone or polished specimens by rock collectors.
Geological facts
The distinct larger crystals are called phenocrysts, which give the rock a 'spotted' appearance often referred to colloquially as 'leopard rock' or similar pattern names depending on the mineral inclusions.
Field identification & locations
Easily identified by its contrasting light-colored crystal patches embedded within a dark, fine-grained volcanic matrix. Commonly found in regions with historical volcanic activity or glacial outwash plains.
