igneous
Porphyritic Andesite
Andesite (Porphyritic texture)
Hardness: 6-7 on Mohs scale; Color: Dark purple to brownish-maroon groundmass with white/light gray phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None visible in matrix.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to sub-vitreous
Identified More igneous →
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Dark purple to brownish-maroon groundmass with white/light gray phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (fine-grained matrix with larger visible crystals); Cleavage: None visible in matrix.
Formation & geological history
Formed from extrusive volcanic activity where magma begins cooling slowly underground (forming large phenocrysts) followed by a rapid eruption and quick cooling at the surface (forming the fine-grained matrix).
Uses & applications
Primarily used in construction as crushed stone, road ballast, or as decorative river rock. Polished specimens may be used in jewelry or as palm stones.
Geological facts
The name Andesite comes from the Andes Mountains where it is abundant. The distinct white dashes in this specimen are likely plagioclase feldspar crystals that grew before identifying the rock was erupted.
Field identification & locations
Identify by looking for 'rice-grain' like white crystals embedded in a dark, fine-grained rock. Often found in volcanic regions or as glaciated 'river rocks' far from the source.
