igneous
Porphyritic Igneous Rock / Rhyolite Porphyry
Porphyritic volcanic rock
Hardness: 6-7 on Mohs scale; Color: Pinkish-brown or greyish with darker phenocrysts; Luster: Dull to sub-vitreous; Texture: Porphyritic (large crystals embedded in a fine-grained matrix); Specific Gravity: ~2.5 - 2.7
- Hardness
- 6-7 on Mohs scale
- Color
- Pinkish-brown or greyish with darker phenocrysts
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 6-7 on Mohs scale; Color: Pinkish-brown or greyish with darker phenocrysts; Luster: Dull to sub-vitreous; Texture: Porphyritic (large crystals embedded in a fine-grained matrix); Specific Gravity: ~2.5 - 2.7
Formation & geological history
Formed via two-stage cooling: slow cooling beneath the Earth's surface formed large crystals (phenocrysts), followed by rapid eruption and cooling at the surface creating the fine-grained groundmass.
Uses & applications
Used as decorative stone, landscaping, and occasionally road aggregate.
Geological facts
The distinct two-sized crystal structure tells a complex story of a magma chamber slowly cooling at depth before being violently ejected in a volcanic eruption.
Field identification & locations
Identify by looking for distinct larger mineral grains (often quartz or feldspar) set within a uniform fine-grained pinkish or grey rock matrix. Commonly found in ancient volcanic regions.
