igneous
Porphyry
Porphyritic Igneous Rock
Hardness: 6-7 on Mohs scale. Color: Grayish-green background matrix (aphanitic to fine-grained) with embedded white to cream feldspar phenocrysts. Luster: Dull to earthy matrix with vitreous feldspar crystals. Specific gravity: ~2.6-2.8.
- Hardness
- 6-7 on Mohs scale
- Luster
- Dull to earthy matrix with vitreous feldspar crystals
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Grayish-green background matrix (aphanitic to fine-grained) with embedded white to cream feldspar phenocrysts. Luster: Dull to earthy matrix with vitreous feldspar crystals. Specific gravity: ~2.6-2.8.
Formation & geological history
Formed via a two-stage cooling process of silica-rich magma. Large crystals (phenocrysts) grow slowly at depth, followed by rapid cooling nearer to the surface which forms the fine-grained matrix. Commonly associated with volcanic arcs and intrusive igneous bodies.
Uses & applications
Historically used as decorative and structural stone, in road aggregate, and as a key indicator rock in mineral exploration for porphyry copper deposits.
Geological facts
The term porphyry comes from the Greek word for purple, as ancient imperial porphyry from Egypt was reddish-purple. It is famously linked to major copper, gold, and molybdenum ore deposits worldwide.
Field identification & locations
Identified by its distinctive porphyritic texture—visible larger crystals (phenocrysts) suspended within a fine-grained groundmass. Widely found in river beds, glacial deposits, and mountain ranges around active or ancient volcanic zones.
