igneous
Rhyolite (Porphyritic / Vesicular)
Rhyolite
Hardness: 6-7 Mohs. Color: reddish-brown to pinkish-grey groundmass with whitish feldspar phenocrysts and small pinhole vesicles. Luster: dull to sub-vitreous. Texture: porphyritic and slightly vesicular. Structure: fine-grained (aphanitic) matrix.
- Hardness
- 6-7 Mohs
- Luster
- dull to sub-vitreous
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Physical properties
Hardness: 6-7 Mohs. Color: reddish-brown to pinkish-grey groundmass with whitish feldspar phenocrysts and small pinhole vesicles. Luster: dull to sub-vitreous. Texture: porphyritic and slightly vesicular. Structure: fine-grained (aphanitic) matrix.
Formation & geological history
Extrusive igneous rock formed from high-silica (felsic) lava during explosive or viscous volcanic eruptions. Rapid cooling creates the fine groundmass while small gas bubbles form vesicles, and pre-crystallized mineral grains form white phenocrysts.
Uses & applications
Commonly used as crushed stone, construction aggregate, decorative landscaping rock, and for geological study and mineral collecting.
Geological facts
Rhyolite has the same chemical composition as granite, but cools much faster at or near the Earth's surface, resulting in much smaller crystals. Highly weathered stream-rounded rhyolite pebbles like this are common in ancient volcanic terrains.
Field identification & locations
Look for a reddish-pinkish fine-grained matrix with scattered small white feldspar or clear quartz crystals and tiny gas pits (vesicles). Commonly found in volcanic regions such as the Lake Superior area, western North America, and ancient volcanic belts worldwide.
