igneous
Porphyritic Andesite
Porphyritic Andesite (Intermediate Volcanic Rock)
Hardness: 6-7 on Mohs scale; Color: Dark gray to greenish-gray 'groundmass' with white to light-colored rectangular crystals; Luster: Dull to sub-vitreous; Crystal Structure: Porphyritic (large crystals in fine-grained matrix); Cleavage: Good in two directions for the feldspar phenocrysts; Specific Gravity: 2.4-2.8
- 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 gray to greenish-gray 'groundmass' with white to light-colored rectangular crystals; Luster: Dull to sub-vitreous; Crystal Structure: Porphyritic (large crystals in fine-grained matrix); Cleavage: Good in two directions for the feldspar phenocrysts; Specific Gravity: 2.4-2.8
Formation & geological history
Formed through a two-stage cooling process of intermediate magma. Initial slow cooling deep underground allows large Plagioclase feldspar crystals (phenocrysts) to grow, followed by rapid cooling during a volcanic eruption which freezes the remaining magma into a fine-grained 'groundmass'. Commonly found in volcanic arcs above subduction zones.
Uses & applications
Used primarily in construction as crushed stone for road beds, railway ballast, and occasionally as decorative facing stone or garden ornaments due to its unique 'starry' or 'snow-on-rock' appearance.
Geological facts
This specific texture is often colloquially called 'Chinese Writing Stone' or 'Chrysanthemum Stone' (though true Chrysanthemum stone is different). It captures a snapshot in time of a volcano's plumbing system right before an eruption.
Field identification & locations
Identify in the field by looking for distinct, lath-shaped white crystals that look like scattered grains of rice or matchsticks embedded in a dark, hard rock. Common in the Andes Mountains, Cascades (USA), and Japan. Collectors should look for high contrast between the white crystals and dark background.
