igneous
Porphyritic Basalt (with Augite Crystals)
Porphyritic Mafic Volcanic Rock (Fe-Mg Silicates)
Hardness: 5-7 (Mohs). Color: Dark gray to black matrix with darker, chunky phenocrysts. Luster: Dull to sub-vitreous. Structure: Porphyritic (large crystals in a fine-grained 'groundmass'). Cleavage: Poorly visible in matrix, two directions at nearly 90 degrees in crystals.
- Hardness
- 5-7 (Mohs)
- Color
- Dark gray to black matrix with darker, chunky phenocrysts
- Luster
- Dull to sub-vitreous
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Physical properties
Hardness: 5-7 (Mohs). Color: Dark gray to black matrix with darker, chunky phenocrysts. Luster: Dull to sub-vitreous. Structure: Porphyritic (large crystals in a fine-grained 'groundmass'). Cleavage: Poorly visible in matrix, two directions at nearly 90 degrees in crystals.
Formation & geological history
Formed from volcanic activity where magma begins cooling slowly underground (forming the larger crystals) and then erupts, cooling rapidly on the surface (forming the fine-grained matrix). Often found in Cenozoic volcanic fields.
Uses & applications
Used primarily in road construction as crushed aggregate, as railroad ballast, and occasionally as decorative garden stones. High-quality specimens are used by geology students for studying igneous textures.
Geological facts
The 'porphyritic' texture is a geologic 'diary' of two-stage cooling: a slow subterranean stage followed by a fast eruptive stage. These rocks are common on the ocean floor and in volcanic island chains like Hawaii.
Field identification & locations
Identify in the field by looking for a 'chocolate chip cookie' appearance—dark, distinct crystal shapes embedded in a uniform, fine-grained background. Commonly found in coastal or volcanic regions like the Pacific Northwest or Scotland.
