igneous
Porphyritic Basalt
Porphyritic Basalt (with Plagioclase phenocrysts)
Hardness: 6-7 on Mohs scale; Color: Dark grey to black groundmass with light-colored (white/cream) spots; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained) matrix with large visible crystals (phenocrysts); Cleavage: Indistinct; Specific Gravity: 2.8-3.0.
- 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 grey to black groundmass with light-colored (white/cream) spots; Luster: Dull to sub-vitreous; Crystal Structure: Aphanitic (fine-grained) matrix with large visible crystals (phenocrysts); Cleavage: Indistinct; Specific Gravity: 2.8-3.0.
Formation & geological history
Formed from the cooling of mafic lava. The large crystals (phenocrysts) formed slowly underground before the remaining lava erupted and cooled rapidly on the surface. This specific texture is called 'porphyritic'. Often associated with volcanic arcs and oceanic crust.
Uses & applications
Used primarily as crushed stone for construction aggregate, road base, and railroad ballast. Exceptionally attractive specimens are sometimes tumbled for decor or used as 'lucky stones' in pockets.
Geological facts
This texture is sometimes referred to as 'Chinese Writing Stone' when the crystals are needle-like, or 'Snowflake Stone'. The larger crystals are most commonly plagioclase feldspar.
Field identification & locations
Identify in the field by the distinct 'spotted' appearance of light crystals against a dark, dense matrix. Commonly found in areas of past volcanic activity, such as the Pacific Northwest, Hawaii, or glacial till in the Great Lakes region.
