igneous
Porphyritic Diabase (Porphyry)
Porphyritic Diabase / Microgabbro
Hardness: 6-7 on Mohs scale. Color: Dark gray to black matrix filled with lighter grayish-white plagioclase feldspar phenocrysts. Luster: Sub-vitreous to dull. Texture: Porphyritic with diabasic (ophitic) groundmass. Cleavage: Feldspars show distinct 2-direction cleavage at nearly 90 degrees.
- Hardness
- 6-7 on Mohs scale
- Luster
- Sub-vitreous to dull
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Physical properties
Hardness: 6-7 on Mohs scale. Color: Dark gray to black matrix filled with lighter grayish-white plagioclase feldspar phenocrysts. Luster: Sub-vitreous to dull. Texture: Porphyritic with diabasic (ophitic) groundmass. Cleavage: Feldspars show distinct 2-direction cleavage at nearly 90 degrees.
Formation & geological history
Formed when magma cools in two distinct stages: large plagioclase crystals (phenocrysts) grow slowly at depth, followed by rapid cooling of the remaining magma near or at shallow sub-surface dike/sill intrusions.
Uses & applications
Used as crushed stone for construction, road base, riprap, and decorative landscaping rock. Sometimes polished for ornamental stones or paperweights by collectors.
Geological facts
The distinct texture of large crystals floating in a fine-grained matrix is known as a porphyritic texture. Dikes of diabase porphyry are famous worldwide, including occurrences in Sweden (Hälleflinta/Ganggraniet) and North America.
Field identification & locations
Identified by the distinct contrast between large, blocky light-colored feldspar phenocrysts and the fine-grained dark basaltic/diabasic matrix. Commonly found in areas with volcanic or intrusive basaltic dike formations, shoreline gravels, and glacial till.
