igneous
Porphyritic Basalt or Diabase
Porphyritic Igneous Rock
Mohs hardness around 6-7; distinct rectangular to lath-shaped white or grey phenocrysts (likely plagioclase feldspar) embedded in a finer-grained dark green to grey groundmass; sub-to-well rounded water-worn pebble; dull to sub-vitreous luster; specific gravity ~2.7-3.0.
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Physical properties
Mohs hardness around 6-7; distinct rectangular to lath-shaped white or grey phenocrysts (likely plagioclase feldspar) embedded in a finer-grained dark green to grey groundmass; sub-to-well rounded water-worn pebble; dull to sub-vitreous luster; specific gravity ~2.7-3.0.
Formation & geological history
Formed through a two-stage cooling process. Initially, magma cools slowly at depth, allowing large crystals (phenocrysts) to form, followed by a rapid cooling phase near or at the surface, creating the fine-grained groundmass. Often found in glacial tills, riverbeds, and ancient volcanic provinces.
Uses & applications
Used primarily in landscaping, crushed stone for construction, road aggregate, and as collectible specimens for amateur geologists.
Geological facts
The distinct texture is known as porphyritic texture. The large lighter crystals are often referred to as 'phenocrysts', standing out sharply against the darker matrix.
Field identification & locations
Easily identifiable in the field by its striking 'cookie-and-cream' or spotted appearance due to the contrasting light feldspar crystals in a dark volcanic matrix. Common in regions with glacial activity or ancient basalt flows.
