igneous

Porphyritic Basalt (with Lath-like Phenocrysts)

Porphyritic Basalt

Hardness: 5-6 (Mohs); Color: Dark grey to black groundmass with light-colored (white/creamy) rectangular phenocrysts; Luster: Dull/earthy matrix with vitreous crystals; Structure: Porphyritic (large crystals in fine-grained matrix); Cleavage: Perfect in certain planes for the phenocrysts.

Hardness: 5-6 (Mohs)
Luster: Dull/earthy matrix with vitreous crystals

Identified April 5, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the two-stage cooling of volcanic magma. The large crystals (phenocrysts) formed slowly at depth, followed by a rapid eruption and cooling of the surrounding liquid matrix (groundmass) at or near the surface.

Uses & applications

Primary uses include road aggregate, concrete manufacture, and crushed stone in construction. Distinctive porphyritic specimens are often collected for geological study and education.

Geological facts

The large white crystals are likely plagioclase feldspar. Porphyritic basalt makes up a significant portion of the oceanic crust and is common in volcanic islands like Hawaii and Iceland.

Field identification & locations

Identify in the field by looking for a fine-grained, dark-colored matrix containing distinct, well-formed, lighter colored lath-shaped crystals. Commonly found in areas of past or present volcanic activity.

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Dark Igneous Rock (Likely Basalt or Diabase)

Basalt (extrusive igneous rock) or Diabase (intrusive igneous rock - also known as Dolerite), largely composed of plagioclase feldspar, pyroxene (augite), and sometimes olivine and amphibole. Exact mineralogy would require thin section analysis.

Igneous

Magnetite

Iron(II,III) oxide (Fe3O4)

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Magnetite

Magnetite (Fe3O4)

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