Igneous

Porphyritic Andesite

Intermediate Volcanic Rock - Andesite

Hardness: 6-7 (Mohs scale); Color: Gray to salt-and-pepper (intermediate); Luster: Dull (aphanitic groundmass) with vitreous phenocrysts; Crystal Structure: Porphyritic (large crystals in a fine-grained matrix); Composition: Plagioclase feldspar, hornblende, and pyroxene.

Hardness: 6-7 (Mohs scale)
Color: Gray to salt-and-pepper (intermediate)
Luster: Dull (aphanitic groundmass) with vitreous phenocrysts

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of intermediate-composition lava at or near the Earth's surface, often in subduction zones above volcanic arcs. The porphyritic texture indicates a two-stage cooling process: slow cooling underground (forming large phenocrysts) followed by a rapid eruption.

Uses & applications

Used primarily in the construction industry as crushed stone for road base, concrete aggregate, and railroad ballast. Due to its durability and resistance to slip, it is also used for paving and floor tiles.

Geological facts

Andesite is named after the Andes Mountains, where it is a primary constituent of the volcanic range. It is chemically intermediate between basalt and rhyolite.

Field identification & locations

Identify in the field by looking for a 'salt-and-pepper' appearance with visible rectangular white feldspar crystals (phenocrysts) embedded in a fine-grained gray matrix. Commonly found in the Pacific Northwest, the Andes, and Japan.

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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.

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Iron(II,III) oxide (Fe3O4)

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Blue Apatite

Apatite (Group), Ca5(PO4)3(F,Cl,OH)

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Magnetite

Magnetite (Fe3O4)

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