igneous

Andesite Porphyry

Andesite (extrusive volcanic rock)

Hardness: 6-7 (Mohs scale); Color: Gray to greenish-gray groundmass with white/light-colored phenocrysts (likely plagioclase feldspar); Luster: Dull/earthy; Structure: Porphyritic (large crystals in a fine-grained matrix); Specific Gravity: 2.4 - 2.8

Hardness: 6-7 (Mohs scale)
Luster: Dull/earthy

Identified March 8, 2026More igneous →

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Physical properties

Formation & geological history

Formed from the rapid cooling of intermediate lava at or near the Earth's surface, typically at subduction zones (convergent plate boundaries). The large crystals (phenocrysts) formed slowly at depth before the eruption, while the fine matrix cooled quickly upon eruption.

Uses & applications

Primarily used as crushed stone for road construction, railroad ballast, and aggregate in concrete. High-quality specimens are sometimes used for landscaping or as decorative stone.

Geological facts

Andesite is named after the Andes Mountains in South America, where it is very common. It represents a chemical middle ground between basalt and rhyolite. Many famous stratovolcanoes, like Mount St. Helens, are primarily composed of andesite.

Field identification & locations

Identify in the field by looking for a 'salt and pepper' appearance where distinct, blocky white crystals are suspended in a solid gray or bluish-gray background. Common in volcanic arcs like the Pacific Northwest of the USA, Japan, and the Andes.

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Fluorapatite (Ca5(PO4)3F)

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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

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

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Magnetite

Magnetite (Fe3O4)

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