igneous

Andesite Porphyry

Andesite (Intermediate Volcanic Rock)

Hardness: 6-7 on Mohs scale; Color: Dark gray to bluish-black groundmass with light-colored white/cream phenocrysts; Luster: Dull to sub-vitreous; Structure: Porphyritic (large crystals in a fine-grained matrix); Cleavage: None (conchoidal fracture likely).

Hardness: 6-7 on Mohs scale
Luster: Dull to sub-vitreous

Identified March 7, 2026More igneous →

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Physical properties

Formation & geological history

Formed through the rapid cooling of intermediate-composition lava at or near the Earth's surface. The larger white crystals (likely plagioclase feldspar) formed slowly underground before a volcanic eruption triggered the fast cooling of the remaining dark matrix. Commonly associated with subduction zones.

Uses & applications

Primarily used as crushed stone for road construction, railroad ballast, and high-quality concrete aggregate due to its durability. Occasionally used for decorative masonry or as stepping stones in landscaping.

Geological facts

Andesite is named after the Andes Mountains where it is found in abundance. It is the volcanic equivalent of the plutonic rock Diorite. The distinct 'salt and pepper' look in porphyritic samples like this one is a classic indicator of a two-stage cooling history.

Field identification & locations

Identify in the field by looking for distinct larger light crystals (phenocrysts) embedded in a fine-grained, dark grey to black background. It is commonly found in volcanic arcs like the Cascades or the Andes. Collectors look for samples with well-defined, intact crystal shapes.

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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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