igneous

Porphyritic Rhyolite

Porphyritic Rhyolite (compositionally similar to granite)

Hardness: 6-7 (Mohs); Color: Light gray to tan with white crystals; Luster: Dull/earthy groundmass with vitreous crystals; Crystal structure: Aphanitic groundmass with rectangular phenocrysts (orthoclase feldspar); Specific gravity: 2.4-2.6

Hardness: 6-7 (Mohs)
Color: Light gray to tan with white crystals
Luster: Dull/earthy groundmass with vitreous crystals

Identified May 20, 2026More igneous →

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Physical properties

Formation & geological history

Formed via rapid cooling of silica-rich volcanic lava at or near the Earth's surface, where larger crystals (phenocrysts) grew slowly at depth before the final eruption. They often date from the Proterozoic to the Cenozoic eras.

Uses & applications

Commonly used as crushed stone for road construction and aggregate; some fine-grained varieties are used as decorative stone or for ancient stone tools (lithics).

Geological facts

Rhyolite is the volcanic equivalent of granite. The larger white crystals visible in this specimen are likely feldspar, which solidified while the magma was still deep underground before it erupted.

Field identification & locations

Identified by light color (silica-rich), fine-grained base, and the presence of visible rectangular crystals. Found in volcanic regions like the American West (Yellowstone), Andes, and Iceland.

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Blue Apatite

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

Magnetite

Iron(II,III) oxide (Fe3O4)

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Sodalite

Sodalite - Na8(Al6Si6O24)Cl2

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Blue Apatite

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

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Magnetite

Magnetite (Fe3O4)

mineral