rhyolite porphyry - teaching hand specimen of rhyolite with small phenocrysts
Rhyolite porphyry is a silica rich igneous rock with a composition similar to that of granite. Porphyries are igneous rocks that have had two stages of cooling. Typically they have phenocrysts, larger crystals (pheno = appear in Greek), of feldspar surrounded by smaller crystals that may be so small that they are not visible without a hand lens. With a long period of underground cooling the phenocrysts become large. When deep underground cooling is only a relatively short period, the phenocrysts are small as in these specimens, since they did not have enough time to grow.
An earth science student should recognize this as a rhyolite, the extrusive equivalent of a granite, with a similar silica-rich composition. I would give additional credit to a student who identified this as a porphyry, since the phenocrysts of feldspar are small. The surrounding mass of microscopic crystals is typical of rhyolites - the texture of rhyolite is normally aphanitic, from the Greek words, a = not and phaneros = visible, referring to the crystals.
Granite can also be porphyritic when feldspar phenocrysts form in a deep magma that was later brought closer to the surface where it finally cooled and crystallized.
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