rhyolite porphyry - light pink rhyolite with small phenocrysts - hand specimen

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. 

In this porphyry, the feldspar phenocrysts have been converted to kaolin, so they are pseudomorphs of kaolin after orthoclase. They retain the shape but, unlike orthoclase, can be scratched by a knife. The composition of the phenocrysts was recently identified by x-ray reflection spectroscopy. Since feldspar alters to clay, this shouldn't have been a surprise. 

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  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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