rhyolite porphyry - teaching hand specimen of a very attractive gray rhyolite porphyry with feldspar and phlogopite phenocrysts
Rhyolite is a silica rich extrusive or volcanic igneous rock with a composition similar to that of granite. This is a particularly attractive rhyolite porphyry, with good size contrast between the large feldspar phenocrysts and the much smaller crystalline groundmass.
Porphyries exhibit two stages of cooling - slow cooling deep underground and then more rapid cooling on or near the surface. The large white crystals are orthoclase, a potassium feldspar that forms crystals at high temperatures. The smaller surrounding crystal matrix of a rhyolite is made up of crystals of quartz, sanidine and biotite, or in this case, phlogopite which grades into biotite.
The large phenocrysts should tell students that this cooled for some time at depth, allowing the white feldspar crystals to form at high temperatures. The magma was then brought to the surface where rapid cooling froze the crystals of the remaining minerals before they had time to grow. Two stages of cooling are typical of porphyries. Phenocrysts, the larger crystals that formed at depth, are named from the Greek word phainos for appear.
This rhyolite porphyry is particularly rich with small brassy phenocrysts of phlogopite mica in addition to the much larger white feldspar phenocrysts. The brassy phlogopite phenocrysts are unusual in this rhyolite and make these specimens particularly attractive. Phlogopite is occasionally characterized by having a brassy or coppery reflection from the cleavage surfaces, and this is a good example.
This material was encountered in a pyroclastic pumice bed and pushed aside by the miners. We never know what we will see when we crack a rhyolite boulder, and this one was a surprise. Phenocrysts (pheno = appear in Greek) are the typical characteristic of a porphyry. Granite can also be porphyritic when feldspar phenocrysts formed in a deep magma that was later brought closer to the surface where it finally cooled and crystallized.
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