rhyolite porphyry - gray rhyolite porphyry with interesting small brassy phlogopite mica phenocrysts. Student unit of 5 specimens
This rhyolite porphyry is particularly rich with small brassy phenocrysts of phlogopite mica in addition to the large white feldspar phenocrysts. 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.
The small 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. Phlogopite grades into biotite, the more common mica in this rhyolite. 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.
Potassium-argon ages from nearby rhyolites in ash flows in the Coso Formation yield ages of around 3.4 million years. These pumice flows represent a single eruption or a series of eruptions in the Coso Range just east of Haiwee Reservoir, the water from which flows to perpetually thirsty Los Angeles. The area is geologically active, with a geothermal plant not far from where these specimens were collected, but inside the China Lake Naval Weapons Test Station, generating enough electricity that the U.S. Navy sells the excess to the commercial grid.
If you are a science or earth science teacher purchasing this as a teaching specimen for your class, consider having your students compare this with a rhyolite porphyry that cooled for a shorter time at depth. We have examples with very small feldspar phenocrysts that spent little time cooling at depth.
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