Porphyritic rhyolite has large, well-formed crystals, mostly alkali feldspar, plagioclase, quartz, and less often hornblende, pyroxene, or mica in a fine-grained texture matrix.
The fine-grained matrix will have alkali feldspar, plagioclase, and quartz with smaller amounts of hornblende, muscovite, and biotite. Sometimes, this matrix may have fayalite and accessory minerals like magnetite and ilmenite.
The large, euhedral to subhedral crystals are known as phenocrysts, while the finer-grained matrix is the groundmass. Euhedral describes well-formed crystals whose faces are sharp and easy to recognize.
In porphyritic rhyolite, the alkali feldspar, usually sanidine, is less often orthoclase or anorthoclase, while plagioclase is oligoclase.
However, the exact phenocrysts in this rock may vary. For instance, nevadite, a highly porphyritic rhyolite, is dominated by quartz phenocrysts.
Sometimes, porphyritic rhyolite is known as pyric rhyolite (US) or rhyolite porphyry. Porphyry is a none that describes minerals with two distinctive grain sizes, i.e., bimodal grain sizes.
Lastly, rhyolite is one of the most strongly porphyritic rocks. Some specimens can have as much as 50% phenocrysts. This is the highest amount of phenocrysts that will still allow magma to move.
_(26755758577).jpg){kind=link}
How does porphyritic rhyolite form?
Rhyolite porphyry forms from two-stage cooling, i.e., a slow, deep inside the Earth’s crust and a faster one when rhyolitic magma erupts.
During the slow cooling stage, nucleation rates are low, and most melt is uncrystallized. This allows the growth of large, well-formed crystals. These crystals form phenocrysts.
Nucleation rates are low due to a low degree of supersaturation in magma cooling deep inside the other crust.
When the magma erupts as lava, cooling rates are high. This supercooling will cause many crystals to nucleate. These many crystals will not have enough time to grow large. Therefore, they will form a fine-grained groundmass.
Besides two cooling histories, porphyritic rocks can also form if crystals nucleate and grow at different rates. However, this is unlikely the cause of rhyolite porphyry.
Neither the early crystallization of minerals outside the eutectic composition explains why rhyolitic rock forms porphyritic texture. This probably occurs in plutonic rocks, which cool slowly.
Lastly, eutectic composition refers to a homogenous mixture whose temperature is below the melting point of any of the components.
Porphyritic rhyolite composition
Porphyritic rhyolite has a composition similar to other rhyolitic rocks. The only difference is that it has large, well-formed crystals in a fine-grained groundmass.
Chemically, this rock is an acidic rock with 69-80 wt.% silica. It is also high in sodium and potassium oxide, i.e., more than 7 wt.%, and low in calcium, magnesium, and iron oxides.
On the other hand, rhyolite has mainly alkali feldspar, quartz, lesser plagioclase, and minor amounts of hornblende, pyroxene, biotite, and fayalite. Also, it may have accessory minerals like ilmenite and magnetite.
Considering the composition of rhyolite, it is the extrusive equivalent of granite.
Where are they found?
Porphyritic rhyolite rocks will occur where rhyolite rocks occur. These include the above subduction zones in continental margins. Also, they occur in some hotspots and rifts.
They occur as lava plugs, mounds, or domes and in lava flows, tuffs, and ignimbrites.
Some places with porphyritic rhyolite include Yellowstone in Wyoming, Moat Volcanics in New Hampshire, Long Valley Systems and Coso Volcanic Field in California, and North Shore Volcanic Group in Minnesota, among other places.
Uses
Rhyolite porphyry doesn’t have many uses in construction size, and it is often vuggy and fractured. It serves as fill or aggregate when there are no other rocks available.
This rock may also host valuable gemstones like topaz, jasper, opal, red beryl, agate, etc., in vugs, geodes, thundereggs, amygdules, or spherulites.
Did you know that thundereggs only occur in rhyolitic lava flows, ash deposits, or tuff? Now you know.