Porphyritic phonolite or phonolite porphyry has well-formed, large leucite, nepheline, and alkali feldspar crystals in a fine-grained matrix.
The larger crystals are known as phenocrysts, while the fine-grained matrix is called groundmass.
In porphyritic phonolite, phenocrysts are mostly feldspathoids like nepheline or leucite and alkali feldspars like anorthoclase, sanidine, or orthoclase.
However, it can have other feldspathoids like sodalite, haüyne, or analcite or minerals like nosean, diopside, and minor amounts of titanite or kaersutite (titanium-bearing amphibole), etc.
On the other hand, the groundmass will have alkali feldspar, feldspathoids, and rarely plagioclase. Also, it will have minor augite, Fe-olivine, diopside, Na-amphibole, biotite, titanite, apatite, and opaque minerals.
Usually, when naming porphyritic phonolite, you should include the name of the dominant feldspathoids. For instance, you can have leucite or nepheline phonolites.
Lastly, considering its chemical and mineral composition, phonolite is the volcanic or extrusive igneous rock of feldspathoid syenite like nepheline, sodalite, or leucite syenites.
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How does porphyritic phonolite form?
Phonolite porphyry forms from two different cooling histories. The large crystals or phenocrysts during a slow cooling stage. This happens deep inside the Earth’s crust, where low levels of supersaturation or subcooling favor few mineral nucleation and larger crystal growth.
During the slow cooling stage, most of the magma is liquid, allowing for the formation of well-formed crystals.
On the other hand, a faster cooling stage happens when the partially crystallized magma with phenocrysts erupts or extrudes.
This eruption brings the magma to the surface, where cooling is fast. Fast cooling causes supercooling, which favors lots of nucleation. This high nucleation forms a fine-grained groundmass.
Besides two-stage cooling, porphyritic rocks can form in other ways. These include when minerals nucleate and their crystals grow at different rates.
Also, the larger crystals can start forming earlier outside the eutectic composition. Such will be the large and well-formed phenocrysts.
Later, cooling at eutectic composition will form the finer-grained groundmass. Groundmass crystals cannot grow as large as crystals that formed early unimpeded.
Porphyry phonolite uses
Porphyritic phonolite rocks are crushed to make aggregate for various uses, including fill, subbase, roading industry, railway ballast, etc.
Also, they are cut to make dimensional stones for building or even roofing tiles replacing shingles.
Furthermore, phonolite porphyries are associated with mineralization elements like gold and silver. A good example is The Cripple Creek Gold District in Colorado, USA, which has mostly phonolites and trachytes.
Where is it found?
Phonolite porphyry occurs in places with phonolite rocks. These places include Devil’s Tower and Missouri Buttes in Wyoming and Cripple Creek, Colorado, USA.
Also, they occur in Hoodoo Mountain and Baldface Mountain in British Colombia in Canada.
Lastly, kenyte, i.e., porphyritic anorthoclase phonolite variety, occurs in Mt. Kenya Volcano in Kenya and Mt. Erebus Volcano in Antarctica.