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Phonolite

Phonolite

Phonolite, also known by the rather uninspired English moniker clinkstone, is an extrusive igneous rock that typically forms in shallow intrusions or as a volcanic extrusive. It’s an uncommon beast, straddling that awkward middle ground in chemical composition between the lighter felsic and the heavier mafic rocks. Its texture can range from the finely grained aphanitic to the more complex porphyritic, where you’ll find a mix of grain sizes. Think of it as a variation on trachyte, but with the distinct absence of quartz and the rather bold inclusion of nepheline or leucite. It boasts an unusually high alkali content, specifically Na₂O + K₂O, which clocks in at 12% or more. This is what lands it squarely in its designated spot within the TAS classification of igneous rocks. Its intrusive, coarser-grained (phaneritic) counterpart is the rather more common nepheline syenite. Phonolite itself, however, is typically fine-grained and remarkably compact.

The name "phonolite" is a rather poetic nod to its origins, derived from the Ancient Greek words for "sounding stone." This appellation stems from the distinctive, almost metallic ring it produces when an unfractured piece is struck – a characteristic that also lends it the synonym "clinkstone." It’s a rock that makes a sound. Remarkable, isn't it?

Formation

This particular section feels… undernourished. It’s as if the author simply shrugged and said, "Eh, it is what it is." Let's try to inject some life into it.

Phonolite’s genesis is, shall we say, peculiar. It originates from magma that’s surprisingly low in silica. This isn't the usual path for such rocks; it’s typically generated by very limited partial melting – less than 10% – of highly aluminous rocks found deep within the Earth's crust. We're talking about rocks like tonalite, monzonite, and various metamorphic rocks. This low-degree melting process is quite selective, favoring the liberation of aluminum, potassium, sodium, and calcium primarily through the melting of feldspar minerals, with a lesser contribution from mafic minerals. The consequence of this silica-undersaturated environment is the near-complete absence of quartz or any other silica-rich crystalline structures. Instead, the rock is dominated by feldspathoid minerals, which are inherently low in silica, and these tend to be more prevalent than feldspar itself.

Geological events and tectonic shifts play a crucial role in creating the conditions necessary for phonolite formation. Intracontinental hotspot volcanism is a prime candidate, often occurring above mantle plumes that are situated beneath thick layers of continental crust. You’ll often find phonolites associated with A-type granites and alkaline igneous provinces. Another possibility involves the low-degree partial melting of the underlying granitic material in the root zones of orogenic belts – essentially, the zones where continents collide.

Mineralogy and Petrology

Phonolite is, in essence, the fine-grained counterpart to nepheline syenite. Both are products of partial melting and are characterized by their silica-undersaturated nature, meaning they contain feldspathoids in their normative mineralogy rather than quartz.

The typical mineral assemblage found in phonolites is rich in feldspathoids such as nepheline, sodalite, hauyne, leucite, and analcite. Alongside these, alkali feldspar minerals like sanidine, anorthoclase, or orthoclase are abundant. Sodic plagioclase is rare. Minor but common constituents include biotite, sodium-rich amphiboles and pyroxenes, and iron-rich olivine. Accessory minerals, appearing in smaller quantities, can include titanite, apatite, corundum, zircon, magnetite, and ilmenite. The characteristic dark hue of phonolite is attributed to its significant concentration of dark pyroxenes, such as aegirine and augite.

A specific variety, blairmorite, is noteworthy for its high analcite content, distinguishing it within the phonolite family.

A phonolite dike, a relatively narrow intrusion, can be observed at Haddinnet in Ethiopia, showcasing its geological presence.

Occurrence

Phonolite, along with its intrusive relative nepheline syenite, is found distributed across the globe. You can find it in Canada, Norway, Greenland, Sweden, the United Kingdom, the Ural Mountains, the Pyrenees, Italy, and famously in Germany's Eifel and Kaiserstuhl regions. It also appears in Brazil, the Transvaal region of South Africa, the Magnet Cove igneous complex in Arkansas, and the Beemerville Complex in New Jersey. Even oceanic islands like the Canary Islands host these formations.

Europe, in particular, has a notable prevalence of phonolite, especially within the Eifel Plateau and around the Laacher See. It's also present in the Czech Republic and the Mediterranean region, notably near Italy. In the United States, phonolite can be encountered in the Black Hills Forest of South Dakota. Perhaps the most iconic geological structure composed of phonolite is the striking Devil's Tower in Wyoming.

Furthermore, nepheline-normative rocks, which include phonolite, are often found in close proximity to the Bushveld Igneous Complex in South Africa. It's theorized that these rocks formed from the partial melting of the surrounding wall rocks adjacent to this massive ultramafic layered intrusion. Phonolite also makes an appearance in related complexes like the Pilanesberg Complex and the Pienaars River Complex.

Examples

Africa

Europe

North America

A porphyritic phonolite sample from Devils Tower offers a visual representation of its texture.

Other Regions

Economic Importance

Phonolite isn't just a geological curiosity; it has practical applications. It can be quarried and used as dimension stone, prized for its unique appearance, or processed into aggregate for gravel.

In rarer instances, phonolite-nepheline syenite alkaline complexes have been found to be economically significant, hosting mineralization of rare-earth elements, uranium, and phosphates, as seen at Phalaborwa, South Africa.

Prehistoric peoples in Germany, specifically around Hohentwiel and Hegau, utilized phonolite tuff as a source of flint for crafting adze heads and similar tools.

Phonolite can also be meticulously separated into slabs of suitable dimensions, making it a viable alternative to roofing slate for constructing roofs. This practice is observed in the French Massif Central region, particularly in the Haute Loire département.

References

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  • ^ BGS map viewer mapapps.bgs.ac.uk
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