Perhaps surprisingly this rock formation is a phonolite. Yes I am talking about \u50e7\u5e3d\u5ca9. The name phonolite comes from the Ancient Greek. It means sounding stone. The English name clinkstone is a synonym. This is due to the metallic sound it produces if hit.
Now consider its formation. Phonolite forms from magma with low silica content. This is generated by low degrees of partial melting. This melting involves aluminous rocks of the lower crust. These rocks include tonalite and monzonite. Metamorphic rocks are also included. The melting of these rocks releases aluminium, potassium, sodium and calcium. This happens through the melting of feldspar. Mafic minerals are also involved. Because the rock is silica-undersaturated it has no quartz. It also has no other silica crystals. Low-silica feldspathoid minerals dominate more than feldspar minerals.
Let’s explore further. A few geological processes can melt the precursor rocks to form phonolite. These include intracontinental hotspot volcanism. This can form above mantle plumes covered by thick continental crust. A-type granites and alkaline igneous provinces usually occur alongside phonolites. Low-degree partial melting of underplates of granitic material in collisional orogenic belts may also produce phonolites.
Here we can see fine-grained rock. Phonolite is a fine-grained equivalent of nepheline syenite. They are products of partial melting. They are silica-undersaturated. Feldspathoids are in their normative mineralogy.
Consider the minerals within \u50e7\u5e3d\u5ca9. Mineral assemblages include abundant feldspathoids. Nepheline, sodalite, hauyne, leucite and analcite are among them. Alkali feldspar is also present. Sanidine, anorthoclase or orthoclase are also common. Biotite is a common minor mineral. Sodium-rich amphiboles and pyroxenes are included. Iron-rich olivine is present too. Accessory phases include titanite and apatite. Corundum, zircon, magnetite and ilmenite can be found. The rock’s dark color comes from dark pyroxenes. Aegirine and augite contribute to this color.
Notably phonolite is found worldwide. It appears in Canada and Norway. Greenland and Sweden also have it. You can find it in the United Kingdom and the Ural Mountains. The Pyrenees and Italy are locations too. Look for it in Eifel and Kaiserstuhl in Germany. It is also in Brazil and the Transvaal region. The Magnet Cove igneous complex of Arkansas has it. You can see it in the Beemerville Complex of New Jersey. Oceanic islands such as the Canary Islands also feature this rock.
In Europe phonolite is common. The Eifel Plateau is one place. Laacher See is another. It is also found in the Czech Republic. The Mediterranean area near Italy contains it. In the United States you can find phonolite in the Black Hills Forest in South Dakota. Devils Tower in Wyoming is a well-known structure composed of phonolite.
The Bushveld Igneous Complex is associated with nepheline-normative rocks. This may have formed from partial melting of wall rocks. This occurred adjacent to that large ultramafic layered intrusion. Phonolite occurs in the related Pilanesberg Complex and Pienaars River Complex.
Interestingly phonolites have economic uses. They can be dimension stone. They are used as aggregate for gravels.
Sometimes economically mineralised phonolite-nepheline syenite alkaline complexes can be associated with rare-earth mineralisation. Uranium mineralisation and phosphates are linked too. Phalaborwa in South Africa is such a place.
Did you know that prehistoric people used phonolite tuff as a source of flint? They used it for adze heads. Hohentwiel and Hegau in Germany were the sources.
Lastly phonolites can be separated into slabs for roofing tiles. They replace roofing slate. The French Massif Central region has such occurrences. The Haute Loire département is one example. \u50e7\u5e3d\u5ca9 is a fascinating rock formation. Its unique origin and composition make it special. Its uses throughout history are equally interesting.