Phonolite is an uncommon shallow intrusive or extrusive rock, of intermediate chemical composition between felsic and mafic, with texture ranging from aphanitic (fine-grained) to porphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous rock trachyte that contains nepheline or leucite rather than quartz.[1] It has an unusually high (12% or more) Na2O + K2O content, defining its position in the TAS classification of igneous rocks. Its coarse grained (phaneritic) intrusive equivalent is nepheline syenite. Phonolite is typically fine grained and compact.
The name phonolite comes from the Ancient Greek meaning "sounding stone" due to the metallic sound it produces if an unfractured plate is hit; hence, the English name clinkstone is given as a synonym.
Unusually, phonolite forms from magma with a relatively low silica content, generated by low degrees of partial melting (less than 10%) of highly aluminous rocks of the lower crust such as tonalite, monzonite and metamorphic rocks. Melting of such rocks to a very low degree promotes the liberation of aluminium, potassium, sodium and calcium by melting of feldspar, with some involvement of mafic minerals. Because the rock is silica-undersaturated, it has no quartzorother silica crystals, and is dominated by low-silica feldspathoid minerals more than feldspar minerals.
A few geological processes and tectonic events can melt the necessary precursor rocks to form phonolite. These include intracontinental hotspot volcanism,[2] such as may 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.
Mineralogy and petrology
[edit]Total alkali vs. silica classification scheme (TAS), as proposed in Le Maitre's 2002 Igneous Rocks – A classification and glossary of terms[3]: 237
Phonolite is a fine-grained equivalent of nepheline syenite. They are products of partial melting, are silica-undersaturated, and have feldspathoids in their normative mineralogy.
Phonolite is common across Europe, particularly within the Eifel Plateau and the Laacher See. It is also found in the Czech Republic and the Mediterranean area near Italy. For localities in the United States, phonolite can be found in the Black Hills Forest in South Dakokta. The most well known phonolite-composed natural structure is the Devil's Tower, found in Wyoming.[1]
Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex, possibly formed from partial melting of the wall rocks adjacent to that large ultramafic layered intrusion. Phonolite occurs in the related Pilanesberg Complex and Pienaars River Complex.[10]
Phonolites can be of interest as dimension stone or as aggregate for gravels.
Rarely, economically mineralised phonolite-nepheline syenite alkaline complexes can be associated with rare-earth mineralisation, uranium mineralisation and phosphates, such as at Phalaborwa, South Africa.
Phonolite tuff was used as a source of flint for adze heads and such by prehistoric people from Hohentwiel and Hegau, Germany.[18]
Phonolites can be separated into slabs of appropriate dimensions to be used as roofing tiles in place of roofing slate. One such occurrence is in the French Massif Central region such as the Haute Loire département.[citation needed]
^Deer, W.A.; Howie, R.A.; Zussman, J. (2013). An Introduction to the Rock-Forming Minerals (3rd ed.). London: Mineralogical Society. ISBN9780903056274.
^Woolley, A.R., 1995. Alkaline rocks and carbonatites of the world., Geological Society of London.
^Eby, G. N., 2012, The Beemerville alkaline complex, northern New Jersey, in Harper, J. A., ed., Journey along the Taconic unconformity, northeastern Pennsylvania, New Jersey, and southeastern New York: Guidebook, 77th Annual Field Conference of Pennsylvania Geologists, Shawnee on Delaware, PA, p. 85-91.
^Bryan, S. E; Cas, R. A. F.; Martı́, J (May 1998). "Lithic breccias in intermediate volume phonolitic ignimbrites, Tenerife (Canary Islands): constraints on pyroclastic flow depositional processes". Journal of Volcanology and Geothermal Research. 81 (3–4): 269–296. Bibcode:1998JVGR...81..269B. doi:10.1016/S0377-0273(98)00004-3.
^Pirajno, Franco (1992). Hydrothermal Mineral Deposits: Principles and Fundamental Concepts for the Exploration Geologist. Berlin: Sringer-Verlag. pp. 267–269. ISBN978-3-642-75673-3.
