Phonolite

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Phonolite
Clinkstone
analcite, sanidine, anorthoclase
Secondarybiotite, amphibole, pyroxene, olivine
Demonstration of sound produced when phonolite is struck, Cerro de la Campana (Hermosillo, Sonora, Mexico)
Lithophone made of Phonolite in Schellerhau botanic garden (Germany)

Phonolite is an uncommon shallow intrusive or

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.

Formation

Unusually, phonolite forms from

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 quartz or other 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

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.

Mineral assemblages in phonolite occurrences are usually abundant

analcite) and alkali feldspar (sanidine, anorthoclase or orthoclase), and rare sodic plagioclase. Biotite, sodium-rich amphiboles and pyroxenes along with iron-rich olivine are common minor minerals. Accessory phases include titanite, apatite, corundum, zircon, magnetite and ilmenite.[4]
Phonolite's characteristic dark color comes from its concentration of dark pyroxenes such as aegirine and augite.

Blairmorite is an analcite-rich variety of phonolite.[5][6]

Phonolite dike in Haddinnet in Ethiopia

Occurrence

Outcrop of phonolite at Beemerville Complex, New Jersey

Nepheline syenites and phonolites occur widely distributed throughout the world

Transvaal region, the Magnet Cove igneous complex of Arkansas, the Beemerville Complex of New Jersey,[8] as well as on oceanic islands such as the Canary Islands.[9]

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]

Examples

North America

Coarse gray rock surface in close-up
Porphyritic phonolite at Devils Tower

Europe

Other

Economic importance

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

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

Haute Loire département.[citation needed
]

References

  1. ^ a b Bonewitz, Ronald (2008). Rock and Gem: The Definitive Guide to Rocks, Minerals, Gems, and Fossils. United States: DK. p. 47.
  2. ISSN 0022-3530
    .
  3. ^ Ridley, W. I., 2012, Petrology of Igneous Rocks, Volcanogenic Massive Sulfide Occurrence Model, USGS Scientific Report 2010-5070-C, Chapter 15.
  4. ISBN 0-7167-2438-3
    .
  5. ^ Peterson, T.D.; Currie, K.L. (1993). Analcite-bearing igneous rocks from the Crowsnest Formation, southwestern Alberta (Current Research report 93-B1) (PDF). Geological Survey of Canada. pp. 51–56.
  6. ISBN 9780903056274
    .
  7. ^ Woolley, A.R., 1995. Alkaline rocks and carbonatites of the world., Geological Society of London.
  8. Shawnee on Delaware, PA
    , p. 85-91.
  9. doi:10.1016/S0377-0273(98)00004-3
    .
  10. ISBN 978-3-642-75673-3
    .
  11. S2CID 3101604
    .
  12. ^ BGS map viewer http://mapapps.bgs.ac.uk/geologyofbritain/home.html
  13. ^ "Gerbier de Jonc et sources de la Loire". Volcans des sucs (in French). Geopark - Parc Naturel Régional des Monts d'Ardèche. Retrieved 15 January 2017.
  14. S2CID 140601047
    .
  15. ^ Marshall, Patrick, 'The occurrence of a mineral hitherto unknown in the phonolites of Dunedin, New Zealand', 1929.
  16. doi:10.1093/petroj/39.5.905
    .
  17. ^ "Levelwood | Saint Helena Island Info: All about St Helena, in the South Atlantic Ocean".
  18. ^ Affolter, J., 2002, Provenance des silex préhistoriques du Jura et des régions limitrophes, Archéologie neuchâteloise, 28.

External links

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