| Extrusive igneous rock | |
 Dark, prismaticphenocrysts ofaegirine in phonolite sample | |
| Composition | |
|---|---|
| Primary | nepheline,sodalite,hauyne,leucite,analcite,sanidine,anorthoclase |
| Secondary | biotite,amphibole,pyroxene,olivine |
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Phonolite is an uncommon shallow intrusive orextrusive rock, ofintermediate chemical composition betweenfelsic andmafic, with texture ranging fromaphanitic (fine-grained) toporphyritic (mixed fine- and coarse-grained). Phonolite is a variation of the igneous rocktrachyte that contains nepheline or leucite rather than quartz.[1] It has an unusually high (12% or more) Na2O + K2O content, defining its position in theTAS classification of igneous rocks. Its coarse grained (phaneritic)intrusive equivalent isnepheline syenite. Phonolite is typically fine grained and compact.The namephonolite comes from theAncient Greek meaning "sounding stone" due to the metallic sound it produces if an unfractured plate is hit; hence, the English nameclinkstone is given as a synonym.
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Unusually, phonolite forms frommagma with a relatively lowsilica content, generated by low degrees ofpartial melting (less than 10%) of highly aluminous rocks of the lowercrust such astonalite,monzonite andmetamorphic rocks. Melting of such rocks to a very low degree promotes the liberation of aluminium, potassium, sodium and calcium by melting offeldspar, with some involvement of mafic minerals. Because the rock is silica-undersaturated, it has noquartz orother silica crystals, and is dominated by low-silicafeldspathoid minerals more than feldspar minerals.
A few geological processes and tectonic events can melt the necessary precursor rocks to form phonolite. These include intracontinentalhotspot volcanism,[2] such as may form abovemantle plumes covered by thickcontinental crust. A-typegranites and alkaline igneous provinces usually occur alongside phonolites. Low-degree partial melting of underplates of granitic material in collisionalorogenic belts may also produce phonolites.
Phonolite is a fine-grained equivalent ofnepheline syenite. They are products of partial melting, are silica-undersaturated, and have feldspathoids in theirnormative mineralogy.
Mineral assemblages in phonolite occurrences are usually abundantfeldspathoids (nepheline,sodalite,hauyne,leucite andanalcite) and alkalifeldspar (sanidine,anorthoclase ororthoclase), and rare sodicplagioclase.Biotite, sodium-richamphiboles andpyroxenes along with iron-richolivine are common minor minerals. Accessory phases includetitanite,apatite,corundum,zircon,magnetite andilmenite.[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]
Nepheline syenites and phonolites occur widely distributed throughout the world[7] in Canada, Norway,Greenland, Sweden, theUnited Kingdom, theUral Mountains, thePyrenees, Italy,Eifel andKaiserstuhl in Germany,Brazil, theTransvaal region, theMagnet Cove igneous complex ofArkansas, the Beemerville Complex ofNew Jersey,[8] as well as on oceanic islands such as theCanary 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 theDevil's Tower, found in Wyoming.[1]
Nepheline-normative rocks occur in close association with theBushveld Igneous Complex, possibly formed from partial melting of the wall rocks adjacent to that largeultramafic layered intrusion. Phonolite occurs in the related Pilanesberg Complex and Pienaars River Complex.[10]
Phonolites can be of interest asdimension stone or as aggregate for gravels.
Rarely, economically mineralised phonolite-nepheline syenite alkaline complexes can be associated withrare-earth mineralisation,uranium mineralisation andphosphates, such as atPhalaborwa, South Africa.
Phonolitetuff was used as a source offlint foradze heads and such by prehistoric people fromHohentwiel andHegau, 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 FrenchMassif Central region such as theHaute Loire département.[citation needed]
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