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| Igneous rock | |
 Hand samples of nepheline syenite of theOrdovician Beemerville Complex, northern New Jersey | |
| Composition | |
|---|---|
| Primary | alkali feldspar,nepheline,clinopyroxene,amphibole,biotite |
| Secondary | magnetite,ilmenite,apatite,titanite |
Nepheline syenite is aholocrystallineplutonic rock that consists largely ofnepheline andalkalifeldspar.[1] The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlikegranites, but dark green varieties are also known.[2]Phonolite is the fine-grainedextrusive equivalent.
Nepheline syenites are silica-undersaturated and some areperalkaline (terms discussed inigneous rock). Nepheline is afeldspathoid, a solid-solution mineral, that does not coexist withquartz; rather, nepheline would react with quartz to produce alkali feldspar.
They are distinguished fromsyenites not only by the presence of nepheline but also by the occurrence of many other minerals rich inalkalis and inrare earths[2] and otherincompatible elements. In nepheline syenites, alkali feldspar dominates, commonly represented byorthoclase and the exsolved lamellaralbite, formperthite. In some rocks the potashfeldspar, in others the soda feldspar predominates. Fresh clearmicrocline is very characteristic of some types of nepheline syenite.
Sodalite, colorless and transparent inthin section, but frequently pale blue in the hand specimens, is the principalfeldspathoid mineral in addition to nepheline. Reddish-brown to blackaenigmatite occurs also in these rocks.[2] Extremely iron-richolivine is rare, but is present in some nepheline syenite. Other minerals common in minor amounts include sodium-richpyroxene,biotite,titanite,iron oxides,apatite,fluorite,melanitegarnet, andzircon.[citation needed]Cancrinite occurs in several nepheline-syenites. A great number of interesting and rare minerals have been recorded from nepheline syenites and thepegmatite veins which intersect them.[2]
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Macroscopic aspects of nepheline syenite are similar to those ofgranite. The presence ofnepheline and absence ofquartz are the fundamental difference.Biotite is generally of low content and the mainmafic minerals areclinopyroxene (±) andamphibole (±). The macroscopic colour is grey, being little darker than granite. There is high-grademetamorphic rock originated from nepheline syenite that is characterized bygneiss texture of very rare occurrence. It is called nepheline syenite gneiss orlitchfieldite. An example is found at Canaã village, State ofRio de Janeiro,Brazil.
The rock is holocrystalline, generallyequigranular, equidirectional, and gross with grain size of 2 mm to 5 mm. In certain rare cases, the rock contains alkaline feldsparphenocrysts of 2 cm to 5 cm in length and 5 mm to 2 cm in thickness. The phenocrysts demonstrate orientation and eventually show cumulative texture.
The main minerals arealkali feldspar,nepheline,clinopyroxene (±),amphibole (±), andbiotite (±).Nepheline is the mainfeldspathoid.Quartz andorthopyroxene are absent. According to the IUGS classification nomenclature (International Union of Geological Sciences, Streckeisen, 1978), nepheline syenite has 10% <F/(F +A +P) < 60% andP/(A +P) < 10% (whereF – feldspathoids,A – alkali feldspar, andP – plagioclase volume fractions).Phonolite is the fine-grained equivalent. In case nepheline is less than 10%, the rock is called alkaline syenite with nepheline orpulaskite. The similar rock without quartz and nepheline is denominated alkaline syenite or syenite. Because of the presence offeldspathoids, nepheline syenite is classified to be a typical alkaline rock.
The alkaline feldspar is not potassic, but generally sodic-potassic, which is characterized by interlockinganorthoclase, calledperthite. In the alkali feldspar almost purealbite domains are observed.Nepheline generally shows partial alteration intonatrolite andcancrinite. Theclinopyroxene is sodic whose composition varies fromhedenbergite toaegirine-augite. This mineral eventually presentsresorption shape. The reaction rim constituted byamphibole and/orbiotite is commonly observed. The amphibole is of high alkali, such as alkalinehornblende andriebeckite. The alkalineclinopyroxene and amphibole are characteristics of typical alkaline rocks. Biotite is annite, with high Fe/Mg ratio.
The accessory minerals aremagnetite,ilmenite,apatite, andtitanite. Eventually,sodalite is found along hydrothermal fractures. Different from granite,zircon is rare and, if present, it is asxenocrysts. On the other hand, nepheline syenite gneiss contains abundant and large zircon crystals.
Silica-undersaturated igneous rocks typically are formed by low degrees of partial melting in theEarth's mantle. Carbon dioxide may dominate over water in source regions. Magmas of such rocks are formed in a variety of environments, including continental rifts, ocean islands, and supra-subduction positions insubduction zones. Nepheline syenite and phonolite may be derived by crystal fractionation from more mafic silica-undersaturated mantle-derived melts, or as partial melts of such rocks. Igneous rocks with nepheline in theirnormative mineralogy commonly are associated with other unusual igneous rocks such ascarbonatite.
Nepheline syenites and phonolites occur, for example, inBrazil,Canada,Cameroon,China,Greenland,Italy,Norway, thePyrenees,Sweden, theTransvaal region, theUral Mountains, in the USAMagnet Cove igneous complex ofArkansas, the mountains of the CentralMontana Alkali Province,[3] and as well as on oceanic islands.
Phonolite lavas formed in the East African rift in particularly large quantity, and the volume there may exceed the volume of all other phonolite occurrences combined, as discussed by Barker (1983).
Nepheline-normative rocks occur in close association with theBushveld Igneous Complex, possibly formed from partial melting of the wall rocks to that largeultramafic layered intrusion.
Nepheline syenites are rare; there is only one occurrence inGreat Britain (Loch Borralan)[4] and one inFrance andPortugal. They are known also inBohemia and in several places inNorway,Sweden andFinland. In theAmericas these rocks have been found inTexas,Arkansas,New Jersey (Beemerville Complex[5]) andMassachusetts, also inOntario,British Columbia andBrazil.South Africa,Madagascar,India,Tasmania,Timor andTurkestan are other localities for the rocks of this series.[2]
Rocks of this class also occur inBrazil (Serra de Tingua) containing sodalite and often much augite, in the westernSahara, on theÎles de Los (Guinea), and theCape Verde Islands; also at Zwarte Koppies in theTransvaal,Madagascar,São Paulo inBrazil, Paisano Pass inWest Texas,United States, andMontreal,Quebec,Canada. The rock ofSalem, Massachusetts, United States, is amica-foyaite rich inalbite andaegirine: it accompaniesgranite andessexite.Litchfieldite is another well-marked type of nepheline-syenite, in which albite is the dominantfeldspar. It is named afterLitchfield, Maine, United States, where it occurs in scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similar nepheline-syenite is known fromHastings County, Ontario, and contains hardly anyorthoclase, but only albite feldspar. Nepheline is very abundant and there is also cancrinite, sodalite, scapolite, calcite, biotite and hornblende. The lujaurites are distinguished from the rocks above described by their dark color, which is due to the abundance of minerals such as augite, aegirine, arfvedsonite and other kinds ofamphibole. Typical examples are known near Lujaur on theWhite Sea, where they occur with umptekites and other very peculiar rocks. Other localities for this group are at Julianehaab inGreenland with sodalite-syenite; at their margins they containpseudomorphs afterleucite. The lujaurites frequently have a parallel-banding orgneissose structure. Sodalite-syenites in which sodalite very largely or completely takes the place of nepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsonite and eudialyte.[2]
Cancrinitesyenite, with a large percentage of cancrinite, has been described fromDalekarlia,Sweden and fromFinland. Urtite from Lujaur Urt on theWhite Sea consists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (from Jacupiranga in Brazil) is a blackish rock composed oftitaniferous augite,magnetite,ilmenite, perofskite and nepheline, with secondary biotite.[2]
There is a wide variety of silica-undersaturated andperalkaline igneous rocks, including many informal place-name varieties named after the locations in which they were first discovered. In many cases these are plain nepheline syenites containing one or more rare minerals or mineraloids, which do not warrant a new formal classification. These include;
Foyaite: foyaites are named after Foya in theSerra de Monchique, in southernPortugal.[2] These are K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usuallypyroxene,hornblende andbiotite.
Laurdalite: The laurdalites, from Laurdal inNorway, are grey or pinkish, and in many ways closely resemble thelarvikites of southern Norway, with which they occur. They containanorthoclase feldspars, biotite or greenish augite, much apatite and in some cases, olivine.[2]
Ditroite: Ditroite derives its name from Ditrau,Transylvania,Romania. It is essentially amicrocline,sodalite andcancrinite variety of nepheline syenite. It contains alsoorthoclase,nepheline, biotite,aegirine,acmite.
The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich inalkalis and inalumina (hence the abundance of felspathoids and alkali feldspars), with silica varying from 50 to 56%, whilelime,magnesia andiron are never present in great quantity, though somewhat more variable than the other components.[2] A worldwide average of the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressed as weight percent oxides.
Nepheline syenite is characterized by high ratio of (Na2O+K2O)/SiO2 and (Na2O+K2O)/Al2O3, which are represented respectively by the existence of nepheline and alkalinemafic minerals. Therefore, it is classified geochemically as alkaline rock. This rock has low Fe and Mg contents, in total about 3wt%, and in this sense it is classified to befelsic rock. However, the SiO2 content is not so high, being 53% to 62wt%, which is equivalent toandesite anddiorite. In this sense, it corresponds tointermediate rock. Lightrare-earth elements are highly concentrated, indicating that the magma is highly differentiated.
Thenormative mineralogy of this average composition contains about 22%nepheline and 66%feldspar.
Because nepheline syenite lacksquartz and is rich in feldspar and nepheline, it is used in the manufacturing of glass and ceramics.
Industrial use of nepheline syenite includes refractories,glass making,ceramics and, in pigments and fillers. In these applications the nepheline syenite is ground and dark minerals are carefully separated leaving a mixture of primarilyfeldspar and nepheline. This mixture is higher in alkali and aluminium and typically lower in iron and silica than feldspars from pegmatites making it a good raw material. In 1994 production of nepheline syenite in Canada and Norway, the largest producing countries, was 600,000 tonnes and 330,000 tonnes respectively.[6]
Requirements for nepheline syenite as a raw material for glass manufacturer include:[7]
The typical mineralogical and chemical analysis of a ceramic grade nepheline syenite are:[8]
| Country | Norway |
|---|---|
| Producing company | Sibelco |
| Albite, % | 11 |
| Microcline, % | 48.5 |
| Analcime, % | 0.6 |
| Nepheline , % | 39.8 |
| SiO2, % | 55.7 |
| Al2O3, % | 24.5 |
| Fe2O3, % | 0.1 |
| TiO2, % | - |
| CaO, % | 1.1 |
| MgO, % | - |
| K2O, % | 8.8 |
| Na2O, % | 8.2 |
| LOI, % | - |
One or more of the preceding sentences incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Nepheline-syenite". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. pp. 383–384.
