Nepheline syenite

  Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive equivalent.

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Petrology

Nepheline syenites are silica-undersaturated and some are peralkaline (terms discussed in igneous rock). Nepheline is a feldspathoid, a solid-solution mineral, that does not coexist with quartz; rather, nepheline would react with quartz to produce alkali feldspar.

They are distinguished from ordinary syenites not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalis and in rare earths and other incompatible elements. Alkali feldspar dominates, commonly represented by orthoclase and the exsolved lamellar albite, form perthite. In some rocks the potash feldspar, in others the soda feldspar predominates. Fresh clear microcline is very characteristic of some types of nepheline syenite.

Sodalite, colorless and transparent in thin section, but frequently pale blue in the hand specimens, is the principal feldspathoid mineral in addition to nepheline. Reddish-brown to black triclinic aenigmatite occurs also in these rocks. Extremely iron-rich olivine is rare, but is present in some nepheline syenite. Other minerals common in minor amounts include sodium-rich pyroxene, biotite, titanite, zircon, iron oxides, apatite, fluorite, melanite garnet, and zircon. Cancrinite occurs in several nepheline-syenites. A great number of interesting and rare minerals have been recorded from nepheline syenites and the pegmatite veins which intersect them.

Genesis

Silica-undersaturated igneous rocks typically are formed by low degrees of partial melting in the Earth'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 in subduction 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 their normative mineralogy commonly are associated with other unusual igneous rocks such as carbonatite.

Distribution

Nepheline syenites and phonolites occur in Canada, Norway, Greenland, Sweden, the Ural Mountains, the Pyrenees, Italy, Brazil, China, the Transvaal region, and Magnet Cove igneous complex of Arkansas, 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 the Bushveld Igneous Complex, possibly formed from partial melting of the wall rocks to that large ultramafic layered intrusion.

Nepheline syenites are rare; there is only one occurrence in Great Britain and one in France and Portugal. They are known also in Bohemia and in several places in Norway, Sweden and Finland. In the Americas these rocks have been found in Texas, Arkansas and Massachusetts, also in Ontario, British Columbia and Brazil. South Africa, Madagascar, India, Tasmania, Timor and Turkestan are other localities for the rocks of this series.

Rocks of this class also occur in Brazil (Serra de Tingua) containing sodalite and often much augite, in the western Sahara and Cape Verde Islands; also at Zwarte Koppies in the Transvaal, Madagascar, São Paulo in Brazil, Paisano Pass in West Texas and Montreal, Canada. The rock of Salem, Massachusetts, United States, is a mica-foyaite rich in albite and aegirine: it accompanies granite and essexite. Litchfieldite is another well-marked type of nepheline-syenite, in which albite is the dominant feldspar. It is named after Litchfield, Maine, United States, where it occurs in scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similar nepheline-syenite is known from Hastings County, Ontario, and contains hardly any orthoclase, 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 of amphibole. Typical examples are known near Lujaur on the White Sea, where they occur with umptekites and other very peculiar rocks. Other localities for this group are at Julianehaab in Greenland with sodalite-syenite; at their margins they contain pseudomorphs after leucite. The lujaurites frequently have a parallel-banding or gneissose 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.

Cancrinite syenite, with a large percentage of cancrinite, has been described from Dalekarlia, Sweden and from Finland. We may also mention urtite from Lujaur Urt on the White Sea, which consists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (from Jacupiranga in Brazil) is a blackish rock composed of titaniferous augite, magnetite, ilmenite, perofskite and nepheline, with secondary biotite.

Nomenclature

There is a wide variety of silica-undersaturated and peralkaline 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 the Serra de Monchique, in southern Portugal. These are K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usually pyroxene-, hornblende- and biotite.

Laurdalite: The laurdalites, from Laurdal in Norway, are grey or pinkish, and in many ways closely resemble the laurvikites of southern Norway, with which they occur. They contain anorthoclase feldspars, biotite or greenish augite, much apatite and in some cases, olivine.

Ditroite: Ditroite derives is name from Ditrau, Transylvania, Romania. It is essentially a microcline, sodalite and cancrinite variety of nepheline syenite. It contains also orthoclase, nepheline, biotite, aegirine, acmite.

Chemical composition

The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich in alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica varying from 50 to 56%, while lime, magnesia and iron are never present in great quantity, though somewhat more variable than the other components. A worldwide average of the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressed as weight percent oxides.

• SiO₂ — 54.99%
• TiO₂ — 0.60%
• Al₂O₃ — 20.96%
• Fe₂O₃ — 2.25%
• FeO — 2.05%
• MnO — 0.15%
• MgO — 0.77%
• CaO — 2.31%
• Na₂O — 8.23%
• K₂O — 5.58%
• H₂O — 1.47%
• P₂O₅ — 0.13%

The normative mineralogy of this average composition contains about 22 percent nepheline and 66 percent feldspar.

Because nepheline syenite lacks quartz and is rich in feldspar and nepheline, it is used in the manufacture of glass and ceramics.

References

• Daniel S. Barker, Igneous Rocks, Prentice-Hall, Inc., 417 p., 1983. ISBN 0-13-450692-8
• Nepheline in Arkansas
• Canada fact sheet Nepheline syenite
• USGS
• Alkaline rock occurrences in the Americas

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.