Nepheline syenite

Petrology

Nepheline syenites are silica-undersaturated and some are peralkaline (terms discussed in quartz; rather, nepheline would react with quartz to produce alkali feldspar.

They are distinguished from ordinary microcline is very characteristic of some types of nepheline syenite.

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 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 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 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.

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 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 acmite.

Chemical composition

The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich in 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 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.