Nepheline syenite

Nephelene-syenite, also called elaeolite-syenite, is a holocrystalline plutonic rock which 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.

Petrology

Nepheline syenites are silica underaturated ultrapotassic and peralkaline rocks. Nepheline is a species of feldspathoid, which is a solid-solution mineral which contains a lower molar volume of silica than feldspars.

From ordinary syenites they are distinguished not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalis or in rare earths. Orthoclase and albite are the principal feldspars; usually they are intergrown to 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 the slides, but frequently pale blue in the hand specimens, is the principal feldspathoid mineral in addition to nepheline.

The dark-brown triclinic hornblende aenigmatite occurs also in these rocks. Olivine is rare, but may be found in some basic forms of nepheline-syenite. The commonest accessories are sphene, zircon, iron ores and apatite. Cancrinite occurs in several nepheline-syenites; in others there is fluor-spar or melanite garnet. A great number of interesting and rare minerals have been recorded from nepheline-syenites and the pegmatite veins which intersect them.

Genesis

Ultrapotassic and peralkaline igneous rocks are formed by very low degrees partial melting. Nepheline-normative melts are generally formed in supra-subduction positions in subduction zones, where thick continental crust melts. Nepheline-normative igneous rocks are associated with igneous provinces of similar ultrapotassic silica-undersaturated rocks such as lamproites, lamprophyres and occasionally carbonatites.

Distribution

Nepheline syenite is a coarse crystalline igneous rock composed primarily of orthoclase alkali-feldspars, nepheline and mafic minerals such as the sodium rich pyroxenes and amphiboles. Accessory minerals are other sodium-feldspathoids, zircon, apatite and sphene. Nepheline syenites occur in Canada, Norway, Greenland, Sweden, the Ural Mountains, the Pyrenees, Italy, Brazil, China, the Transvaal region, and Magnet Cove area of Arkansas.

Nepheline-normative and other ultrapotassic 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 rocks; 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 Massachussetts, 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 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, Sao 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. The chemical peculiarities of the nepheline-syenites are well marked, as will be seen from the following analyses. 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. As a group, also, these rocks have a low specific gravity.

Nomenclature

There is a wide variety of ultrapotassic 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-nepeline 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 sometimes olivine.

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

References

• Nepheline in Arkansas
• Canada fact sheet Nepheline syenite
• USGS
• Alkaline rock occurrences in the Americas

This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. {{cite encyclopedia}}: Missing or empty |title= (help) Template:Geol-stub