Nepheline syenite

Macroscopic aspects

Macroscopic aspects of nepheline syenite are similar to those of granite. The presence of nepheline and absence of quartz are the fundamental difference. Biotite is generally of low content and the main mafic minerals are clinopyroxene (±) and amphibole (±). The macroscopic colour is grey, being little darker than granite. There is high-grade metamorphic rock originated from nepheline syenite that is characterized by gneiss texture of very rare occurrence. It is called nepheline syenite gneiss or litchfieldite. An example is found at Canaã village, State of Rio de Janeiro, Brazil.

Texture

The rock is holocrystalline and commonly equigranular, with a typical grain size of 2 to 5 mm (0.079 to 0.197 in). Some varieties contain oriented alkali-feldspar phenocrysts several centimetres long and may display a cumulate texture.

Mineral composition

The principal minerals are alkali feldspar and nepheline, with variable clinopyroxene, amphibole and biotite. Quartz and orthopyroxene are absent. In the IUGS QAPF classification, nepheline syenite contains 10–60% feldspathoids among the felsic minerals and less than 10% plagioclase relative to total feldspar; F, A and P denote feldspathoids, alkali feldspar and plagioclase, respectively.

The alkaline feldspar is not potassic, but generally sodic-potassic, which is characterized by interlocking anorthoclase, called perthite. In the alkali feldspar almost pure albite domains are observed. Nepheline generally shows partial alteration into natrolite and cancrinite. The clinopyroxene is sodic whose composition varies from hedenbergite to aegirine-augite. This mineral eventually presents resorption shape. The reaction rim constituted by amphibole and/or biotite is commonly observed. The amphibole is of high alkali, such as alkaline hornblende and riebeckite. The alkaline clinopyroxene and amphibole are characteristics of typical alkaline rocks. Biotite is annite, with high Fe/Mg ratio.

The accessory minerals are magnetite, ilmenite, apatite, and titanite. Eventually, sodalite is found along hydrothermal fractures. Different from granite, zircon is rare and, if present, it is as xenocrysts. On the other hand, nepheline syenite gneiss contains abundant and large zircon crystals.

Genesis

Silica-undersaturated magmas can form by low degrees of partial melting in the mantle and occur in several tectonic settings, including continental rifts and ocean islands. Nepheline syenite and phonolite may evolve by fractional crystallisation of more mafic alkaline magmas, while some may represent partial melts of silica-undersaturated source rocks. They are commonly associated with other alkaline igneous rocks and, in some provinces, with carbonatite.

Distribution

A well-studied occurrence is the approximately 1.4-billion-year-old Pilanesberg Complex in South Africa, which contains extensive nepheline-syenitic and related peralkaline intrusive and volcanic rocks.

Nepheline syenites and phonolites occur, for example, in Brazil, Canada, Cameroon, China, Greenland, Italy, Norway, the Pyrenees, Sweden, the Transvaal region, the Ural Mountains, in the USA Magnet Cove igneous complex of Arkansas, the mountains of the Central Montana Alkali Province, 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 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 (Loch Borralan) 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, New Jersey (Beemerville Complex) 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, on the Îles de Los (Guinea), and the Cape Verde Islands; also at Zwarte Koppies in the Transvaal, Madagascar, São Paulo in Brazil, Paisano Pass in West Texas, United States, and Montreal, Quebec, 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. Urtite from Lujaur Urt on the White Sea 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 larvikites 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 its 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.

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 alkaline mafic 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 be felsic rock. However, the SiO₂ content is not so high, being 53% to 62wt%, which is equivalent to andesite and diorite. In this sense, it corresponds to intermediate rock. Light rare-earth elements are highly concentrated, indicating that the magma is highly differentiated.

• 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% nepheline and 66% feldspar.

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

Applications

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 primarily feldspar 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.

Requirements for nepheline syenite as a raw material for glass manufacturer include:

• Al₂O₃ >23%; >14% Na₂O + K₂O; Fe₂O₃ <0.1%,
• Absence of refractory minerals.
• Coarsely ground, typically -40# to +200# mesh.

The typical mineralogical and chemical analysis of a ceramic grade nepheline syenite are: