Nepheline 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.

Petrology

thumb|right|Nepheline syenite of the Intrusive Complex of Tanguá, State of Rio de Janeiro, Brazil (Motoki et al., 2011a)

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 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 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.

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

  • Al<sub>2</sub>O<sub>3</sub> >23%; >14% Na<sub>2</sub>O + K<sub>2</sub>O; Fe<sub>2</sub>O<sub>3</sub> <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:

{| class="wikitable collapsible"

! Country

| Norway

|-

! Producing company

| Sibelco

|-

!

|

|-

! Albite, %

| 11

|-

! Microcline, %

| 48.5

|-

! Analcime, %

| 0.6

|-

! Nepheline , %

| 39.8

|-

!

|

|-

! SiO<sub>2</sub>, %

| 55.7

|-

! Al<sub>2</sub>O<sub>3</sub>, %

| 24.5

|-

! Fe<sub>2</sub>O<sub>3</sub>, %

| 0.1

|-

! TiO<sub>2</sub>, %

| -

|-

! CaO, %

| 1.1

|-

! MgO, %

| -

|-

! K<sub>2</sub>O, %

| 8.8

|-

! Na<sub>2</sub>O, %

| 8.2

|-

! LOI, %

| -

|}

Notes

References

  • Sørensen, H. 1974. The alkaline rocks. 1st Edition. John Wiley & Sons Ltd. 634 p. .
  • Streckeisen, A. L. 1978. IUGS Subcommission on the Systematics of Igneous Rocks. Classification and Nomenclature of Volcanic Rocks, Lamprophyres, Carbonatites and Melilite Rocks. Recommendations and Suggestions. Neues Jahrbuch für Mineralogie, Abhandlungen, 141, 1–14.
  • Motoki, A., Sichel, S. E., Vargas, T., Aires, J. R., Iwanuch, W., Mello, S. L. M., Motoki, K. F., Silva, S., Balmant, A., Gonçalves, J. 2010. Geochemical evolution of the felsic alkaline rocks of Tanguá, Rio Bonito, and Itaúna intrusive bodies, State of Rio de Janeiro, Brazil. Geociências, Rio Claro, 29–3, 291–310.
  • Motoki, A., Araújo, A. L., Sichel, S. E., Motoki, K. F., Silva, S. Nepheline syenite magma differentiation process by continental crustal assimilation for the Cabo Frio Island intrusive complex, State of Rio de Janeiro, Brazil. Geociências, Rio Claro, 2011, in press.

Further reading

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