Nepheline syenite

Source: Wikipedia, the free encyclopedia.
Nepheline syenite
clinopyroxene, amphibole, biotite
Secondarymagnetite, ilmenite, apatite, titanite

Nepheline syenite is a

plutonic rock that consists largely of nepheline and alkali feldspar.[1] 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

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

rare earths and other incompatible elements. In nepheline syenites, 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.

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.

Macroscopic aspects

Kakortokite (eudialytic nepheline syenite) Kangerdluarssaq Fjord, far-southern Greenland. Slab is 9.5 cm tall.

Macroscopic aspects of nepheline syenite are similar to those of

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, generally

phenocrysts
of 2 cm to 5 cm in length and 5 mm to 2 cm in thickness. The phenocrysts demonstrate orientation and eventually show cumulative texture.

Mineral composition

The main minerals are

orthopyroxene are absent. According to the IUGS classification nomenclature (International Union of Geological Sciences, Streckeisen, 1978), nepheline syenite has 10% < F/(F + A + P) < 60% and P/(A + P) < 10% (where F – feldspathoids, A – alkali feldspar, and P – plagioclase volume fractions). Phonolite is the fine-grained equivalent. In case nepheline is less than 10%, the rock is called alkaline syenite with nepheline or pulaskite. The similar rock without quartz and nepheline is denominated alkaline syenite or syenite. Because of the presence of feldspathoids
, nepheline syenite is classified to be a typical alkaline rock.

The alkaline feldspar is not potassic, but generally sodic-potassic, which is characterized by interlocking

clinopyroxene
and amphibole are characteristics of typical alkaline rocks. Biotite is annite, with high Fe/Mg ratio.

The accessory minerals are

xenocrysts
. On the other hand, nepheline syenite gneiss contains abundant and large zircon crystals.

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

Pseudoleucite nepheline syenite of the Intrusive Complex of Morro de São João, State of Rio de Janeiro, Brazil (Motoki et al., 2011b)

Nepheline syenites and phonolites occur, for example, in

Transvaal region, the Ural Mountains, in the USA Magnet Cove igneous complex of Arkansas, the mountains of the Central Montana Alkali Province,[2]
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)[3] 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[4]) 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

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.

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,

acmite
.

Chemical composition

Thin section image of the nepheline syenite of the Intrusive Complex of Tanguá, State of Rio de Janeiro, Brazil (Motoki et al., 2011a)

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

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

intermediate rock. Light rare-earth elements
are highly concentrated, indicating that the magma is highly differentiated.

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,

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.[5]

Requirements for nepheline syenite as a raw material for glass manufacturer include:[6]

  • 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:[7]

Country Norway
Producing company Sibelco
Albite, % 11
Microcline, % 48.5
Analcime, % 0.6
Nepheline , % 39.8
SiO2, % 55.7
Al2O3, % 24.5
Fe2O3, % 0.1
TiO2, % -
CaO, % 1.1
MgO, % -
K2O, % 8.8
Na2O, % 8.2
LOI, % -

Notes

  1. S2CID 135266142
    .
  2. ^ S. W. Wallace (1953). The petrology of the Judith Mountains, Fergus County, Montana (Report). U.S. Geological Survey.
  3. ^ Sutherland, D.S. (editor) (1982) Igneous Rocks of the British Isles, page 211
  4. Shawnee on Delaware, PA
    , p. 85–91.
  5. Industrial Minerals
    , Pg. 268
  6. ^ ’Industrial Minerals And Their Uses - A Handbook And Formular’ P. A. Ciullo. William Andrew, 1996. Pg. 43
  7. ^ "Nepheline Syenite – Improve the Functional and Aesthetic Characteristics of Matt Glazes" (PDF). Sibelco. Retrieved 11 April 2023.

References

Attribution

Further reading

External links

Wikimedia Commons has media related to Nepheline syenite.
Retrieved from "https://en.wikipedia.org/w/index.php?title=Nepheline_syenite&oldid=1151202299"