Nepheline syenite is a coarse-grained, pale-colored (white, gray, or pinkish) intrusive igneous rock that resembles granite. Its chemical composition is intermediate to felsic and has mainly alkali feldspar and nepheline with minor amphiboles and clinopyroxenes. Also, it may rarely have biotite and other minerals.
Nepheline syenite forms from the slow cooling of highly differentiated magma deep inside the Earth’s crust.
According to Bailey (1989), this rock was first named foyaite by Professor Johann Reinhard Blum of Heidelberg. The name came from Foya (Portugal), where it was first described. However, in 1877, Harry Rosenbusch, also of Heidelberg, proposed nepheline syenite as an alternative name owing to its high alkali oxides.
Today, foyaite is only a hypersolvus nepheline syenite variety. It has mainly orthoclase and a trachytic texture due to platy crystals of this alkali feldspar. Also, foyaite may have hornblende, pyroxene, sodalite, mica, and accessories like zircon.
Discover more on nepheline syenite. We will discuss its chemical and mineral composition and uses. Also, we will tell you how it forms, where it is found, and more.
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Quick facts and properties
- Name: Nepheline syenite
- Rock type: Igneous
- Subcategory: Peralkaline or peraluminous rock
- Origin: Intrusive or plutonic
- Colors: Pale colored, usually grey to pink, sometimes dark green
- Color index: Leucocratic
- Texture: Coarse-grained or phaneritic, i.e., with grains about 1/16mm to 3 cm
- Mohs hardness scale: 6 – 6.5
- Density: 2.55-2.76 g/cm3
- Melting point: 1200-1450 °C
- Extrusive equivalent: Phonolite, a fine-grained volcanic rock
- Tectonic environment: Intracontinental rifts or hotspots, volcanic arcs, and less often in oceanic islands
What does nepheline syenite look like?
To identify this rock in the field, consider texture and fabric. Usually, nepheline syenite is a massive, whitish, gray, pink, or brownish, less often dark green with a coarse-grained texture and closely resembles granite.
This rock has holocrystalline with equigranular, equidirectional crystals 0.2-0.5cm with primarily light and fewer scattered dark minerals.
The lighter-colored minerals are alkali feldspar, often intergrown (perthitic) with albite plagioclase and feldspathoids, mainly nepheline (sometimes altered). In contrast, darker minerals (green, blue, brown, or black) are usually pyroxenes, amphiboles, and biotite. However, this rock may have many other minerals.
Besides a coarse-grained texture, in rare cases, some nepheline syenite may show a porphyritic texture. This texture is characterized by 2-5cm long and 0.5-2cm thick phenocrysts. These phenocrysts show preferred orientation and cumulative texture.
Also, some specimens may be pegmatitic, while those with medium-grained are known as nepheline microsyenite.
Besides the massive forms, some may show layering with other rocks, such as in the Ilimaussaq intrusion in southern Greenland and the Lovozero Massif in the Kola Peninsula, Russia.
Lastly, nepheline syenite varies significantly in habit, fabric, and mineralogy from one location to another, making it challenging to identify.
Nepheline syenite composition
To understand this rock more, let us look at its chemical and mineral composition.
1. Chemical composition
Nepheline syenite is an intermediate to basic or felsic rock with > 52% silica. It is relatively high in aluminum, potassium, and sodium oxides and lower in calcium, magnesium, and iron oxides.
Data from Le Maitre (1976) shows the percentage weight of nepheline syenite chemical composition as 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%, and P2O5: 0.13%.
In this rock, the combined sodium and potassium oxide ratio to silica and aluminum is high. Therefore, the rock can have alkali amphiboles and pyroxenes, making it an alkaline rock geochemically.
Lastly, this rock often has incompatible elements like niobium, thorium, titanium, uranium, and rare earth elements.
2. Mineral composition
Mineralogically, nepheline syenite has predominantly alkali feldspar (mainly orthoclase) and nepheline with minor amounts of hornblende, clinopyroxenes, and rarely biotite (annite). It may have a small amount of plagioclase and other minerals but has no quartz.
Accessory minerals include sphene (titanite), zircon, apatite, melanite, magnetite, beryl, conundrum, eudialyte, titanite, andradite, perovskite, fluorites, and sulfides. Some, like astrophyllite and pectolite (zeolite), may occur in cavities or as amygdule.
The main felsic minerals are orthoclase (which may have exsolved lamella of albite forming a perthite or sometimes co-exist with albite plagioclase) and nepheline.
Nepheline is the dominant feldspathoid. It has a stubby euhedral prism to anhedral or maybe poikilitic. Also, it can occur as phenocrysts in porphyritic texture together with alkali feldspar.
Besides nepheline, this rock may have other foids like sodalite, cancrinite, vishnevite, haüyne, or leucite. However, according to Britannica, a rock with over 30% nepheline or mafic minerals isn’t known as nepheline syenite.
On the other hand, mafic minerals in this rock are alkali amphiboles like eckermannite, arfvedsonite, and riebeckite and alkali clinopyroxenes like aegirine, acmite, hedenbergite, and aegirine-augite. Some of the clinopyroxenes may have biotite or amphibole rims.
Also, this rock has minerals like aenigmatite, phlogopite, carletonite, poudrettite (in breccias), rinkite, villiaumite, vesuvianite (idocrase), and iron-rich olivine, and many others, including where pegmatites intersect these rocks.
On the QAPF diagram of plutonic rocks, syenite nepheline is a foid-syenite rock in which feldspathoids account for 10-60% QAPF content by volume with nepheline the dominant foid, has no quartz, and alkali feldspar is at least 90% of the total feldspars.
Lastly, according to Blatt et al. (2006), mineral alteration in this rock nclude biotite → sericite; nepheline → cancrinite, natrolite, sericite, sodalite, calcite, or analcime; sodalite → analcime, cancrinite or calcite; potash feldspar → nepheline, kaolinite, sodalite, sericite, and calcite.
Types or varieties
There are many varieties and local names for nepheline syenite rock. Such have slightly different compositions, with some having rare minerals, including accessory minerals.
These varieties and local names include:
1. Lujavrite
Lujavrite is a dark-colored (melanocratic) agpaitic nepheline syenite variety high in aegirine, eudialyte arfvedsonite, and other amphiboles. Its alkali feldspar may be perthitic or occur separately as microcline or albite. Also, it may show parallel banding or gneissose structure.
Lujavrite occurs in Russia at Lovozero and Odikhincha Massifs and Selsurt, Kuivchorr, Engporr and Karnasurt Mountains and in Greenland at Igaliku, límaussaq, and Kvanefjeld complexes.
Other places with lujavrite are the USA (Stettin Pluton, Wisconsin), South Africa (Pilanesberg Alkaline Ring Complex), China (Saima Complex), DRC Congo (Kirumba Complex), and Brazil (Poços de Caldas alkaline complex).
2. Litchfieldite
Litchfieldite is also known as nepheline syenite gneiss. It’s named after Litchfield, Maine, USA. It has a gneissic and foliated appearance with mainly albite and some microcline.
Also, litchfieldite rock has nepheline, cancrinite, sodalite, and calcite, while mafic minerals are magnetite and iron-rich biotite.
Besides the US, litchfieldite also occurs in Canada (French River and Blue Mountain), Norway (Soroy and Val River), Portugal (Cevadais), and some locations in Brazil.
Litchfieldite rock from Precambrian Canaã alkaline massif. Photo credit: Eurico Zimbres, CC BY-SA 2.5, via Wikimedia Commons
3. Malignite (Foid syenite) and shonkinite
Malignite is a mesocratic foid syenite that was initially considered a variety of nepheline syenite. It has aegirine augite and equal amounts of orthoclase and nepheline. Also, it may have garnet, biotite, and amphibole. In contrast, shonkinite refers to melanocratic varieties.
4. Agpaitic
Agpaitic are a variety of nepheline syenite with complex zirconium and titanium minerals, not simpler ones like eudialyte, zircon, or ilmenite.
5. Kakortokite
Kakortokite is an igneous rock named after Qaqortoq, Kujalleq, Greenland. It is an agpaitic variety with conspicuous cumulate textures and layering, i.e., repeated layers of arfvedsonite, eudialyte (reddish), and alkali feldspar.
Kakortokite also has aegirine and arfvedsonite and is often used to make ornamental objects, including cut into cabochon.
6. Borolanite
Borolanite is a local name for the coarse-grained nepheline-syenite variety with alkali feldspar and nepheline (alteration products) with biotite and melanite. It may also have pseudo leucite, a leucite pseudomorph, and occurs in the Loch Borralan Complex in Scotland.
How is nepheline syenite formed?
Nepheline syenite rock forms from the slow cooling of differentiated or evolved silica-undersaturated magma enriched with incompatible elements deep in the Earth’s crust. Such magmas are common in ocean islands, continental rifts, and the subduction zones’ supra-subduction positions.
These magmas originate from the crystal fractionation of mafic, silica-undersaturated melts from low-grade upper mantle melting or alkali basaltic magmas. The occurrence of feldspathoids like nepheline shows that this rock is from silica-undersaturated magma. Otherwise, feldspathoids would react with free silica to form alkali feldspar.
However, upper mantle partial melting may occur with contamination and assimilation of crustal materials. These may be a reason for this rock’s many incompatible and rare earth elements.
The association of unusual rocks like carbonatite with rocks with normative nepheline shows that partially melting some mantle rocks may occur where carbon dioxide is more than water.
Also, the close association of this rock in the Bushveld Igneous Complex (South Africa) with ultramafic layered intrusion confirms their formation is from the partial melting of mantle rocks.
Alternatively, extremely low degree partial melting of some crustal igneous or granitic protolith may produce an undersaturated silica melt. Such a melt will form nepheline syenite by replacing orthoclase with nepheline, leucite, or analcime feldspathoids.
Where is nepheline syenite found?
Nepheline syenite is a rare or uncommon rock compared to other plutonic rocks. It may occur in intrusions like lenses, pods, pipes, dikes, batholiths (like Itarantim in Brazil), and or parts of massifs and is associated with carbonatites in some locations. Also, it may occur in layered intrusions.
In the US, it occurs in Arkansas (Pulaski, Garland, and Hot Spring Counties), Texas, Massachusetts, New Hampshire (Red Hill Syenite), and the Beemerville Complex in New Jersey. Other countries in the Americas with this rock are Canada (Ontario and British Colombia) and Brazil.
In Europe, it occurs in Norway, Sweden, Russia (including at the Khibiny Massif), and Finland, with the UK, Portugal, and France only having one location. Also, it is Bohemia in the Czech Republic.
Elsewhere, it occurs in South Africa, India, Kazakhstan (Turkestan), Australia (Tasmania), Madagascar, and Timor Island.
What is nepheline syenite used for?
This rock is weathering and chemical resistant and high in strength. Also, it is high in alumina, silica-undersaturated, and inert.
Some of the uses of nepheline syenite include making aggregate, roofing granules, industrial glass, and ceramic manufacture, and as fillers or extenders in various products. Also, you can use it in landscaping, riprap, paving, or as a stone for making buildings, sculptures, monuments, etc.
Here is more on these various uses:
- In ceramic and glass manufacturing, it substitutes feldspar. It promotes fluxing at lower temperatures, reduces energy costs, lengthens furnace lifespan, and improves glass quality and yield. Also, it is used in glazes and enamel and makes refractories.
- It serves as fillers and extenders in paints, wood varnishes, coatings, inks, sealants, paper, foam rubber, rubber, and plastics or polymers. It improves tint retention, weatherability, color, and sheen uniformity in paints. Also, it improves their stain resistance and lowers volatile organic use (making them environmentally friendly and safer).
- This rock is crushed to make roofing granules and aggregate for construction, asphalt, concrete, roads, and railways. Resultant fines are good compaction fill, fluxing in brick manufacturing, and colorizing.
- It is cut and polished to make dimensional stones for building, paving, and other architectural uses.
- Landscaping and riprap for road embarkments or riverbanks
- Making monuments and sculptures
- It makes White Lightning, which is an industrial sandblasting or traction media. Also, it can work as a swimming pool filter sand.
Mining of nepheline syenite
Canada pioneered the use of nepheline syenite in filler, ceramic, and glass industries. According to the Encyclopedia of Canada, annual amounts in Canada exceeded 700,000 tons annually in 2015. Mining is via open pit followed by grinding and separation of iron-bearing minerals. Afterward, it is graded and packaged.
Today, it is the preferred source of alumina and alkali glass manufacture, replacing feldspar. Many countries in North and South America, Europe, Asia, the Middle East, and Africa where it occurs mine and use it.
Lastly, according to Fortune Business Insights, the nepheline syenite market size in 2019 was USD 79.7 million, with forecast growth of 1.3% up to USD 87.6 million. Prices per 50-pound bag are US$ 20-40.
Frequently Asked Questions (FAQs)
Nepheline syenite is a plutonic peralkaline or peraluminous rock. It is peralkaline if total alkali oxides (Na2O+ K2O) > aluminum oxide) or peraluminous if aluminum oxide > total alkali oxides. This rock is a variety of syenite in which nepheline is the dominant feldspathoid. Also, it has alkali-rich minerals, rare Earth, and some incompatible elements.
Nepheline syenite resembles granite in its grains and colors. However, it is darker in color, lacks quartz, is silica undersaturated, has less biotite, and is richer in alkali minerals than granites. In contrast, granite is lighter, has quartz, is silica-saturated, and has more biotite.
Nepheline syenite is an intermediate to felsic rock depending on the mineral content of felsic (lighter), ferromagnesium (darker), and the amount of silica. It is not a mafic rock. Usually, lighter-colored samples, i.e., with more light-colored minerals and more than 63% silica, are often felsic, and darker ones with 52-63% silica are intermediate.
No. Nepheline syenite is inert and safe. The EPA has given it a green circle, meaning it is a chemical verified experimentally and on modeled data to be of low concern. One concern from Unimin Corporation (Canada) MSDS is on respiratory passages, nose, and throat of grounded form. Also, it can potentially cause eye irritation or even injury. However, this is a problem typical of any rock dust.
We recommend you use gloves, protective glasses, dust masks, and other safety clothing when handling the powdered form of this rock.
References
- Bailey, D. K. (1989). Alkaline rocks: undersaturated. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology. New York: Van Nostrand Reinhold.
- Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company
- Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (3rd ed.). The Mineralogical Society.
- Frost, B. R. (2014). Essentials of igneous and metamorphic petrology. Cambridge University Press.
- O’Donoghue, M. (ed.), (2006). Gems: Their sources, descriptions, and identification (6th ed.). Butterworth-Heinemann
- Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.
- Le Maitre, R. W. (1976). The chemical variability of some common igneous rocks. Journal of Petrology, 17(4), 589–598. https://doi.org/10.1093/petrology/17.4.589
- Manutchehr-Danai, M. (2009). Dictionary of gems and gemology (3rd ed.). Springer.
- Office of the State Geologist. (2023). Nepheline syenite. Arkansas.gov. https://www.geology.arkansas.gov/minerals/industrial/nepheline-syenite.html.