Syenite is a coarse-grained, mostly light-colored intrusive rock with an intermediate to felsic composition. It has mainly alkali feldspar, lesser plagioclase, and one or more mafic minerals. Also, it may have a small amount of quartz.

Like other plutonic rocks, it forms from the slow cooling of intermediate silicic alkali-rich magmas deep inside the Earth’s crust.

Where did the name originate from? Pliny the Elder coined the name syenite to mean hornblende granite, popularly used for construction in ancient times. This rock was sourced from Syene, Egypt, now Aswan. However, the meaning is now different.

Learn more about syenite rock, including its properties, facts, and chemical and mineral composition. We will also cover how it forms, where it is found, its uses, and the various varieties or types.    

Quick facts and properties

• Name: Syenite
• Rock type: Igneous
• Origin: Intrusive
• Color: Mostly off-white, gray, or pink but can have greenish or reddish-brown hues.   
• Color index: Leucocratic
• Texture: Coarse-grained or phaneritic, sometimes porphyritic textured, and rarely pegmatitic.
• Cooling history: Slow cooling rate deep in the Earth’s crust  
• Volcanic equivalent: Trachyte, i.e., has the same composition as volcanic or extrusive rock trachyte.
• Density: 2.63-2.80 g/cm³ (source)
• Silica content: 53%-65%.
• Mohs scale hardness: 6 – 6.5
• Tectonic environment: Mostly thick continental crusts or at cordilleran (extensive mountain range) subduction zones

What does syenite look like?

Usually, syenite is a massive, coarse-grained, off-white, gray, pink, reddish-brown, or greenish rock with interlocking mostly light (felsic) and fewer darker (mafic) speckles. However, it may be violet-grayish on weathering. Also, it may have other textures like porphyritic, microsyenite, or even show layering.

A hand specimen of this rock will somewhat resemble granitoids (granite, granodiorite, alkali feldspar granite, and tonalite) or rocks like quartz monzonite and quartz-diorite. It may not be easy to tell these rocks apart. However, granitoids are often lighter and will have visible quartz minerals unlike syenite.

Please note that syenite is not granitoid since quartz does not account for 20-60% of the QAPF content by volume. Also, it is less common than granitoids or other plutonic rocks like diorite, gabbro, etc.

Let us talk more about microsyenite and porphyritic textures, hypersolvus vs. subsolvus, and layering. It will further help you identify this rock.

1. Microsyenite

Microsyenite is a medium-grained syenite formed at relatively shallow depths or near intrusion margins. Its grain size will be larger than aphanitic rocks but smaller than coarse (phaneritic) rocks.  

2. Porphyritic

Porphyritic syenite has phenocrysts (larger crystals), usually orthoclase in a finer but coarse-grained matrix or groundmass. The groundmass may have perthite feldspars, and at least one mafic mineral.

This texture indicates two cooling stages. An earlier stage that formed phenocryst and later groundmass.

3. Hypersolvus vs. subsolvus

Hypersolvus syenite has one perthitic feldspar. It forms when crystallization happens at a high temperature under low water vapor or drier magma.

On the other hand, subsolvus form in hydrous magmas under high vapor pressure. These conditions allow alkali feldspar and albite-rich plagioclase to crystallize directly from the melt. Thus, it will have two feldspars.

4. Layering

Sometimes, syenite may be layered with other rocks. An excellent example is the Ilímaussaq alkaline complex in Greenland with thin (inch-scale) layers of augite syenite and pulaskite. Pulaskite is a nepheline-bearing alkali feldspar syenite.

Another example is at Lovozero Peralkaline Intrusions in the Kola Peninsula, Russia. This intrusion has nearly horizontal rhythmic layers of urtite at the bottom. As you go up, alkali feldspar increases, forming leucocratic nepheline syenite with the top rhythm having melanocratic variety and trachytoid.

Also, syenite layering is common in intrusions with overall basaltic composition. For instance, such intrusions will have olivine- or pyroxene-rich ultramafic rocks at the bottom. Plagioclase-rich mafic (gabbros or norites) will follow, and anorthosites will be in the middle. Finally, the upper parts will have highly fractionated felsic rocks like syenite and granophyre.

Syenite composition?

As already mentioned, this rock has an intermediate to felsic composition. Here is more on its chemical and mineral composition.

1. Chemical composition

Syenite is an intermediate to felsic rock, depending on the silica and mafic mineral content. Those low in mafic minerals and with more than 63 wt.% silica are felsic. In contrast, syenites moderate in silica, i.e., 52-63 wt.% and relatively higher mafic content, are intermediate.

A typical weight percentage chemical composition of syenite using data from Le Maitre (1976) is SiO₂: 59.63%, TiO₂: 0.86%. Al₂O₃: 16.94%, Fe₂O₃: 3.09%, FeO: 3.18%, MnO: 0.13%, MgO: 1.90%, CaO: 3.59%, Na₂O: 5.33%, K₂O: 5.04% and P₂O₅: 0.30%. This specimen is intermediate.

Considering the Alumina saturation index (ANSI), most syenites are peralkaline or peraluminous rocks. Peralkaline syenite has more molar alkali oxides than alumina (Na₂O+ K₂O > Al₂O₃). Such rocks will have aluminum-free minerals.

On the other hand, peraluminous are aluminum oversaturated relative to alkalis (Al₂O₃ > Na₂O+ K₂O), thus will have aluminous phase minerals.

2. Mineral composition

Syenite has mainly alkali feldspar, a minor amount of plagioclase, none or small amounts of quartz, muscovite, and one or more mafic minerals.

The common mafic minerals are hornblende, augite, and less often biotite (or annite), or rarely fayalite. Biotite is rare since feldspar formation consumes most aluminum. However, this rock can instead have aluminum-rich annite, a mica, just like biotite.

Accessory minerals are apatite, zircon, titanite/sphene, monazite, conundrum, magnetite, and pyrite.

Let us now look at felsic and mafic minerals, alteration, QAPF diagram, and naming of syenites.

i. Felsic and mafic minerals

In syenites, total feldspars account for more than 65% of the composition by volume, while mafic minerals account for 10-30%.

On a syenite rock hand specimen, the lighter minerals are mainly alkali feldspar (usually, k-feldspar) with a minor amount of sodium plagioclase. Also, this rock may have other felsic minerals like muscovite and quartz.

However, any quartz present will be interstitial, myrmekitic, or micrographic and it indicates crystallization occurred during the last stage.

On the other hand, the darker minerals are mafic. For instance, biotite is brown or green, and hornblende forms subhedral prismatic green crystals, while augite will be black, brownish, or sometimes greenish.

Usually, as Blatt et al. (2006) note, alkali feldspar has subhedral to tubular orthoclase, sanidine, microcline, or anorthoclase on a thin section. Plagioclase, on the other hand, is often subhedral.

However, these feldspars may have a perthitic texture discussed earlier. Usually, perthitic feldspars often have striation, and these two have slight color variations. 

ii. Alterations

Possible alteration in syenite includes feldspar → kaolinite and sericite; biotite → chlorite and sphene; amphibole → calcite, iron oxides, or chlorite.

Also, altered syenite may have zeolite, analcime, allanite-(Ce), astrophyllite, prehnite, fluorite, and natrolite or calcite. They occur as secondary minerals, especially in vesicles.

3. QAPF Diagram

On the QAPF diagram of plutonic rocks, syenite is defined as a plutonic rock in which alkali feldspar is 65-90% of total feldspars, and quartz is no more than 5% of QAPF content by volume. This rock has no feldspathoids.

As alkali feldspar reduces, it grades into monzonite, while an increase in alkali feldspar grades this rock into alkali feldspar syenite.

On the other hand, an increase in quartz gives you quartz-syenite, while an increase in foids up to 10% and 20-60 vol. % of QAPF grades syenite into foid-bearing and foid syenites, respectively. Quartz and foids don’t co-exist.

Naming these rocks

Specimens with one dominant mafic mineral will have the prefix of the specific mineral. For instance, hornblende syenite indicates that hornblende is the dominant mafic mineral. This applies to hedenbergite, biotite, or augite syenite.

Also, some syenites with a certain abundant feldspathoid mineral may have the name of the mineral as a prefix. If foid bearing, use a hyphen between the foid and word bearing such as nepheline-bearing syenite.

Related rocks

As alkali feldspar, foids, plagioclase, or quartz volume varies, you will have the following:

1. Quartz syenite

Quartz syenite has 5%-20% of QAPF content by volume, and alkali feldspar accounts for at least 65% of the total feldspar content. This plutonic rock has no foids. ,

2. Foid-bearing syenite

Foids account for up to 10 vol.% of QAPF content in foid-bearing syenite, with alkali feldspar at least 65% of the total feldspars.

Remember, foids belong to the tectosilicate group of silicate minerals, just like feldspar. However, they are often lower in silica, have a different structure, and don’t coexist with quartz.

3. Alkali feldspar-rich syenites

An increase in alkali feldspar to over 90% of the total feldspars grades syenites into alkali feldspar-rich syenites. Naming will depend on the amount of quartz and foids minerals.

We can further categorize these rocks into:

• Alkali feldspar syenite: In this plutonic rock, alkali feldspar accounts for over 90% of the total feldspars, and quartz is no more than 5% by volume of QAPF content.
• Quartz alkali feldspar syenite: It has 5-20% quartz by volume of QAPF content with alkali feldspar over 90% of the total feldspars.
• Foid-bearing alkali syenite: This plutonic rock has up to 10% foids by volume of QAPF content and alkali feldspar accounts for over 90% of the total feldspars.
• Foid syenite: It is an alkali feldspar syenite with 10-60% foids by volume of the QAPF content with alkali feldspar over 90% of total feldspar.

4. Foid monzosyenite

Foid monzosyenite composition is between monzonite and syenite with foids and no quartz. Alkali feldspar accounts for at least 50% of total feldspars, with foids 10-60% by volume of the QAPF content.

5. Foid syenite

In these rocks, alkali feldspar accounts for at least 50% of the total feldspars with foids accounting for 20-60% by volume of the QAPF diagram. Common ones are:

i. Nepheline syenite

Nepheline syenite is a coarse-grained, light-colored (usually gray or pink and less commonly dark green) plutonic or intrusive igneous rock. This foid syenite is either silica under-saturated, or a peralkaline, and its fine-grained or extrusive equivalent is phonolite.

It has mainly alkali feldspar (orthoclase) and nepheline (feldspathoid) and one or more mafic minerals. Mafic minerals are usually amphiboles like arfvedsonite, riebeckite, barkevikite, and pyroxenes, especially aegirine or aegirine-augite. Also, it may have aenigmatite and melanite.

Lastly, nepheline abundance, minerals rich in alkali, and incompatible or rare earth elements distinguish this rock from other syenites.

ii. Sodalite syenite  (fluorescent)

Sodalite syenite describes a foid syenite with predominantly alkali feldspar, especially orthoclase, and a considerable amount of sodalite. Other minor minerals include aegirine, aegirine-augite (alkali pyroxenes), alkali amphiboles (arfvedsonite or riebeckite), biotite, eudialyte, nepheline, etc.

In 2018, Mineral News, The Mineral Collector’s Newsletter reported about Erik Rintamaki’s discovery of florescent sodalite-rich syenite clasts or rocks from the shores of Luce and Chippewa Counties, Michigan, USA. When illuminated by long-wave UV light, these rocks have an intense glow, i.e., hackmanite-like fluorescent, creating yellowish-orange veins. Rintamaki nicknamed them Yooperlites.

Analysis by Michigan Technological University and the University of Saskatchewan reveal that Yooperlites were syenite rock rich with fluorescent sodalite. Their main composition is sodium, aluminum, silicon, chlorine, and oxygen. Also, these rocks had some altered orange-red natrolite resulting from altered nepheline.

The origin of these rocks rich with fluorescent sodalite is likely from the Coldwell Alkaline Complex in Ontario, Canada. Probably, continental glaciers transported them to the shores of Lake Superior. Also, sodalite syenite occurs in Russia, India, Greenland, Russia, Malawi, and other states in the US.

Lastly, these glowing sodalite-rich rocks sold as Yooperlites sell for US$ 5-150 per piece.

6. Alkali syenite

According to Le Maitre (2002), alkali syenite is a term people often use to mean peralkaline syenite. These varieties contain alkali amphiboles like riebeckite and arfvedsonite or amphibole aegirine and aegirine-augite. Also, they have K-feldspar, are poor or lack plagioclase, and have no quartz.

However, this term doesn’t mean the same as alkali feldspar syenite. Also, its use isn’t recommended.

7. Episyenite

According to Suikkanen & Rämö (2019), episyenite is a term coined by Alfred Lacroix for the rock he couldn’t identify its original magma. It describes quartz-depleted stones, rich in alkali feldspar, that occur as lenses, veins, or pods in granitoids or migmatites.

Usually, episyenite rocks have diverse characteristics. However, their porosity and the brick or red color due to K-feldspar alteration or white color from albitization are distinguishing traits noted on their outcrops.

Another unique trait of episyenite is that it undergoes quartz depletion at sub-solidus temperatures. This depletion happens by weakly saline hydrothermal fluids. Sometimes, there is the addition of alkali metal oxides (alkali metasomatism) and, at times, rare earth elements in resultant cavities. Also, dissolution of mafic minerals may occur.

Barren (non-mineralized) episyenite rocks occur in Bohus granite and Forsmark in Sweden, Central Iberian Massif in Spain, Pyrenees and Mont Blanc Massif in France), Gerês in Portugal, Tauern tectonic window in Austria, Bohemian Massif in Germany and Toki granite in Japan.

On the other hand, mineralized episyenites occur in Precambrian granitoids in New Mexico, the USA, Ukraine, Brazil, and Australia.

Lastly, episyenite is a source of rare elements that may host uranium and other valuable minerals.

How is syenite formed?

Syenite rock forms from the slow cooling of alkali-rich, relatively silica-poor magma deep inside the Earth’s crust. The slow cooling allows the growth of minerals and, consequently, a coarse-grained texture.

Let us look at tectonic settings and magma origin.

1. Tectonic environment

Syenite rocks form mainly on 1) continental areas with thick crusts or 2) subduction zones on cordilleras (extensive mountain ranges).

Examples of cordilleras are the Andes in South America and North American Cordillera. These two form the main volcanic arc or belt on the eastern half of the Pacific Ring of Fire.

2. Magma origin

Syenite magma probably originates from the anatexis of subcrustal rocks or the upper mantle partial melting. Also, they may form from magma fractionation. However, these magmas are not in Bowen’s series of crystallization.

Additionally, contamination or assimilation of crustal materials and other crustal interactions play some role in forming these magmas.

A common theory of syenite formation is from low-temperature partial melting of igneous or granitic protolith rock. This condition favors the melting of incompatible minerals like potassium. In contrast, higher partial melting temperatures will form granite, tonalite, or adamellite since more sodium and calcium would melt, creating plagioclase.

Lastly, some conditions may favor anorthite (CaAl₂Si₂O₈) crystallization from highly molten magma. Anorthite will then form cumulates as cooling occurs, leaving a melt low in silica, favorable for syenite formation.

Where is syenite found?

Syenite rocks occur as intrusions such as dikes or at edges of granite massifs. In the US, this rock is in Arkansas, Montana, New England, New York (metamorphized form), and South Carolina (great syenite dyke). For more places in the US, see more geologic units with syenite.

In Europe, it occurs in Norway, Germany, Switzerland, Sweden, Scotland, Bulgaria (in Plovdiv), Portugal, and Romania (near Ditrău city). Also, Lovozero Massif and the Khibiny Mountains in Russia have a nepheline-rich variety.

Elsewhere, this rock occurs in Aswan in Egypt, Mulanje Mountain Forest Reserve in Malawi, and small intrusions in all states of Australia. However, New South Wales, Australia, has a large intrusion. Also, this rock occurs in Kangerluluk and Paatusoq fjords, SE Greenland.

What is syenite used for?

Syenite is a tough, durable rock that resists weathering. Therefore, it is ideal for making aggregate, dimensional stones, sculptures, landscaping, cemetery markers, etc.

Some of the uses of syenite include:

1. According to Haldar &Tisľjar (2014), syenite has better fire resistance qualities and is thus preferred over granite. It is cut to make dimension stones for foyers, facing, paving, building, and other architectural uses. Also, it can make refractories.
2. This rock is crushed to make aggregate for road construction and building sectors. You can also use the aggregate on unpaved walkways, patios, or pathways.
3. It also makes monuments and sculptures like the 3,500-year-old Quay with Sphinxes in Saint Petersburg, Russia, with syenite sourced from Egypt.
4. More uses are curbing, landscaping, garden decoration, cemetery markers, and cement additives.

Lastly, this rock and granite, granodiorite, gneiss, monzonite, larvikite, and anorthosite, are traded as decorative building stones under commercial granite. Don’t get confused if you find it labeled so.

Frequently asked questions

References

• Blatt, H., Tracy, R. J., & Owens, B. E. (2006). Petrology: Igneous, sedimentary, and metamorphic (3rd ed.). W.H. Freeman and Company
• Haldar, S. K., & Tisľjar, J. (2014a). Introduction to Minerology and petrology (1st ed.). Elsevier.
• Parsons, I. (1989). Volcanic glass. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology. New York: Van Nostrand Reinhold.
• Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
• Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
• Syenite. (2023, June 29). In Wikipedia. https://en.wikipedia.org/w/index.php?title=Syenite&oldid=1162436057
• Le Maitre, R. W. (Ed.) (2002). Igneous rocks: A classification and glossary of terms (2nd ed.). Cambridge University Press.
• Suikkanen, E., & Rämö, O. T. (2019). Episyenites—characteristics, genetic constraints, and mineral potential. Mining, Metallurgy & Exploration, 36(5), 861–878. https://doi.org/10.1007/s42461-019-00120-9
• Nesse, W. D., & Nesse, W. D. (1991). Introduction to optical mineralogy (2nd ed.). Oxford Univ. 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