Syenite

The term Syenite is derived from the original word "Syene" that comes from Egypt. It is associated with plutonic groups because it is one of the types of intrusive igneous rocks. It is considered to be similar to granite as well but some difference is there in terms of syenite minerals In this article, we will talk about this rock. We will learn syenite composition, distribution, uses, classification and syenite metaphysical properties and some examples as well. This topic is useful to increase our understanding of igneous rocks especially the Intrusive rocks which form below the surface of the Earth.

Meaning

It is an intrusive igneous rock that consists of ferromagnesian mineral & alkali feldspar and usually coarse-grained. If we talk about its composition, that is generally the same as granite. It contains medium quantities of silica & large amounts of alkalis and alumina. The texture of syenites is like granite, which is granular but these rocks differ from granite due to the lack of quartz. 

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Definitions

Some of the standard definitions are mentioned below:

• "It is an igneous rock composed chiefly of feldspar. " - Merriam-Webster

• "It is a light-coloured coarse-grained plutonic igneous rock consisting of feldspars with hornblende or biotite." Or " a greyish, intrusive igneous rock usually containing feldspar, hornblende, and some quartz. " - Collins dictionary.

• "It is a type of intrusive igneous rock which consists of ferromagnesian minerals as well as alkali feldspar." - General Definition.

Syenite Composition

• If we talk about syenite mineral composition, then the predominant mineral is alkaline.

• Plagioclase feldspar may present but with a small amount, generally less than 10%.

• If ferromagnesian minerals are present, most of all in syenite, then they usually occur in hornblende, amphibole or clinopyroxene, whereas Biotite is rare. 

• Some other common accessory minerals are titanite, apatite, zircon as well as opaques.

• Most of the syenites are considered as either peralkaline or peraluminous.

Classification

We can see the classification on the QAPF diagram. Here, we can observe that various types of syenitic rock cover a large part in the diagram but true syenite rock is that rock that fits into the area marked as red. These types of rocks are usually composed of alkali feldspar (A) along with minor amounts of quartz (Q). On the other hand, Plagioclase feldspar (P) is less important than the former i.e alkali feldspar whereas foid syenites contain a significant amount of rare silicate minerals which are generally known as feldspathoids (F), which in simple words also termed as foids. Here we should note that Q, as well as F, are mutually exclusive. The rock that contains quartz can not contain feldspathoids or vice versa.

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Some of the types of syenite are explained below:

Nepheline Syenite

Nepheline syenite is a medium coarse-grained, intrusive igneous rock member. It is a holocrystalline plutonic rock. It consists of nepheline and alkali feldspar in large quantities. The rock of these is mostly in pale coloured, grey and pink colour. It is poor in silica and rich in alkaline. Generally, they are not similar to granite but because of their dark green varieties, they are known. Nepheline mineral is used to replace feldspar to manufacture ceramic glass and products.

Quartz Syenite

If we talk about this kind of syenite, then it is considered as a group of those plutonic rocks which are having the features of syenite along with a much larger amount of quartz, which is around 5 to 20%. Usually, syenite consists of less amount of quartz that is around less than 5% and when this quartz composition is between 5 to 20%, it is considered as quartz syenite.

Hornblende Syenite

Hornblende Syenite is the subgroup of amphibole minerals. It contains important amounts of calcium and is monoclinic. It is quite common. These rocks consist of a wide variety of rocks, such as syenites, granites, gneisses and gabbros. The minerals are typically opaque. It is dark green to black in colour with a vitreous to dull lustre. The crystals of hornblende are long and thin and they sometimes occur in massive granular aggregates. It can be found in New York ( USA ), Norway, Italy as well as Ontario ( Canada).

Syenite Properties

Physical properties of rocks are studied because they help not only in identifying the type of rocks but also to discover more about them. There are various syenite rock properties such as hardness, streak, lustre, compressive strength, specific gravity, etc. which defines this rock. The properties are important in determining its texture as well as uses. The hardness of Syenite on the Mohs hardness scale is 5.5 - 6 and compressive strength is 150.00 N/mm². If we talk about Streak which is considered as the colour of rock when it is crushed/ powdered, then it is white. Lustre is subvitreous to dull whereas cleavage is perfect. The specific gravity is said to be 2.6-2.7. Besides these, it is considered opaque in nature. Chemically, these types of rocks contain silica of moderate amounts, and relatively large amounts of alkalies, and alumina.

Distribution

It is not a common rock and thus some of the examples of the regions where it is found are mentioned below:

• Two giant nepheline syenite bodies lead to the making up of the Lovozero Massif as well as the Khibiny Mountains in the Kola Peninsula of Russia.

• Arkansas & Montana are regions where it is found in North America along with some parts of New England as well as New York and many other regions.

• Switzerland, Norway, Germany, Sweden, Portugal, Bulgaria and Romania are the countries where it is found in Europe.

• In Australia, it is found mostly in all of the states whereas in the Aswan & Malawi regions of Africa. 

Uses

• This rock is used for flooring, homes, hotels and interior decoration.

• It is used as a building stone, as a Facing Stone or paving stone along for garden decoration or the purposes of office buildings.

• It is also useful for curbing purposes. 

• It is used as a dimension stone, for cement manufacturing, construction or road aggregate, for landscaping, etc. 

• It is used for manufacturing natural cement as well as Magnesium & Dolomite Refractories.

• It is used for artefacts, monuments, sculpture making or small figurines, cemetery markers and the creation of various artwork.

Conclusion

Thus to conclude, in the end, we can say that syenite is an example of intrusive rock that forms beneath the surface of the Earth and consists of feldspar as its chief content. We learned about syenite rock and all related aspects along with some examples such as nepheline rock, nepheline syenite composition, quartz or hornblende, etc. We came to know that syenite is not a common rock found in some of the regions of the world and it is very useful for various purposes such as cement manufacturing, decoration, homes or buildings, sculpture or artworks making, etc. This topic is useful in Geography, Chemistry, Geomorphology, Physical Geography, Geology, Petrology and Earth Sciences. 

Let's look at some frequently asked questions related to syenite: