Syenite is an igneous rock that solidified slowly in the crust in a similar manner
to granite. A true syenite (sensu stricto) is also compositionally resembling granite. The most notable difference is the absence or very low quantity of quartz in syenite while it is an essential component of granite. The dominant mineral in syenite is alkali feldspar, usually orthoclase. Syenites are found in a wide variety of colors.
The term “syenite” in a wider sense involves similar rock types like quartz syenite, alkali feldspar syenite, foid syenite, foid monzosyenite, etc. The classification principles are shown on the diagram below.
This is the QAPF diagram which is used to classify plutonic igneous rocks like granite, gabbro, and also syenite. As you can see different rock types known as syenites cover a large part of the diagram. The true syenites are rocks that fit into the area annotated in red. Please take a look at the article about the QAPF diagram if you are not familiar with it. Syenites are mostly composed of alkali feldspar (A) with a minor amount of quartz (Q). Plagioclase feldspar (P) is clearly less important than alkali deldspar. Some syenites (foid syenites) contain significant amount of relatively rare silicate minerals known as feldspathoids (F) which are called foids for simplicity. Note that Q and F are mutually exclusive. Syenitic rock that contains quartz can not contain feldspathoids and vice versa — foid-bearing and foid syenites contain no quartz.
The diagram above may leave us an impression that the true syenite is a relatively insignificant part of the whole syenite family. Actually, this is not true. Syenite sensu stricto and quartz syenites as well are the most common of the whole group although syenite is a relatively rare rock type anyway, especially when compared with granite or other granitoids. Syenitic rocks like nepheline or sodalite syenites (foid syenites) are actually curiosities that please the eye of a geologist but are by no means common rocks we are likely to see all around us.
Alkali feldspar forming the majority of most syenitic rocks is usually intergrown with sodium-rich plagioclase feldspar (usually oligoclase). Such feldspar intergrowth is named perthite and this is the reason why syenite is more common rock type than alkali feldspar syenite which contains almost no plagioclase. Plagioclase may appear in syenitic rocks in addition to perthitic alkali feldspar also as a separate phase. Dark mica biotite and amphibole hornblende are usual mafic constituents of normal syenites. Alkali syenites may contain somewhat unusual minerals like aegirine, arfvedsonite, etc. Common accessory minerals are zircon, apatite, sphene, magnetite, and ilmenite.
The approximate extrusive (volcanic) equivalent of syenite (more precisely alkali syenite which is rich in sodium) is trachyte. Phonolite is an approximate equivalent of foid syenite. I said “approximate” because the classification principles used to classify plutonic and volcanic rocks are in most cases different. While the classification of plutonic rocks is based on mineralogical composition, the same approach is not practical with fine-grained or even partially glassy volcanic rocks. They are classified according to their chemistry and the results are plotted on the TAS diagram.
Syenitic rocks are usually associated with other plutonic rocks. They form relatively small intrusive bodies or parts of larger intrusions. Most syenites seem to be associated with extensional tectonic regime (rifting continents). At least some syenites are believed to be fractionation products of alkali-rich basaltic magmas but there are several different mechanisms responsible for the genesis of syenitic magma. In this sense syenite is again similar to granite which is also defined solely by its mineralogical composition although there are very different possible ways how granitic magma can evolve. It is perhaps inevitable because minerals are something we can determine, unlike genesis which in many cases is still a mystery.
An immense number of terms describing many varieties of syenite have been used in the past. Some of them are mentioned (foyaite, canadite, durbachite, umptekite) to describe the rocks shown below. In addition to these, nordmarkite (quartz syenite) and perthosite (alkali feldspar syenite) are perhaps the best known unofficial names of syenite varieties. Shonkinite is a dark-colored variety of foid syenite which generally contains more pyroxene than alkali feldspar. Larvikite is a famous decorative stone from Norway which is often named syenite. It exhibits a characteristic schiller but larvikite contains more plagioclase than alkali feldspar and is therefore monzonite, not syenite. Many so-called syenites are actually monzonites. For some reason the term “monzonite” seem to be little known and rarely used.
The term “syenite” is actually an ancient one. There is a town named Syene (nowadays: Aswan) in Egypt. However, the rocks near that town after which syenite got its name are actually hornblende-bearing granites according to modern classification principles.
Quartz alkali feldspar syenite from Estonia. The width of the sample is 8 cm.