Syenite
Syenite is a coarse-grained plutonic rock dominated by alkali feldspar, with little or no quartz.
Syenite is a coarse-grained plutonic rock dominated by alkali feldspar, with little or no quartz. It resembles granite but is poorer in silica and richer in alkalis, and it typically forms in continental rifts and alkaline intrusive complexes. The name comes from Syene, the ancient name of Aswan in Egypt.
🪨 Geology Guide Specimen: Open Syenite in the CRV Science Rock & Mineral ID Guide to log the specimen, compare it with the other 7 subgroups of Igneous, and test your eye in LithoClade.
Alkali-Rich Magma: How Does Syenite Form?
Syenite magmas form by low-degree partial melting of the mantle or by crystal fractionation of alkaline basaltic liquids, usually far from subduction zones, in rifts and within continental plates. Rising and cooling, they crystallize alkali feldspar with minor hornblende, pyroxene or biotite, and silica stays too low for much quartz to appear.
Textures and Feldspars: Why Does Syenite Shimmer?
Alkali feldspars in syenite often contain fine intergrowths of sodium- and potassium-rich feldspar called perthite, which scatter light and give a pearly or bluish sheen. The Norwegian rock larvikite, prized as a decorative facing stone, owes its blue iridescence to this effect.
Economic Value: Why Do Geologists Prospect Syenite Complexes?
Nepheline syenite, a quartz-free variety, is quarried for glass and ceramics because it supplies alumina and alkalis. Alkaline complexes can also host rare-earth elements, niobium and other critical metals, which is why syenites are widely explored.
Syenite at a Glance
| Property | Value |
|---|---|
| Type / variety | Alkali Feldspar Syenite |
| Composition | Orthoclase/Perthite (>65%) + Hornblende/Aegirine (<5% Quartz) |
| Geology Guide group | Igneous Rocks (Plutonic & Volcanic) |
| Rock Classification | Intermediate (Diorite–Andesite) |
| Mohs hardness | Mohs 6 |
| Texture / habit | Coarse interlocking tabular feldspar crystals |
| Density | 2.65–2.80 g/cm³ |
| Lustre and cleavage | Prominent pink, grey, or iridescent blue feldspar cleavage |
| Geological setting | Continental rift zones and alkaline ring complexes |
| Diagnostic feature (Geology Guide) | Coarse-grained intrusive rock that superficially resembles pink or grey granite, yet contains almost no glassy quartz—consisting instead of >65% alkali feldspar laths with minor hornblende. |
| Named for | Syene (Aswan), Egypt |
| Extrusive equivalent | Trachyte |