Syenite is an intrusive igneous rock that has formed by the cooling of magma deep in the Earth's crust. Because of the slow cooling associated with the high temperatures at great depth, syenite has large crystals and is a coarse-grained rock. The crystals can be seen with the naked eye.
Syenite may look rather like granite, but by studying the minerals it contains, differences can be seen.
Syenite will usually appear darker-colored than granite, but not as dark as gabbro. Some syenites may be pink or gray or tinged with violet.
One well-known type of syenite is called larvikite. It is from Norway and is commonly cut into slabs and polished to make a pearly blue-green ornamental stone. This has been used as a facing stone on many buildings throughout the U.K.
As well as occurring in large intrusions, syenite is also found in sills and dykes. These rocks tend to have smaller crystals.
Many syenites have large crystals set into a finer mass, giving an attractive appearance. This is called a porphyritic texture.
As well as feldspar and quartz, syenite can contain hornblende, pyroxene, and the dark mica biotite. This composition contains features of both granite and gabbro.
Microsyenite is an igneous rock with the composition of syenite but contains smaller crystals.
Rhomb porphyry is a type of microsyenite found commonly in Norway. Pebbles of it, which have been carried by ice sheets across the North Sea area, are often found on the east coast of England.