Syenite (hornblende syenite) from the Cretaceous of Vermont, USA.

Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Syenite is an intrusive igneous rock. It is similar to granite and gabbro in that is has phaneritic texture, with all or almost all crystals between 1 m and 1 cm in size each. Phaneritic texture forms as a result of relatively slow cooling of magma deep underground.

Syenite has a chemical composition between that of granite's felsic chemistry and gabbro's mafic chemistry. Syenite is an example of an intermediate igneous rock, which has 52 to 65% silica (= SiO2 chemistry) (intermediate has also been defined as 55 to 65% silica). Intermediate igneous rocks are sometimes light-colored, sometimes dark-colored, and sometimes have medium colors.

The mineral content of syenite is dominated by feldspar with little to no quartz. Syenite is defined as alkali feldspar-rich, with little to no plagioclase feldspar. The minor plagioclase feldspar component is often intergrown with the alkali feldspar to form perthite.

In the syenite shown above, the whitish-gray component is perthitic alkali feldspar. The black crystals are hornblende amphibole and biotite mica. Minor minerals in this material include pyrite, magnetite, titanite/sphene, apatite, and zircon.

Geologic unit & age: Cuttingsville Complex, mid-Cretaceous, 97 Ma

Locality: unrecorded locality near Cuttingsville, southern Vermont, USA (but likely from an old quarry at Granite Hill, in the southwestern part of the Cuttingsville Complex, ~1.5 to 1.7 km southwest of the town of Cuttingsville)

Some info. synthesized from:

Eby & McHone (1997) - Plutonic and hypabyssal intrusion of the Early Cretaceous Cuttingsville Complex, Vermont. in: Guidebook to field trips in Vermont and adjacent New Hampshire and New York. New England Intercollegiate Geological Conference, 89th Annual Meeting, 1997: B2-1 to B2-17.