Foyaite is a variety of nepheline syenite. It is a rare igneous rock. The term does not fit into the modern nomenclature of igneous rocks and therefore its use is not anymore recommended. The rock was named after Foya in Portugal. Originally the term was used as a synonym of nepheline syenite. Slightly altered meaning was given to the rock by a Norwegian geologist Waldemar Christofer Brøgger. In his interpretation foyaite was a nepheline syenite with trachytic texture.
Nepheline syenite from southern Portugal (type locality). Light-colored crystals are alkali feldspar, nepheline is dark gray. Width of sample 12 cm. TUG 1608-2598
It must be said that the texture actually only resembles that of what is typical to many trachytic rocks. Nepheline syenite is a plutonic rock, unlike trachyte which is volcanic. Trachytic texture means that abundant tiny oriented alkali feldspar crystals (microlites) occur within glassy or cryptocrystalline groundmass. Alkali feldspar crystals are easily noticeable with a naked eye and the material between them (which is mostly nepheline) is not glassy or cryptocrystalline.
Foyaite is a variety of nepheline syenite where alkali feldspar clearly dominates over nepheline and first catches the eye because of mostly slender, light-colored, and often sub-parallel crystals.
The genesis of foyaite appears to be connected to continental rifting. In Portugal foyaite is found among the rocks of the Central Atlantic Magmatic Province which formed during the Pangean intracontinental rifting (201 Ma). Norwegian foyaite is somewhat older (formed during the formation of the Oslo Rift). Foyaite occurs in several other locations as well although it is not always named so because the term is obsolete now and should be used in historical context only.
Nepheline syenite from Norway. Although foyaite was named after a locality in Portugal, the rock from Norway is also important because the current meaning (nepheline syenite with a trachytic texture) was probably formulated after the appearance of this rock type from the Oslo Rift. Width of sample 16 cm.