Ijolite

Ijolite (derived from the first syllable of the Finnish words Ii-vaara, Iijoki, &c., common as geographical names in Finland, and the Gr. Xiflos, a stone) is an igneous rock consisting essentially of nepheline and augite. Ijolite is a rare rock type of considerable importance from a mineralogical and petrological standpoint. It occurs in various parts of the Kainuu-region, northeast-Finland and in the Kola Peninsula in northwest Russia on the shores of the White Sea.

The pyroxene is morphic, yellow or green, and is surrounded by formless areas of nepheline. The accessory minerals are apatite, cancrinite, calcite, titanite and iivaarite, a dark-brown titaniferous variety of melanite-garnet. This rock is the plutonic and holo-crystalline analogue of the nephelinites and nepheline-dolerites; it bears the same relation to them as the nepheline syenites have to the phonolites. It is worth mentioning that a leucite-augite rock, resembling ijolite except in containing leucite in place of nepheline, is known to occur at Shonkin Creek, near Fort Benton, Montana, and has been called missourite.

References

•  This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.