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 Kola-peninsula in northwest Russia on the shores of the White Sea.

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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 the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.