II.5. Alkali Si-undersaturated igneous rocks (Foidolites)

IJOLITE

Appearance: Dark coloured, fine- or medium-grained alkali, Si-undersaturated intrusive igneous rocks. It contains approximately equal amount of clinopyroxene (Ti-rich agugite, aegirinaugite, aegirine) and nepheline. In addition, olivine, phlogopite and further feldpathoids could also occur. It can be found often accompanying with carbonatites mostly in continental rift zones and ocean islands. It is the intrusive equivalent of nephelinite at least based on chemical composition.

Mineral content:

Essential minerals: clinopyroxene (Ti-augite, aegirinaugite, aegirine), nefeline

Accessory minerals: olivine, flogopite, leucite, zircon, apatite, magnetite and/or ilmenite, carbonate minerals, garnet (melanite)

Secondary minerals: sericite, epidote, chlorite

Rock types:

Figure II.269. – Ijolite with carbonatites (Fuerteventura, Canary islands; photo: Szabolcs Harangi).

Figure II.270. – Ijolite with carbonatites (Fuerteventura, Canary islands; photo: Szabolcs Harangi).

Locations in the Carpathian-Pannonian region: not known

NEPHELINITE, LEUCITITE, MELILITITE

Appearance: Dark coloured, usually aphyric volcanic rock. It contains abundant feldspathoids (nephelinie or leucite), in addition to olivine, clinopyroxene and phlogopite phenocrysts. The feldspathoids could occur also as phenocrysts. Feldspars are lacking or are in minor amount. They are typical of continental rift zones and could occur also in oceanic islands.

Mineral content:

Essential minerals: olivine, clinopyroxene, feldpathoids (nepheline, leucite, melilite)

Accessory minerals: zircon, apatite, magnetite and/or ilmenite, analcime, carbonate minerals, perovskite, sodalite

Secondary minerals: analcime, sericite, chlorite

Locations in the Carpathian-Pannonian region: Stradner Kogel (Stájer-medence; nephelinite); Bár (olivine-leucitite)

Figure II.271. – Olivine-leucitite (Bár).

Figure II.272. – Characteristic microscopic picture of olivine-leucitite from Bár. Left with one nicol, right with crossed nicols.

Figure II.273. – Characteristic microscopic picture of olivine-leucitite from Bár. Note the iddingsitized olivine phenocryst and the leucites in the groundmass. Photo with one nicol.

Figure II.274. – Characteristic microscopic picture of nephelinite from Stradner Kogel (Styrian basin). Left with one nicol, right with crossed nicols.