Ijolite

Foidolites are coarse-grained intrusive igneous rock with a feldspathoid mineral content greater than 60%. On the APF diagram, these rocks plot in the corner closest to F. The term "foidolite" is too general and, wherever possible, should be replaced with a name that clarifies which of the feldspathoid minerals is predominant and whether other minerals are present in any significant amount. For example, nephelinolites are foidolites comprising mostly nepheline; they can be further categorized as melteigite, ijolite or urtite depending on the relative proportion of dark-colored minerals (most of all, clinopyroxene and biotite) in their composition.
The volcanic equivalents are termed foidite and phonolitic or tephritic foidites. According to the mineralogy can be divided into:

• Ferusite For Fergus, Montana, USA: A variety of foidolite containing essential clinopyroxene and 30-70% modal leucite (typically replaced by a mixture of nepheline, alkali feldspar, kalsilite and analcime).
• Ijolite For Iijoki, northern Finland: A variety of foidolite containing essential clinopyroxene (diopside, aegirine-augite or aegirine) and 30-70% modal nepheline.
• Italite For Italy: A variety of foidolite in which leucite is the predominant feldspathoid mineral and ferromagnesian silicates comprise 30 modal % or less of the rock.
• Melilitite For Melteig, Norway: A collective name for extrusive and shallow intrusive rocks containing > 10% modal melilite.
• Missourite For Missouri, USA: A variety of foidolite containing essential clinopyroxene and 10-30% modal leucite; olivine is also commonly present.
• Tawite For Tavajok, Lovozero Mts., Kola, northwestern Russia: A variety of foidolite in which sodalite is the predominant feldspathoid mineral.
• Urtite For Lujavr Urt, Lovozero Mts. Kola, northwestern Russia: A variety of foidolite containing > 70% modal nepheline; clinopyroxene, apatite, titanite, magnetite and perovskite are commonly present in subordinate amounts.

Ijolite: nepheline (grey), pyroxene (dark). Iron Hill Carbonatite Complex, Colorado, USA. From James St. John.

Ijolite: nepheline (grey), pyroxene (dark). Oka Carbonatite Complex,Quebec, Canada. From James St. John.

Samples doned by F.Stoppa (Chieti University, Italy).

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.

Photo
Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)	Aegirine, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. XPL image, 2x (Field of view = 7mm)	Aegirine (green), titanite, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)
Aegirine, titanite, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. XPL image, 2x (Field of view = 7mm)	Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)	Aegirine (green), titanite, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)
Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)	Aegirine, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. XPL image, 2x (Field of view = 7mm)	Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)
Aegirine, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. XPL image, 2x (Field of view = 7mm)	Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)	Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)
Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)	Aegirine (green), altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)	Aegirine (green), titanite, altered nepheline and feldspar (dusty). Caletta de la cruz, Fuerteventura, Canary Islands. PPL image, 2x (Field of view = 7mm)