Foidolite with Nepheline (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, devoid of kalsilite and, if other feldspathoid minerals are present, F < 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 P-la, northwestern Russia) A variety of foidolite in which sodalite is the predominant feldspathoid mineral.
- Urtite (for For Lujavr Urt, (Lovozero Mts.), Kola P-la, northwestern Russia) A variety of foidolite containing > 70% modal nepheline; clinopyroxene, apatite, titanite, magnetite and perovskite are commonly present in subordinate amounts.


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QAPF Diagram Foidolite field in blue




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Ijolite Sample. Froma Anton R. Chakhmouradian.





Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore
• Eric A.K.Middlemost (1985): Magmas and Magmatic Rocks. Longman, London
• Ron H. Vernon (2004): A pratical guide to rock microstructure. Cambridge editore


Photo
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Augite (pink), Nepheline (colorless) and Apatite (elongated, colorless crystals). PPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite (pink) and Nepheline (colorless). PPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite (pink) and Nepheline (colorless). PPL image, 2x (Field of view = 7mm)
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Augite (pink) and Nepheline (colorless). PPL image, 2x (Field of view = 7mm)
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Augite (pink) and Nepheline (colorless). PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite and Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite (pink) and Nepheline (colorless). PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Augite (pink) and Nepheline (colorless). PPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Nepheline (I order gray). XPL image, 2x (Field of view = 7mm)
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Basal sections of Apatite and Augite crystals. PPL image, 10x (Field of view = 2mm)
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Nepheline with apatite inclusions (colorless and high relief). PPL image, 10x (Field of view = 2mm)
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Nepheline (I order gray). XPL image, 10x (Field of view = 2mm)
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Nepheline (I order gray) and Titanite. XPL image, 2x (Field of view = 7mm)
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Nepheline and Pyroxene crystals. PPL image, 2x (Field of view = 7mm)
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Nepheline and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline and Pyroxene crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. PPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. PPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. PPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 2x (Field of view = 7mm)
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Nepheline. XPL image, 10x (Field of view = 2mm)
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Nepheline, Pyroxene and Titanite crystals. PPL image, 10x (Field of view = 2mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 10x (Field of view = 2mm)
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Nepheline, Pyroxene and Titanite crystals. XPL image, 10x (Field of view = 2mm)