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Foid syenite

Foid syenites are alkali-rich silica-undersaturated varieties of syenites essentially consisting of alkali feldspar, feldspathoids, mafic minerals (hornblende, pyroxene, sodic pyroxene/amphibole), and various accessory minerals, some of which typical of syenitic rocks. According to the QAPF diagram, foid syenites are characterized by a content of alkali feldspar > 90% of all feldspars and a feldspathoid mode (F) of 10 to 60% of all the felsic minerals present. These rocks are rare but show a wide range of different textures, contain various feldspathoids (nepheline, cancrinite, sodalite, leucite…), as well as rare accessory minerals (e.g. aenigmatite, eudyalite). Consequently, different authors assigned foid syenites many local names based on the different mineralogy and textures shown by these rocks (like foyaite, kakortokite, lichfieldite, just to name a few). Just some of these names are actually approved by the IUGS commission: malignite (named after Maligne River, Ontario, Canada) indicates a foid syenite with a modal content of mafic minerals (M) between 30 and 60%, while in a shonkinite (from Shonkin, the Indian name for the Highwood Mountains of Montana, USA) the content of mafic minerals exceeds the 60 % (but it is less than 90%). Finally, foid syenites are named according to the most abundant feldspathoid present (e.g. nepheline syenite, sodalite syenite).

Nepheline syenite

Nepheline syenite containing greenish alkali feldspar and nepheline and dark-colored pyroxene, alkali amphibole, aenigmatite, and garnet var melanite. Mt. Kenya Volcano. Width: 8.3 cm. Photo © James St. John.

Nepheline syenite

Nepheline syenite with greenish to white nepheline and alkali feldspar, black amphibole, and yellowish titanite. Ice River Complex, British Columbia, Canada. Width: 5.2 cm. Photo © James St. John.

Foid syenite
Plutonic igneous rock
Felsic minerals:
alkali feldspar
feldspathoids
Mafic minerals:
hornblende
pyroxene

QAPF classification:
F = 10 – 60%
Plagioclase/feldspars = 0 – 10%
Colored varieties:
• malignite (M = 30 – 60%)
• shonkinite (M = 60 – 90%)
Local names: laurdalite, foyaite, ditroite, lichfieldite, lujavrite, kakortokite, naujaite, mariupolite
Extrusive equivalent: phonolite

Lujavrite

Villiaumitic lujavrite, a variety of nepheline syenite with amphibole, aegirine, nepheline, alkali feldspar, and villiaumite. Illímaussaq Complex, Greenland. Width: 1.7 cm. Photo © James St. John.

Kakortokite

Kakortokite, a variety of agpaitic nepheline syenite with eudyalite (pink), arfvedsonite (black), nepheline and alkali feldspar (white). Ilímaussaq Intrusion, Greenland. Height: 9.5 cm. Photo © James St. John.

References

        

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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