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Geology is the Way

Foid monzosyenite

Foid monzosyenite is a rare variety of syenite that contains abundant feldspathoids and alkali feldspar, minor plagioclase, mafic minerals (biotite, hornblende, pyroxene, sodic pyroxene/amphibole), and various accessory minerals. Foid monzosyenites are characterized by a higher content of plagioclase with respect to foid syenites (10 to 50% plagioclase of all feldspar) and feldspathoids constitute 10 to 60% of the felsic minerals present in the rock. These rocks most commonly contain the feldspathoid nepheline and, less commonly, other feldspathoids. In general, the name of the most abundant feldspathoid present should appear in the name of the rock (e.g. nepheline monzosyenite). Miaskite (named after Miask, in the Urals of Russia) is a special name for a variety of nepheline monzosyenite containing biotite, plagioclase var oligoclase and perthitic alkali feldspar. The modal content of mafic minerals (color index) of foid monzosyenites typically ranges between 15 and 45%.

Nepheline monzosyenite

Nepheline monzosyenite from Sweden consisting of albite, nepheline, and mafic minerals. Width of sample 17 cm. Photo © Siim Sepp.

Foid monzosyenite
Plutonic igneous rock
Felsic minerals:
alkali feldspar
plagioclase
feldspathoids
Mafic minerals:
biotite
hornblende
pyroxene

QAPF classification:
F = 10 – 60%
Plagioclase/feldspars = 10 – 50%
Colored varieties:
• foid leucomonzosyenite (M < 15%)
• foid melamonzosyenite (M > 45%)
Other varieties:
miaskite: leucocratic biotite oligoclase nepheline monzosyenite with perthitic orthoclase
Extrusive equivalent: tephritic phonolite

References

        

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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