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Geology is the Way

Foid monzodiorite and foid monzogabbro

Foid monzodiorite and foid monzogabbro are silica-undersaturated plutonic rocks that are closely related to monzonite, monzodioritee monzogabbro. These rocks contain essential feldspathoids, plagioclase, alkali feldspar, mafic minerals (hornblende, pyroxene), and various accessory minerals. Feldspathoids constitute 10 to 60% of all the felsic minerals present. In comparison with foid monzosyenites, foid monzodiorites and monzogabbros are modally dominated by plagioclase, which constitutes 50 to 90% of all feldspars. Foid monzodiorites are characterized by sodic plagioclasio, with composition < An50, whereas foid monzogabbros contain calcic plagioclasio (> An50). Foid monzodiorite and foid monzogabbro are finally named according to the most abundant feldspathoid present (e.g. sodalite monzogabbro). The typical modal content of mafic minerals (color index) of these rocks is comprised between 20 and 60%. Essexite (type locality: Essex County, Massachusetts, USA) is a variety of nepheline monzodiorite or monzogabbro containing titanian augite, kaersutite and/or biotite, plagioclase, and alkali feldspar.

Essexite (nepheline monzogabbro)
Essexite (nepheline monzogabbro) with black crystals of kaersutite and augite surrounded by light-colored plagioclase and nepheline. Mount Johnson, Quebec, Canada. Photo © James St. John.
Essexite (nepheline monzogabbro)
Essexite (nepheline monzogabbro) with euhedral crystals of augite (black) surrounded by plagioclase and nepheline (white). Craighead Quarry, Crawfordjohn, South Lanarkshire, Scotland. Photo © Ian Stimpson.

Foid monzodiorite/monzogabbro
Plutonic igneous rock
Felsic minerals:
alkali feldspar
Mafic minerals:

QAPF classification:
Plagioclase composition: foid monzodiorite(< An50), foid monzogabbro (> An50)
F = 10 – 60%
Plagioclase/feldspars = 50 – 90%
Colored varieties:
• foid leucomonzodiorite/leucomonzogabbro (M < 20%)
• foid melamonzodiorite/melamonzogabbro (M > 60%)
Other varieties:
essexite: nepheline monzodiorite/monzogabbro
Extrusive equivalent: phonolitic basanite, phonolitic tephrite



Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies


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