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Geology is the Way

Foid diorite and foid gabbro

Foid diorite and foid gabbro are silica-undersaturated plutonic rocks consisting of feldspathoids, plagioclase, mafic minerals (hornblende, pyroxene), and various accessories. They contain more feldspathoids (10 to 60% of all sialic minerals) with respect to diorite and gabbro, but, like the latter, lack or show very little alkali feldspar. The difference between foid monzodiorite and foid monzogabbro is the composition of plagioclase, more sodic in monzodiorite (< An50) and more calcic in monzogabbro (> An50). The most abundant feldspathoid present should be specified in the rock name (e.g. leucite diorite). The modal content of mafic minerals is generally high, between 30 and 70%.

Varieties
There are two terms that are still in use today to indicate two varieties of foid monzogabbro. Theralite is a nepheline gabbro containing titanian augite, labradorite, and nepheline. The name originates from the Greek therein, ‘to search for’. Teschenite (or teschinite; after Teschen, Czech Republic) is a variety of analcime gabbro with olivine, titanian augite, labradorite, and analcime.

Lugarite nepheline gabbro

Nepheline gabbro with black, elongated crystals of kaersutite amphibole associated with Ti-augite and surrounded by a white groundmass of calcic plagioclase, nepheline, and analcime. Locally this rock is known as lugarite. Lugar sill, Ayrshire, Scotland. Width: 13.5 cm. Photo © James St. John.

Lugarite nepheline gabbro

Nepheline gabbro with black, elongated crystals of kaersutite amphibole associated with Ti-augite and surrounded by a white groundmass of calcic plagioclase, nepheline, and analcime. Locally this rock is known as lugarite. Lugar sill, Ayrshire, Scotland. Width: 5.2 cm. Photo © James St. John.

Foid diorite/gabbro
Plutonic igneous rock
Felsic minerals:
plagioclase
feldspathoids
Mafic minerals:
hornblende
pyroxene

QAPF classification:
Plagioclase composition: foid diorite(< An50), foid gabbro (> An50)
F = 10 – 60%
Plagioclase/feldspars = 90 – 100%
Colored varieties:
• foid leucodiorite/leucogabbro (M < 30%)
• foid meladiorite/melagabbro (M > 70%)
Other varieties:
theralite: nepheline gabbro
teschenite: analcime gabbro
Extrusive equivalent: basanite, tephrite

References

        

it_IT Italiano
Igneous Minerals
Igneous Textures
Plutonic Rocks
Igneous Bodies

 

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