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