Return to Alkaline Rocks
Feldspathoid syenites are one of the most common and most variegated (both mineralogically and texturally) groups of alkaline rocks. They are broadly defined as plutonic rocks in which feldspathoid minerals account for 10-60% of all felsic minerals, and alkali feldspars are at least 10 times more abundant than Na-Ca plagioclase. Depending on the predominant feldspathoid mineral, this group of rocks can be further subdivided into nepheline syenites, sodalite syenites, cancrinite syenites, etc. Nepheline syenites and "mixed" sodalite-nepheline syenites are relatively common, whereas such rocks as kalsilite syenites are petrographic oddities. In addition to nepheline and other foids, these rocks contain potassium feldspar, albite (commonly as perthitic intergrowths with K feldspar), sodic amphiboles, aegirine, biotite, titanite and an assortment of rare minerals (like eudialyte, lamprophyllite or loparite).
Extensive fractionation of nepheline and ferromagnesian silicates from a nepheline-syenitic melt generates evolved magmas enriched in H2O, F, P2O5 and such rare elements as Nb, Sr, Ba, Li and Cs. These evolved alkaline magmas and their derivative fluids give rise to pegmatitic veins and lenses containing an astonishing variety of exotic minerals. It is not uncommon for a single pegmatite vein to contain over 50 different minerals, some of which may be so exceedingly rare that they are not known to occur anywhere else in the world. The silvery-white Na-Ti-Zn silicate kukisvumite, for example, has so far been observed only at its type locality at Mt. Kukisvumchorr in the Khibiny Mts (Kola, Russia).
If, on the other hand, nepheline-syenitic magma crystallizes fast slightly below or at the Earth's surface, it produces a fine-grained (sometimes porphyritic) rock termed phonolite. Phonolites may contain cavities (amygdales) lined with all sorts of interesting minerals, some of which precipitated from blobs of fluid trapped in the phonolite. The St. Amable sill near Mont Saint-Hilaire is one example of such amygdaloidal rock.
[Upper right] Trachytoidal nepheline syenite, Mt. Kukisvumchorr (Khibiny, Russia). Viktor Yakovenchuk's collection.
[Middle right] Lujavrite (trachytoidal melanocratic sodalite-nepheline syenite), Mt. Alluaiv (Lovozero, Russia).
[Lower right] Porphyritic nepheline syenite (dark gray) crosscut by a vein of sodalite-nepheine-syenitic pegmatite, Mont Saint-Hilaire (Québec, Canada).
|[Below] Nepheline-sodalite syenite in plane-polarized light (PPL, left) and crossed polars (XP, right), Mont Saint-Hilaire (Québec, Canada). Note large crystals of aegirine-augite (yellow to green in PPL) and sodalite (black in XP).