Return to Alkaline Rocks
Large intrusions of feldspathoid syenites are commonly accompanied by pegmatites, i.e. veins, lenses and pods of coarse-crystalline material comprising the same minerals as found in the surrounding rocks (nepheline, sodalite, alkali feldspars, aegirine and sodic amphiboles), as well as copious quantities of rare minerals enriched in water and "incompatible" trace elements: Sr (lamprophyllite, belovite), Ba (barytolamprophyllite, batisite), REE (rinkolite, belovite), Zr (eudialyte, lovozerite, catapleiite), Nb (labuntsovite, loparite), Li (mangan-neptunite, tainiolite), Be (leucophanite, epididymite, chkalovite) and Th (steenstrupine). There is little doubt that some of these minerals crystallized from super-evolved melts "left over" after the formation of the surrounding alkaline rock. However, this early magmatic stage is commonly followed by hydrothermal processes involving aqueous (and, in some cases, F- or CO3-rich) fluids, which leave their distinct mineralogical overprint, like the crystallization of fine-grained (sugary) albite, fibrous or spherulitic aegirine, exotic carbonate minerals, natrolite and various other zeolites. In some cases, the primary mineral assemblage and texture (such as a comb-like arrangement of crystals along the contact) are completely obliterated by late-stage hydrothermal reworking (metasomatism). These complex associations of minerals are described collectively as pegmatites or, to distinsguish them from their granitic counterparts, alkaline pegmatites. A single pegmatite body may be tens of meters long and contain dozens of different minerals, from such simple compounds as NaF (villiaumite) to such chemically and structurally complex as, for example, lemmleinite, Na4K4(Ba,K)2-4(Mg,Fe,Mn)0-1Ti8(Si4O12)4(O,OH)8·10H2O. Shown here are:
[Upper right] This pegmatite vein in the Karnasurt loparite mine (Lovozero, Russia) was intersected by a stroke of luck in 1970 and named "Yubileinaya" ("Jubilee") to commemorate the 20th anniversary of the Lovozero Mining and Processing Plant (or, according to other accounts, the 100th anniversary of Vladimir Lenin). This vein is the type locality for raite, zorite, vuonnemite and 10 other minerals. Among its 50-plus minerals, the most spectacular are, perhaps, magnesio-arfvedsonite, found as radiating clusters of sword-like black crystals at the contact, and dark-brown plates of lomonosovite. The dark-colored rock underlying the pegmatite is lujavrite. Since 1978, this unique pegmatite has been protected by the state. See Igor Pekov's Lovozero for more information.
[Middle right] Simple pegmatite vein crosscutting medium-grained nepheline syenite on Mt. Alluaiv, Lovozero (Russia). It consists mostly of alkali feldspar (white), eudialyte (brownish red) and magnesio-arfvedsonite (black); note also its conspicuously symmetrical comb-like structure.
[Lower right] Agpaitic nepheline-syenitic pegmatite from the Niorkpakhk apatite mine, Khibiny (Russia). This 14-cm specimen contains abundant eudialyte (raspberry-red) and lamprophyllite (bronzy-brown).