Nepheline crystals inside lapilli, Ol Doinyo Lengai, East Africa

  This summer I had a great opportunity to work for the Geology Department, University of Illinois, Urbana-Champaign. In the department we have studied samples of rocks from recent eruption of Oldoinyo Lengai volcano. Despite the research being focused on the study of isotopes using ICP-MS techniques to the young volcanic rocks from, some petrographic studies were also done. It is worth noting that Oldoinyo Lengai is the only one known volcano that erupts natrocarbonate lava (natrocarbonatite). It is located in the East African Rift, the active rifting system. Nowadays it erupts mostly nephelinitic material, however it has been found that type of eruptions changes regularly from silicate to carbonaceous.

   Almost all the samples we have are loose aggregates of rounded lapilli 2-5 mm in diameter . The carbonatite tephra is very loose and it is extremely hard to prepare these samples for petrographic study. Each of the lapilli contains one or several phenocryst surrounded by fine-grained groundmass. The most common mineral that forms the phenocrysts is nepheline. The phenocrysts of wollastonite are less abundant. Rarely, phenocrysts of clinopyroxene and garnet could be found. Mostly, all the phenocrysts surrounded by some amount of fine-, medium-grained dark groundmass (Fig. 1).

Figure 1. Three large phenocrysts of nepheline. Thin section plane is perpendicular to c axis. PPL.

Figure 2. Nepheline phenocrysts.
Thin section plane is parallel to c axis for the largest crystal. PPL.

  Phenocrysts of nephiline form short hexagonal prisms. On Figure 2 section plane is parallel to c axis of the crystal (as well as to its optical axial plane) and perpendicular to it on Figure 1. This orientation is also confirmed by emplacement of the inclusions. They are parallel to both (0001) and sides of prisms.
  Large crystals of nepheline have large amount of inclusions that arrange according to the zonality. The arrangement Aegerine-augite is the most abundant mineral among the inclusions. Fe-oxides and hydroxides, garnet grains, volcanic glass (melt inclusions) are also common as inclusions.
  Groundmass is very dark in color. Mainly, it is made up of nepheline, aegirine-augite, wollastonite, apatite and opaque minerals (magnetite and/or perovskite) and volcanic glass (?) (Fig. 3).
   Some phenocrysts are free of any kind of matrix and could be found in the volcanic tephra (Fig. 4).