Sector zoned aegirine from the Ilímaussaq alkaline intrusion, South Greenland

Sector zoned aegirine crystals occur in the interstices of peralkaline nepheline syenites in Ilímaussaq. The crystals have grass-green [001] sectors enriched in Ca and Fe²⁺ (as CaFeSi₂O₆), Mn and Zr; pale green {010} sectors enriched in Al (as NaAlSi₂O₆); blue-green {110} sectors enriched in Ti (as NaTi_0.5Fe_0.5²⁺Si₂O₆); and light green {100} sectors enriched in Fe₃₊ (as NaFe³⁺ Si₂O₆). The crystals grew in the liquid with a rate that did not exceed the diffusion rate of most elements in the liquid. However. Fe³⁺ seems to have had diffusion rates lower than the crystal growth rate, and this probably caused the development of some sectors enriched in acmite and others enriched in the hedenbergite component. For Al, Ti and Zr a crystal structural control is envisaged although a recent structure-based model for sector zoning fails to explain the efficient separation of these elements into different sectors. Three more occurrences of sector zoned aegirine are noted, all from peralkaline nepheline syenites. The phenomenon is probably more widespread than hitherto realised.