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Sector zoned aegirine from the Ilímaussaq alkaline intrusion, South Greenland

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Abstract

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 NaTi0.5Fe 0.5 2+ 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.

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Contribution to the mineralogy of Ilímaussaq no. 62

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Larsen, L.M. Sector zoned aegirine from the Ilímaussaq alkaline intrusion, South Greenland. Contr. Mineral. and Petrol. 76, 285–291 (1981). https://doi.org/10.1007/BF00375455

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Keywords

  • Growth Rate
  • Mineral Resource
  • Crystal Growth
  • Diffusion Rate
  • Structural Control