Naujakasite from the Ilímaussaq alkaline complex, South Greenland, and the Lovozero alkaline complex, Kola Peninsula, Russia: a comparison

Naujakasite, Na6(Fe,Mn)Al4Si8O26, long known from the Ilímaussaq alkaline complex, South Greenland, was not reported until 1999 from other occurrences of alkaline rocks in spite of the fact that the mineral is composed of common elements. In 1999, a variety of naujakasite rich in Mn was found in the Lovozero alkaline complex in the Kola Peninsula, Russia. This variety has been approved by the IMA as a new mineral, manganonaujakasite, Na6(Mn0.53Fe2+ 0.47)Al4Si8O26. At Ilímaussaq naujakasite is a rock-forming mineral in the highly evolved rock naujakasite lujavrite in which it may make up more than 75 vol.%; at Lovozero manganonaujakasite is a very rare constituent in mineralised lovozerite–lomonosovite lujavrite. Naujakasite appears to take the place of nepheline in hyper-agpaitic nepheline syenites characterised by exceptionally high Na/K ratios. The nepheline syenites at Ilímaussaq have an average Na/K (atomic) ratio of 3.08, and the naujakasite lujavrites have the extreme ratio 4.56. The nepheline syenites of the Khibina and Lovozero complexes are characterised by lower Na/K ratios, 1.27 for Khibina and 1.67 for Lovozero, and thus nepheline is stable in the hyper-agpaitic rocks and naujakasite occurs only in pegmatites.