Narsarsukite: A New Occurrence in Peralkaline Trachyte, South Greenland

B. G. J. Upton, P. G. Hill, B. Jefferies, C. E. Ford and R. Macdonald*
Department of Geology, Edinburgh, EH9 3JW
* Department of Environmental Sciences, Lancaster, LA1 4YR

Summary: A flinty, spherulitic dyke of peralkaiine trachyte in a Precambrian dyke-swarm consists mainly of albite and aegirine. Within this, euhedral crystals of narsarsukite, Na2(Ti,Fe)Si4O10(F), occur as short prisms elongate parallel to c. This distinguishes them from all previously described narsarsukites, which have tabular habits. Minor phases present include micas, apatite, quartz, pectolite, nordite, and a thorium silicate. Probe analyses are presented for narsarsukite, aegirine, aibite, biotite, apatite, pectolite, nordite, and the thorium silicate.

On mineralogical and geochemical grounds it is unlikely that the narsarsukite-bearing dyke is a differentiate of supposedly mantle-derived basalt-trachyte-comendite magmas in the region. The dyke is colinear with a comendite dyke exposed at lower altitude and it is suggested that the narsarsukite-bearing dyke developed as a rheomorphic fenite generated by reaction between quartz diorite basement and fugitive alkaline solutions from the subjacent comendite.

Mineralogical Magazine; September 1976 v. 40; no. 315; p. 737-746; DOI: 10.1180/minmag.1976.040.315.07
© 1976, The Mineralogical Society
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