Conversion of Nepheline to Sodalite During Subsolidus Processes in Alkaline Rocks

Adrian A. Finch*
Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, U.K.
*Present address: Johnson Matthey Technology Centre, Blount's Court, Sonning Common, Reading RG4 9NH, U.K.

Abstract: Cathodoluminescence (CL) petrography of nepheline syenites of the Igaliko complex, Gardar province, South Greenland shows that sodalites possess embayed contacts against nepheline, and have formed by a process of metasomatic replacement. This texture is demonstrated clearly by CL, since sodalite luminesces bright orange and nepheline is poorly luminescent. The transformation from nepheline to sodalite results in a volume change which leads to a network of fractures in which deep-blue luminescent fluorite is precipitated. Fluorite is formed since the chlorination process involved in the transformation causes localised reductions of the salinity of the fluid and therefore a decrease in the solubility of fluorite. Sodalite-fluorite textures observed using CL allow sodalites of secondary origin in alkaline igneous rocks to be identified.

Nephelines and sodalites, when observed using scanning electron microscopy, possess small micropores. By analogy with recent work on alkali feldspars, pervasive alteration of nephelines may occur by fluid flow assisted by a permeable micropore network.

Keywords: feldspathoids • cathodoluminescence • microporosity • Igaliko • Gardar • Greenland • alkaline rocks

Mineralogical Magazine; September 1991 v. 55; no. 380; p. 459-463; DOI: 10.1180/minmag.1991.055.380.15
© 1991, The Mineralogical Society
Mineralogical Society (