The transformation of nepheline and albite into sodalite in pegmatitic mariupolite of the Oktiabrski Massif (SE Ukraine)

• Sodalite from pegmatite variety of mariupolite is of secondary origin.
• It was formed during the post-magmatic evolution of the host rock.
• Three generations of sodalite in the rocks were distinguished.
• Two older sodalite varieties have been formed as a result of conversion of nepheline and albite.
• The youngest generation was formed via fluid-mediated dissolution–recrystallization process.

Sodalite, Na8Al6Si6Cl2, from a pegmatitic variety of mariupolite in the Oktiabrski Massif (SE Ukraine) was studied using electron microprobe, electron microscopy, spectroscopic cathodoluminescence and Raman techniques to determine its growth history during the evolution of the host rock. Three generations of the mineral were distinguished: (1) the oldest forms patches with a pink-violet cathodoluminescence colour, (2) a younger one, with a dark blue colour, forms the matrix of the crystals, and (3) the youngest generation forms veins with light blue cathodoluminescence in the older sodalite generations; all are undoubtedly secondary phases formed during the post-magmatic evolution of the host rock. The close spatial association of the sodalite with coexisting albite, nepheline, natrolite and K-feldspar, forming inclusions in each other, and the embayed contacts of sodalite with nepheline and albite, and the patchy appearance of sodalite under CL, together suggest that the two older sodalite varieties formed from the conversion of nepheline and albite under the action of Na-, Cl- and Al-bearing, but Si undersaturated basic fluids released during cooling of the host. The excess of SiO2 (aq.) released as a result of albite metasomatism could be accommodated by natrolite occurring as tiny inclusions within the sodalite crystals. The youngest, veinlet, generation was probably formed via a fluid-mediated dissolution–recrystallization process, perhaps simultaneously with the coexisting veins of natrolite.

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