Chlorine-Deficient Analog of Taseqite from Odikhincha Massif (Russia): Genesis and Relation with Other Sr-Rich Eudialyte-Group Minerals

Abstract

Eudialyte-group minerals are important accessory minerals of peralkaline rocks of nepheline-syenite massifs and alkaline–ultramafic complexes. Here, we report the complex study of a eudialyte-group mineral (EGM) from peralkaline pegmatite of the alkaline-ultrabasic Odikhincha massif (Polar Siberia). The chemical composition of the studied EGM is intermediate between those of taseqite and eudialyte, with small admixtures of other members of the eudialyte group. The crystals of EGMs were formed during the postmagmatic stage in the temperature range of 300–350 °C and partly replaced by late eudialite along cracks during the zeolite stage (~230 °C). The chemical compositions, structural features and mineral association of the studied EGM are similar to those of Sr-Nb-dominant EGM found in other nepheline-syenite massifs, such as Khibiny, Lovozero and Pilansberg. The EGM studied in this work is a Cl-deficient taseqite variety (“monochlore taseqite”), which differs from “dichlorotaseqite” (found only in the Ilimaussaq massif) by a lower amount of chlorine.