Compositions and petrogenetic significance of the eudialyte group minerals from Sushina, Purulia, West Bengal

Abstract

The eudialyte-group of minerals (EGM) is one of the most important index minerals of the peralkaline (agpaitic) nepheline syenites. They crystallize in varied physico-chemical conditions ranging from the early-magmatic (orthomagmatic) to late-magmatic and even in the post-magmatic (hydrothermal) stage. In India, the only agpaitic nepheline syenite gneisses of the Sushina Hill region contain both late-magmatic as well as hydrothermal eudialytes. Compositionally these are Mn-Nb-Ca rich eudialytes and are comparable to the other EGM occurrences such as Ilimaussaq (Greenland), Tamazeght (Morocco), Mont-Saint Hilaire (Canada) and Pilansberg (South Africa). High Mn content (>6.5 wt.%) for both varieties of the Sushina EGM indicates that they are highly evolved in nature. In terms of the calculated site occupancy, particularly the [M(3)] and [M(2)], the Sushina eudialytes mimic some Pilansberg eudialytes. In addition to the eudialyte, the host nepheline syenite gneiss also contains an unknown Na-Zr-silicate (NZS) which is often found to be replacing both types of eudialytes. Compositionally these NZS can be tentatively represented as Na2Zr2S6O17. These NZS are characterized by much higher Zr, but lower Mn and Nb concentrations compared to the associated eudialytes. Two distinct varieties of eudialyte and NZS indicate subtle changes in the alkalinity during their formations. The formation of the late-magmatic as well as hydrothermal eudialyte essentially took place at somewhat elevated pH conditions. The replacement or alteration of eudialytes by NZS indicates a decreasing pH condition. In terms of the chemical composition the late-magmatic eudialytes can be represented as a solid-solution series between the kentbrooksite-taseqite-aqualite while the hydrothermal eudialyte represents solid-solution between kentbrooksitetaseqite -Ce-zirsilite.