Chakrabarty, and two co-authors, reported in 2012 that 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 Ilímaussaq (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 kentbrooksite-taseqite -Ce-zirsilite
The authors concluded that eudialyte-bearing Sushina syenite gneissic complex represents unique EGM chemical compositions. Two different textural modes are observed for the Sushina eudialyte viz. a viz. late-magmatic and hydrothermal/post-magmatic eudialytes. The late-magmatic eudialytes are relatively smaller in size and at places relicts of relatively less altered grains are also present. The hydrothermal eudialytes on the other hand dominant over the late-magmatic one and frequently form complex aggregates composed of albite, nepheline and aegirine. In terms of their chemical compositions the following conclusions can be drawn:
- Sushina eudialytes are enriched in Mn-Nb-Ca. The lower limit of enrichment for these elements is much higher compared to all other EGM reported world-wide. The extremely high Mn content of the Sushina eudialytes indicates that they are highly evolved in nature.
- Owing to their high Mn-Nb content, the calculated site occupancy makes the Sushina eudialytes the second to be reported, after some Pilansberg eudialyte, which contain significant Nb and Mn at [ M(3)] and [M(2) ]sites respectively.
- REE and Sr behave incompatibly in the agpaitic system and are incorporated preferentially in the hydrothermal eudialytes at least in this case.
- Compositional variability between the two different types of Sushina eudialytes and their replacement by NZS can be used to understand changes in alkalinity(pH) during the evolution of the magma from the late-magmatic to post-magmatic/hydrothermal stages. Eudialyte formation indicates elevated pH conditions and the formation of NZS takes place at decreasing pH conditions.
- The deuteric fluid associated with the unmetamorphosed agpaitic syenite is suggested to be enriched in Mn, Nb, Ca, Zr and Cl.
- The present study is in agreement with observations made by Schilling et al. (2011) that the eudialyte compositions can be used in metamorphosed agpaitic rock to trace out its magmatic history as metamorphic fluids do not destabilized EGM up to the amphibolite facies of metamorphism.
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