Accessory rare earth element (REE) minerals occur in small quantities in agpaitic and miaskitic nepheline syenite gneisses of the Sushina Hill Complex, India. The REE-rich minerals restricted mainly to the agpaitic rocks are rinkite, cerianite-(Ce), and cerian thorite. Rinkite, formed at the ortho-magmatic stage predates other REE-rich phases and is the most Nd-F-rich rinkite (6.62–7.45 wt.% Nd2O3; 8.75–9.74 wt.% F) with very high Nd/Ce (>2.46) ratios reported to date. Hydrothermal cerianite-(Ce), formed by the decomposition of eudialyte in the agpaitic rocks, occurs as small rounded crystals rich in Ce (∼63–74 wt.% CeO2) and Y (6.03–11.69 wt.% Y2O3). The presence of cerianite-(Ce) indicates formation in an evolving hydrothermal fluid in an oxidizing milieu. Hingganite-(Ce) is present in the miaskitic unit and is considered to represent the superposition of an agpaitic mineral on an initial miaskitic assemblage. Hingganite-(Ce) is characterized by elevated contents of Ce (18.03–21.94 wt.% Ce2O3), and Nd (13.90–15.40 wt.% Nd2O3). Experimental data, coupled with the observed assemblage, suggest that the hingganite-(Ce) precipitated from the hydrothermal fluid between 400 and 300°C followed by cerianite-(Ce) (<∼300°C). This conclusion implies that eudialyte decomposition was probably initiated above 400°C.

Eudialyte-group minerals (EGM) represent the most important index minerals of persodic agpaitic systems. Results are presented here of a combined EPMA, Mössbauer spectroscopy and LA-ICP-MS study and EGM which crystallized in various fractionation stages from different parental melts and mineral assemblages in silica over- and undersaturated systems are compared. Compositional variability is closely related to texture, allowing for reconstruction of locally acting magmatic to hydrothermal processes. Early-magmatic EGM are invariably dominated by Fe whereas hydrothermal EGM can be virtually Fe-free and form pure Mn end-members. Hence the Mn/Fe ratio is the most suitable fractionation indicator, although crystal chemistry effects and co-crystallizing phases play a secondary role in the incorporation of Fe and Mn into EGM. Mössbauer spectroscopy of EGM from three selected occurrences indicates the Fe3+/ΣFe ratio to be governed by the hydration state of EGM rather than by the oxygen fugacity of the coexisting melt. Negative Eu anomalies are restricted to EGM that crystallized from alkali basaltic parental melts while EGM from nephelinitic parental melts invariably lack negative Eu anomalies. Even after extensive differentiation intervals, EGM reflect properties of their respective parental melts and the fractionation of plagioclase and other minerals such as Fe-Ti oxides, amphibole and sulphides.

The layered agpaitic nepheline syenites (kakortokites) of the Ilímaussaq complex, South Greenland, host voluminous accumulations of eudialyte-group minerals (EGM). These complex Na-Ca-zirconosilicates contain economically attractive levels of Zr, Nb and rare-earth elements (REE), but have commonly undergone extensive autometasomatic/hydrothermal alteration to a variety of secondary mineral assemblages. Three EGM alteration assemblages are recognized, characterized by the secondary zirconosilicates catapleiite, zircon and gittinsite. Theoretical petrogenetic grid models are constructed to assess mineral stabilities in terms of component activities in the late-stage melts and fluids. Widespread alteration of EGM to catapleiite records an overall increase in water activity, and reflects interaction of EGM with late-magmatic Na-, Cl- and F-rich aqueous fluids at the final stages of kakortokite crystallization. Localized alteration of EGM and catapleiite to the rare Ca-Zr silicate gittinsite, previously unidentified at Ilímaussaq, requires an increase in CaO activity and suggests post-magmatic interaction with Ca-Sr bearing aqueous fluids. The pseudomorphic replacement of EGM in the kakortokites was not found to be associated with significant remobilization of the primary Zr, Nb and REE mineralization, regardless of the high concentrations of potential transporting ligands such as F and Cl. We infer that the immobile behaviour essentially reflects the neutral to basic character of the late-magmatic fluids, in which REE-F compounds are insoluble and remobilization of REE as Cl complexes is inhibited by precipitation of nacareniobsite-(Ce) and various Ca-REE silicates. A subsequent decrease in F– activity would furthermore restrict the mobility of Zr as hydroxyl-fluoride complexes, and promote precipitation of the secondary zirconosilicates within the confines of the replaced EGM domains.