Abstract

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.

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