FIRST DISCOVERY OF LOPARITE-(Ce) IN CARBONATITE OF THE GULI COMPLEX (POLAR SIBERIA, RUSSIA): ZONING OF CRYSTALS, ISOMORPHIC SUBSTITUTIONS AND GENETIC FEATURES

For the first time, loparite-(Ce) was found in a xenolith of phoscorite from the calcite carbonate of the Guli complex of the Maimecha-Kotui alkaline province (Krasnoyarsk Krai, Russia). Its composition is close to the end member (Na_0.5LREE_0.5)TiO₃. Crystals of loparite-(Ce) occur in magnetite as relict inclusions, their intermediate and marginal zones are characterized by a zonal distribution of rare earth elements (REE). Local areas enriched in Nb and Th are recorded in the marginal zones. Compositional variations of loparite-(Ce) crystals are described by the following substitution schemes: Ca²⁺ + Ti⁴⁺ ↔ Na⁺ + (Nb + Ta)⁵⁺ and REE³⁺ + Ti⁴⁺ ↔ Ca²⁺ + (Nb + Ta)⁵⁺. Based on the results of Raman spectroscopy, areas differing in structure and chemical composition of REE were identified in loparite crystals. It is concluded that isomorphism affects the distortion of the crystal structure and the symmetry of vibrational modes. The obtained results indicate that loparite-(Ce) was formed at the early stages of crystallization of an alkaline-carbonatite immiscible melt, and the later crystallization of pyrochlore and zirconolite is consistent with the fact that the residual carbonate melt was enriched in fluorine, rare and radioactive elements.