The aeschynite-group minerals occurring in an aegirine-fluorite-dolomite- aeschynite vein in the Bayan Obo Nb-REE-Fe deposit show complex compositional zoning in BSE images. Microprobe analyses revealed that the concentrations of REE2O3 are globally high in aeschynite-group minerals, ranging from 36.72 to 40.32 wt.%. Four species were identified in the same sample, these being aeschynite-(Ce), aeschynite-(Nd), nioboaeschynite-(Ce) and nioboaeschynite-(Nd). The correlations among cations suggest two special coupled substitutions with charge-balanced described by the exchange vectors as CaNb(Nd,Sm,Pr)-1 Ti-1 and LaNbTh-1 Ti-1. The chondrite-normalized REE patterns are hump-shaped with the maximum at Pr or Nd. The chondrite-nomalized La/Nd ratios vary in the range ∼0.15 to 0.40. It indicates that at Bayan Obo aeschynite minerals are relatively strongly La-depleted and Nd-enriched. The compositional variations in aeschynite-group minerals were controlled by the crystal chemistry and affected by fluid composition.

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