Compositional variation of aeschynite-group minerals in the Bayan Obo Nb-REE-Fe ore deposit, Inner Mongolia, China

The aeschynite-group minerals occurring in an aegirine-fluorite-dolomite- a eschynite vein in the Bayan Obo Nb-REE-Fe deposit show complex compositiona l zoning in BSE images, Microprobe analyses revealed that the concentration s 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 b eing aeschynite-(Ce), aeschynite-(Nd), nioboaeschynite-(Ce) and nioboaeschy nite-(Nd). The correlations among cations suggest two special coupled subst itutions with charge-balanced described by the exchange vectors as CaNb(Nd, Sm,Pr)(-1)Ti-1 and LaNbTh-1Ti-1. The chondrite-normalized REE patterns are hump-shaped with the maximum at Pr or Nd. The chondrite-nomalized La/Nd rat ios vary in the range similar to0.15 to 0.40. It indicates that at Bayan Ob o aeschynite minerals are relatively strongly La-depleted and Nd-enriched. The compositional variations in aeschynite-group minerals were controlled b y the crystal chemistry and affected by fluid composition.