易解石族矿物的稀土成分变化特征

Rare Earth Compositional Variation of Aeschynite-Group Minerals

白云鄂博Nb RE Fe矿床中,产于霓石 萤石 白云石 易解石脉中的易解石族矿物,在背散射电子图象中显示复杂带状结构特征。电子探针分析表明,相对于世界其它产地易解石,白云鄂博易解石的RE2O3含量较高,其质量分数变化范围为36.72%～40.32%。区分4个矿物种:铈易解石、钕易解石、铌铈易解石和铌钕易解石。矿物成分中阳离子的相关关系表明,存在特殊的耦合类质同象替换,可用交换矢量表示为:CaNb(Nd,Sm,Pr)-1Ti-1和LaNbTh-1Ti-1。易解石的稀土分布模式为单峰型,最大值为Pr或Nd。陨石标准化La Nd比的变化范围为0 15～0.40,表明白云鄂博的易解石为相对La衰减和Nd富集。易解石族矿物的化学成分变化受晶体化学控制和含矿热液化学变化的影响。

The aeschynite-group minerals occurring in an aegirine-fluorite-dolomite-aeschynite vein in the Bayan Obo Nb-RE-Fe deposit show complex compositional zoning in BSE images. Microprobe analyses reveal that the concentrations of RE_2O_3 are globally high in aeschynite-group minerals, ranging from 3672% to 40.32%. Four species are 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-balance described by the exchange vectors as CaNb(Nd,Sm,Pr)_(-1)Ti_(-1) and LaNbTh_(-1)Ti_(-1). The chondrite-normalized RE patterns are hump-shaped with the maximum at Pr or Nd. The chondrite normalized La/Nd ratios vary in the range from 0.15 to 0.40. It indicates that in the Bayan Obo aeschynite there are relatively strongly La-depletion and Nd-enrichment. The compositional variations in aeschynite-group minerals are controlled by the crystal chemistry and affected by fluid composition.