Compositional relationships among polysomes of bastnaesite-synchysite series from the Maoniuping REE deposit, Southwest China

Chemical compositions of bastnaesite-synchysite polysomatic series minerals from the Maoniuping aegirine-alkali granite massif have been studied by el ectron microprobe. Syntaxial intergrowth of the polysomatic minerals is com monly observed. Bastnaesite with enriched LREE occurs replacing the polysom es. For the polysomes with the mixed-layer structures the average values of CaO (wt.%), REE2O3 (wt.%) and CaO/REE2O3 (weight ratio) are 8.21, 63.68, 0 .131 from compositional analyses; and 9.02, 62.84, 0.144 from calculation. These values are lower than those of parisite. On the basis of stoichiometr ically collinear relationship this suggests that the activities of Ca2+ and CO32- in the parent fluid were too low to allow the formation of typical p arisite. The limited compositional range of the polysomes can be used to pr edict new polysomes. Because there are only very limited regions for each p olysome, it is considered that these polysomes have the compositional chara cteristics of short-range ordered structures.