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Differential REE uptake by sector growth of monazite

Abstract Monazite-(Ce) from a dolomite carbonatite at Kangankunde, Malawi, is sector-zoned with variation in La2O3 of up to 6.0 wt.% and in Nd2O3 of up to 3.9 wt.% between sectors. Single crystal X-ray diffraction, backscattered electron imaging and microprobe analysis have been used to establish the relationship between the morphology and sector chemistry of this low-Th monazite, (Ce,La,Nd)PO4. Uptake of La by {011} sector surfaces is enhanced relative to that of {1̅01} and {100} sectors; Ce shows no partitioning differences; and uptake of Nd is more easily facilitated on {1̅01} and {100} surfaces relative to {011}. There appears to be a distinct relationship between the size of the REE ion and the probability of uptake via the different growth surfaces. Interpretation of this uptake behaviour, based on theories involving ‘protosites’, involves an investigation of the possible kink site geometries at edge-steps during growth. Part-formed kink sites with small entrance sizes are calculated to occur with higher frequency on {1̅01} relative to {011}, and this correlates with an increase in the smaller- sized REE (Nd) uptake by {1̅01} growth surfaces. The overall morphology and sector growth is suggested to be a function of uptake chemistry.