Differential REE uptake by sector growth of monazite

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 elect ron imaging and microprobe analysis have been used to establish the relatio nship 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 t o that of {(1) over bar 01) and {100) sectors; Ce shows no partitioning dif ferences; and uptake of Nd is more easily facilitated on {(1) over bar 01) and {100) surfaces relative to {011). There appears to be a distinct relati onship between the size of the REE ion and the probability of uptake via th e different growth surfaces. Interpretation of this uptake behaviour, based on theories involving 'protosites', involves an investigation of the possi ble kink site geometries at edge-steps during growth. Part-formed kink site s with small entrance sizes are calculated to occur with higher frequency o n {(1) over bar 01} relative to {011}, and this con;elates with an increase in the smaller-sized REE (Nd) uptake by {(1) over bar 01} growth surfaces. The overall morphology and sector growth is suggested to be a function of uptake chemistry.