Abstract

Nacareniobsite-(Ce), Na3Ca3(REE)Nb(Si2O7)2OF3, and britholite-(Ce), (Na,Ca,REE)10(Si,P)6O24(OH,F)2, formed during the late stages of crystallization of the most evolved peralkaline granites from the Morro Redondo Complex, Graciosa Province (Southern Brazil). Nacareniobsite-(Ce) compositions are very close to ideal, with Na2O and Nb2O5 contents up to 11.1 and 15.5 wt.%, respectively, corresponding to an average formula of Na3.0(Ca2.75Sr0.01)∑2.76 (REE,Y)1.16 (Nb0.95Ta0.02Ti0.04)∑1.01 Si3.99O14(O1.11F2.89)∑4. Compared to the host rock composition, trace-element patterns for nacareniobsite-(Ce) indicate enrichment in REEs and Sr, and depletion in alkali metals and Zr. The available chemical data are in excellent agreement with a complete solid solution series between rinkite and nacareniobsite-(Ce), controlled by [M+4M2+]−1[M5+Na+] and, in part, [M2+]−2[REE3+Na+] and [M+4M2+]−1[REE3+]2 exchange vectors. Britholite-(Ce) compositions are amongst the most (REE,Y)-enriched reported to date, with REE + Y contents up to 73.6 wt.% oxide. REE and trace element distributions relative to the host rock exhibit some similarities with those observed for nacareniobsite-(Ce); however, Ba, Th, U, and Pb contents are relatively higher. The observed compositions agree very well with [PM2+F]−1[Si(REE,Y)(OH)], [M2+]−2[Na(REE,Y)], and [M2+(Th,U)]−1[(REE,Y)]2 exchange vectors. Crystallization of both minerals was favored by the F-, Na-, and HFSE-enriched nature and moderate-to-strong peralkaline signature of the melt, at temperatures below 600 °C in a relatively reduced environment.

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