• journal_title：The Canadian Mineralogist
• Contributor：Frederico C. J. Vilalva ; Silvio R.F. Vlach ; Antonio Simonetti
• Publisher：Mineralogical Association of Canada
• Date：2013-04-01
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.51.2.313
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：51
• issue：2
• firstpage：313
• section：Articles

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 Na_3.0(Ca_2.75Sr_0.01)_∑2.76 (REE,Y)_1.16 (Nb_0.95Ta_0.02Ti_0.04)_∑1.01 Si_3.99O₁₄(O_1.11F_2.89)_∑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.