Crystal Chemistry of Eudialyte Group Minerals from Rouma Island, Los Archipelago, Guinea

We herein present a comprehensive investigation of the eudialyte group minerals from the nepheline syenites of Rouma Island in the Los Archipelago, Conakry region, Guinea. Two distinct mineral phases were identified: an oneillite-like phase, associated with the agpaitic rock suite, and, for the first time in this locality, kentbrooksite, occurring in pegmatites. The oneillite-like phase crystallizes in the trigonal system (space group R3), with unit cell parameters a = 14.1489(2) Å, c = 30.1283(5) Å and an idealized crystal chemical formula of Na15(Mn,REE)3(Ca,Mn)3(Fe,Mn)3Zr3(Zr,Si,Al,Nb,Ti)1 (Si25O73)(O,OH,H2O)3(OH,Cl,F)2. Kentbrooksite also exhibits trigonal symmetry (space group R3m), with unit cell parameters a = 14.2037(3) Å c = 30.1507(9) Å and an idealized formula of (Na,REE)15(Ca,Mn)6(Mn,Fe)3Zr3(Nb,Si)1(Si25O73)(O,OH,H2O)3(F,Cl,OH)2. Compared to the oneillite-like phase, kentbrooksite is markedly enriched in Mn and rare earth elements (REE). This geochemical distinction aligns with the progressive mineralogical evolution of the system, transitioning from the miaskitic to agpaitic suite (oneillite-like phase) and subsequently to pegmatites (kentbrooksite). These findings are consistent with the broader-scale observations regarding the syenite ring structure of the Los Archipelago.