Hydrothermal REE-rich eudialyte from the Pilanesberg Complex, South Africa

The Pilanesberg Complex, in South Africa, one of the world's largest alkali ne complexes, contains large resources of Zr and the rare-earth elements (R EE). Eudialyte is the main carrier of Zr, and contains appreciable concentr ations of REE. It is particularly abundant in the green nepheline syenite ( 20% by volume). It forms complexly zoned poikilitic domains that enclose ae girine, albite, microcline, nepheline, zircon, pectolite, sodalite, and an unidentified Na-Zr silicate. These domains mantle corroded silicates and re place outer zones of microcline euhedra. The eudialyte is partially replace d by fergusonite-(Y) and britholite. Compositionally, the eudialyte at Pila nesberg is unusual, it contains the highest concentration of Nb (up to 3.8 wt% Nb2O5) and the lowest concentrations of Na (<11.4 wt% Na2O) and Fe (< 0 .4 wt% FeO) reported in the literature. In addition, it has one of thr high est contents of REE (up to 7.6 wt% REE2O3, mainly Ce and La and traces of S m and Nd) and Mn (up to 7.5 wt%) reported in the literature. On the basis o f 78 anions, its formula is (Na12.30REE1.39Ca0.89 K-0.20)(S14.78) (Ca5.42Mn 0.58)(S6) (Mn2.96Fe0.04)(S3) (Zr2.66Nb0.28Hf0.06)(S3) (Nb0.66Si0.28 Ta-0.06 )(S1) Si-25.64 O-74 (OH1.01Cl0.92F0.07)(S2).2H(2)O. On the basis of textura l relationships, the Pilanesberg eudialyte is interpreted to be hydrotherma l. It seems to have formed from an orthomagmatic Na-Nb-REE-Cl-F-bearing hyd rothermal fluid that exsolved from an agpaitic syenitic magma. In this syst em, zirconium was probably remobilized from magmatic zircon as a F-complex (e.g., ZrF62-), and REE were introduced as Cl- and F-complexes. The incorpo ration of the REE in eudialyte probably occurred in response to the reduced Cl- and F-activities that accompanied precipitation of sodalite and the un identified (F-bearing) Na-Zr silicate.