Oneillite: A new Ca-deficient and REE-rich member of the eudialyte group from Mont Saint-Hilaire, Quebec, Canada

Oneillite, ideally Na15Ca3 Mn3Fe3 Zr3Nb (Si25O73) (O,OH,H2O)(3)(OH,Cl)(2), is a new member of the eudialyte group from Mont Saint-Hilaire, Quebec. It occurs as yellowish brown anhedral grains up to 2 mm in diameter. Associate d minerals include albite, sodalite, pyrite and aegirine. It is transparent to translucent, with a vitreous luster and white streak. It is brittle, wi th a hardness of 5-6 (Mohs scale). It has no cleavage, no parting, and an u neven fracture. It is uniaxial negative with omega 1.6450(3) and epsilon 1. 6406(3). It is trigonal, space group R3, a 14.192(1) and c 29.983(3) Angstr om, V 5230(1) Angstrom(3), Z = 3. The strongest X-ray powder-diffraction li nes [d in Angstrom(I)(hkl)] are: 11.348(44.2)(101), 7.100(33)(110), 6.021(3 6.1)(021), 5.683(30.8)(202), 4.291(36.5)(205), 3.389(42.9)((1) over bar 41) , 3.199(30.8)(208), 3.150(34.9)(237), 2.964(100)((3) over bar 45) and 2.844 (89.1)(404). The infrared spectrum is given. An electron-microprobe analysi s of the grain of oneillite chosen for refinement of the structure gave Na2 O 13.60, K2O 0.28, CaO 2.90, MnO 7.70, FeO 3.00, SrO 0.09, Al2O3 0.18, Y2O3 0.78, La2O3 2.88, Ce2O3 5.14, Pr2O3 0.48, Nd2O3 1.45, Gd2O3 0.20, SiO2 43. 46, ZrO2 11.44, HfO2 0.16, Nb2O5 3.48, Ta2O5 0.14, Cl 0.76, H2O 0.63, O = C l 0.17, total 98.58 wt.%. The proportion of H2O was calculated from stoichi ometry on the basis of the crystal-structure analysis. The empirical formul a of oneillite, based on 78.09 anions as determined in the crystal-structur e analysis is: (Na14.37REE1.53K0.20Sr0.03)(Sigma 16.13) (Ca1.77REE0.59Na0.6 6)(Sigma 3.02) (Mn2.76Y0.24)(Sigma 3.00) (Fe1.43Mn0.96Zr0.25)(Sigma 2.64) ( Zr2.93Nb0.05Hf0.03)(Sigma 3.01) (Nb0.85Ta0.02)(Sigma 0.87) (Si24.77Al0.12)( Sigma 24.89) O-73(O,OH,H2O)(3.09) (OH1.27Cl0.73)(Sigma 2.00) D-meas. = 3.20 (3), D-calc. = 3.22(3) g/cm(3). Compared to the eudialyte structure, the un ique feature in oneillite is the ordering of Mn and Ca + REE at the MI site , which lowers the symmetry from R3m to R3. Determination of the crystal st ructure was dependent on the recognition of a merohedral twin. The REE cont ent is the highest hitherto reported, with REE occupying almost 50% of Na(4 ).