Home > Mineral Abstracts > CanMin Vol. 37, pp. 1295-1301 (1999)




Vol. 37, pp. 1295-1301 (1999)



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


Ole Johnsen*
Geological Museum, University of Copenhagen,
Oster Voldgade 5-7, DK-1350 Copenhagen, Denmark



Joel D. Grice and Robert A. Gault
Research Division, Canadian Museum of Nature,
P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada


ABSTRACT

     Oneillite, ideally Na15Ca3Mn3Fe3Zr3Nb(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 2mm in diameter. Associated minerals include albite, sodalite, pyrite and aegirine. It is transparent to translucent, with a vitreous luster and white streak. It is brittle, with a hardness of 5-6 (Mohs scale). It has no cleavage, no parting, and an uneven fracture. It is uniaxial negative with w = 1.6450(3) and
e = 1.6406(3). It is trigonal, space group R3, a 14.192(1) and c 29.983(3) Å, V5230(1) Å3, Z = 3.

     The strongest X-ray powder-diffraction lines [d in Å (I)(hkl)] are:


11.348(44.2)(101)
7.100(33)(110)
6.021(36.1)(021)
5.683(30.8)(202)

4.291(36.5)(205)
3.389(42.9)(-141)
3.199(30.8)(208)
3.150(34.9)(-237)

 
2.964(100)(-345)
2.844(89.1)(404)


     The infrared spectrum is given. An electron-microprobe analysis of the grain of oneillite chosen for refinement of the structure gave:


Na2O - 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 = Cl - 0.17
total 98.58 wt.%


     The proportion of H2O was calculated from stoichiometry on the basis of the crystal-structure analysis. The empirical formula of oneillite, based on 78.09 anions as determined in the crystal-structure analysis is:


(Na14.37REE1.53K0.20Sr0.03)S16.13(Ca1.77REE0.59Na0.66)S3.02


(Mn2.76Y0.24)S3.00 (Fe1.43Mn0.96Zr0.25)S2.64 (Zr2.93Nb0.05Hf0.03)S3.01


(Nb0.85Ta0.02)S0.87 (Si24.77Al0.12)S24.89 O73(O,OH,H2O)3.09


(OH1.27Cl0.73)S2.00. Dmeas. = 3.20(3) g/cm3 and Dcalc. = 3.22(3) g/cm3.

     Compared to the eudialyte structure, the unique feature in oneillite is the ordering of Mn and Ca + REE at the M1 site, which lowers the symmetry from R3m to R3. Determination of the crystal structure was dependent on the recognition of a merohedral twin. The REE content is the highest hitherto reported, with REE occupying almost 50% of Na(4).

Keywords: oneillite, eudialyte, new mineral species, crystal structure, merohedral twin, Mont Saint-Hilaire, Quebec, Canada.

     * E-mail address: oj@savik.geomus.ku.dk



© 1999  The Canadian Mineralogist