CanMin 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, DenmarkJoel 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:

(Na_14.37REE_1.53K_0.20Sr_0.03)_S16.13(Ca_1.77REE_0.59Na_0.66)_S3.02

(Mn_2.76Y_0.24)_S3.00 (Fe_1.43Mn_0.96Zr_0.25)_S2.64 (Zr_2.93Nb_0.05Hf_0.03)_S3.01

(Nb_0.85Ta_0.02)_S0.87 (Si_24.77Al_0.12)_S24.89 O₇₃(O,OH,H₂O)_3.09

(OH_1.27Cl_0.73)_S2.00. D_meas. = 3.20(3) g/cm³ and D_calc. = 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.