Home > Mineral Abstracts > CanMin Vol. 25, pp. 181-183 (1987)




Vol. 25, pp. 181-183 (1987)



Thornasite,
a new hydrous sodium thorium silicate
from Mont St-Hilaire, Quebec


Valerie E. Ansell and George Y. Chao
Department of Geology, Carleton University,
Ottawa , Ontario K1S 5B6


ABSTRACT

          Thornasite, from Mont Saint-Hilaire, Quebec, occurs as small, euhedral crystals in patches of white, powdery brockite associated with yofortierite and analcime.  The mineral is rhombohedral, space group R3m or R32, with cell parameters a 29.08(1), c 17.30(1) Å, Z = 18.  The strongest lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are:


14.54 (20)(110)
8.17 (30)(012)
7.27 (100)(220)
5.09 (20)(042)

4.17 (70)(422)
3.239 (30)(262)
2.959 (20)(802)
2.890 (25)(811,713)


      The grains are colorless to very pale green with a white streak and are translucent to transparent with a vitreous to waxy lustre.  Thornasite is brittle with a subconchoidal to uneven fracture.  Under long- and short-wave ultraviolet radiation, the mineral fluoresces a bright apple-green.  Density 2.62(2)(meas.), 2.627 g/cm3(calc.). Optically, the mineral is uniaxial (+) with w 1.510(10 and e 1.512(1).  The average of five electron- microprobe analyses gives:


SiO2 - 57.93
Al23 - 1.42
ThO2 - 22.45
UO2 - 0.88

Na2O - 1.55
K2O - 1.10
CaO - 0.17
F - 0.40

Cl - 0.05
H2O- - 8.62
H2O+ - 5.14
(F + Cl) = O - 0.018


     Sum 99.53 wt.%.  Corresponding to:


(Na0.56K0.26Ca0.03)S0.85 (Th0.94U0.04)S0.98(Si10.69Al0.31)S11.00


O24.12F0.23Cl0.02 · 8.5H2O or, simply, (Na,K)ThSi11(O,H2O,F,Cl)33.

     The compatibility index is 0.096.  Thornasite is slightly metamict.

Keywords:  thornasite, hydrous sodium thorium silicate, new mineral species, Mont St-Hilaire, Quebec.


© 1987  The Canadian Mineralogist