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:

      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:

     Sum 99.53 wt.%.  Corresponding to:


(Na_0.56K_0.26Ca_0.03)_S0.85 (Th_0.94U_0.04)_S0.98(Si_10.69A_l0.31)_S11.00


O_24.12F_0.23Cl_0.02 · 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.