ABSTRACT

     The new mineral monteregianite occurs as needle-shaped crystals in radiating clusters and as groups of tabular crystals in miarolitic cavities, metamorphosed inclusions and rheomorphic breccias in the nepheline syenite at Mont St-Hilaire, Québec.  The mineral is orthorhombic, Bmab or B2ab, a 14.014 (4), b 23.910 (5), c 13.096 (2) Å.  A pronounced pseudocell with a and c halved has Pmmb, P2mb or Pm2b symmetry.  Strongest eight lines of the X-ray powder diffraction pattern are:

     The mineral is colorless, white, grey or rarely mauve or pale green with white streak and vitreous to silky lustre.  Crystals, varying in size from 0.05 to 3 mm, are bounded by {010}, {001} and {100}; some show additional {101}.  Cleavages:  {010} perfect, {001} very good and {100} fair.  Hardness 3½.  The mineral is readily etched by 1:1 HCl, HNO₃ and H₂SO₄.  Optically, the mineral is biaxial positive, a 1.510 (1), b 1.513 (1), g 1.517 (1), 2V meas. 87°, calc. 82°, dispersion inconspicuous.  Orientation:  X = c,Y = a, Z = b.  Chemical analysis gave:

     corresponding to (Na_4.66K_1.80) (Y_1.68Ca_0.18Mg_0.06Ba_0.04)


(Si_15.87Al_l0.16) O₃₈ · 10.02H₂O or (Na,K)₆ Y₂Si₁₆O₃₈ · 10 H₂O.


     Z = 4, D(meas.) 2.42, (calc.) 2.391 g/cm³.  TGA and DTG show three distinct stages of dehydration at 25-80, 80-200, and 200-400°º C.  Water in the mineral is zeolitic, and recovereable after heating to 650° C.  Infrared spectra confirm the presence of H₂O in the mineral.