ABSTRACT

          Tetranatrolite is a new name for "tetragonal natrolite", a mineral first reported from Ilimaussaq, Greenland by Andersen et al. (1969).  It is a wide-spread constituent in the pegmatite dykes and miaroles in the nepheline syenite at Mont St-Hilaire, Québec, occurring most commonly as epitactic overgrowths on natrolite crystals and as small euhedral crystals or silky fibrous sprays on natrolite, analcime or microcline.  It occurs less commonly as irregular earthy patches on natrolite or analcime.  The mineral is white, translucent to opaque with vitreous to dull lustre.  The hardness could not be determined, and cleavage was not observed.  The mineral is readily soluble in 1:1 HCl, slowly in HNO₃ and is only slightly attacked by 1:1 H₂SO₄.  Optically the mineral is uniaxial positive, w 1.481 and e 1.496.   Single-crystal photographs show the mineral to be tetragonal, I42d or I4₁md, a 13.098(2), c 6.635(2) Å.  With D_meas 2.276 ,we find Z = 4, whence D_calc = 2.234.  Strongest lines in the powder X-ray diffraction pattern are:

    Electron-microprobe analysis gave:

     The analysis corresponds to (Na_1.75Co_0.10K_0.09)Fe_0.01Al_1.95Si_3.0210 · 2.06H₂O or, ideally Na₂Al₂Si₃O₁₀ · 2H₂O.

     A TGA curve shows a minor weight-loss between room temperature and 135° C and a major weight-loss between 135 and 435° C.  The infrared spectrum of the mineral stronly suggests (Si,Al) disorder.  Hydrothermally the mineral decomposes to nepheline and analcime at 445° C and 1.0 kbar, and analcime at 330° C and 0.5 kbar.  No change was observed at 265° C and 0.5 kbar.  The mineral is a dehydration product of a higher hydrate.

Keywords:  tetranatrolite, St-Hilaire, Québec, natrolite, pegmatite, epitactic overgrowth, Si, Al, disorder, nepheline syenite.

© 1980  The Canadian Mineralogist