Gonnardite

Tetragonal  Images    

   
   
  Formula (Na,Ca0.5)8-10Al8+xSi12-xO40 ∙ 12H2O;  x = 0-2
  Optic class & sign Uniaxial positive or negative
  Relief Moderate- to low-negative
  Refractive indices no = 1.480 -1.506
ne = 1.493 -1.508
-
  Birefringence   (max.)0.002  - 0.013
   -
  Sign of elongation Length-slow, l (+); if Ca-rich also l (-)
  Interference figure Commonly poor figures due to fibrous habit and low Δn
  Colour /   pleochroism Colourless
  Zoning 

  Form  Habit Acicular, radiating aggregates
  Surface 
  Cleavage {110}, 2 sets
  Twinning 
  Extinction Straight to traces of prism faces and cleavage in prismatic sections; symmetrical to traces of {110} faces and cleavage in sections ⊥c

  Reaction textures  
  Alteration /   decomposition Thomsonite; gonnardite is an alteration product of nepheline

  Occurence     Ign 
  Met 
  Sed 
  Hyd Cavities in basalt and leucite tephrite; altered skarn
  Other 

  Distinctive   properties               
  Additional   comments Also described as orthorhombic, with 2V = 50°.
n-Δn chart: Edi – edingtonite, Gon – gonnardite, Lmt – laumontite, Mes – mesolite, Mor – mordenite, Ntr – natrolite, Sco – scolecite, Thm - thomsonite