Stilbite (desmine)

Monoclinic  Images    
   
   
  Formula (Ca0.5,Na,K)9Al9Si27O72 ∙ 30H2O
  Optic class & sign Biaxial negative
  Optical orientation X near a, Y = b, Z near c
  Optical plane (010)
  Relief Moderate- to low-negative
  Refractive indices nx = 1.482 -1.500
ny = 1.491 -1.507
nz = 1.493 -1.513
n increases with substitution of Ca for Na
  Birefringence   (max.)0.009  - 0.014
   
  Optic Angle 2Vx = 30 - 50°
2Vz
  Sign of elongation Length-slow, l (+), and length-fast, l (-), in elongate sections approximately orthogonal to (010) platelets
  Interference figure 
  Colour /   pleochroism Colourless
  Zoning -

  Form  Habit Thin tabular; radiating, sheaf-like aggregates
  Surface 
  Cleavage {010}
  Twinning {001}, {100}, contact and penetration twins, generally present; apart from further sector-growth-zoning, this typically creates a “fourling” twin structure.
  Extinction 

  Reaction textures  
  Alteration /   decomposition Montmorillonite, other zeolites

  Occurence     Ign 
  Met 
  Sed Hot-springs deposits, cementing material in sandstones and conglomerates
  Hyd Cavities and veins in basalt, andesites, and in granite and various metamorphic rocks
  Other 

  Distinctive   properties               Sheaf-like aggregates of tabular crystals; very low n, mid to upper first-order interference colours
  Additional   comments Twinned crystals with growth sectors of different crystallographic orientation are common.
n-Δn charts A, B: Bre - brewsterite, Cpt - clinoptilolite, Epi - epistilbite, Hul - heulandite,
Lev - levyne, Stb - stilbite, Yug - yugawaralite