Amblygonite - Montebrasite

Triclinic  Images    

   
   
  Formula LiAl(PO)4(F,OH)
  Optic class & sign Biaxial positive or negative
  Optical orientation X near b, Y near a, Z near c
  Optical plane Approx parallel to b
  Relief Moderate- positive
  Refractive indices nx = 1.577 -1.615
ny = 1.592 -1.624
nz = 1.596 -1.645
n increases with replacement of F by OH
  Birefringence   (max.)0.019  - 0.031
   Δ:n increases with replacement of F by OH
  Optic Angle 2Vx = 66 - 90°
2Vz = 90 - 40°
  Sign of elongation Length-slow, l (+)
  Interference figure 
  Colour /   pleochroism Colourless
  Zoning 

  Form  Habit Equant, short- to long-prismatic
  Surface 
  Cleavage {100}, {110}
  Twinning Multiple twins on {111} common
  Extinction Inclined

  Reaction textures  
  Alteration /   decomposition 

  Occurence     Ign Granite pegmatite
  Met 
  Sed 
  Hyd Veins, greisen
  Other 

  Distinctive   properties               Relief, Δn, twinning, low symmetry, restricted occurrence
  Additional   comments