Serpentine: Antigorite, Lizardite, Chrysotile

Monoclinic  Images    
   
   
  Formula Mg3Si2O5(OH)4
  Optic class & sign Biaxial negative
  Optical orientation X = c, Y = a or b, Z = b or a
  Optical plane (010)
  Relief Low-negative to moderate-positive
  Refractive indices nx = 1.529 -1.595
ny = 1.530 -1.603
nz = 1.537 -1.604
  Birefringence   (max.)0.004  - 0.014
   
  Optic Angle 2Vx = 20 - 80°
2Vz
  Sign of elongation Length-slow, l (+) and length-fast, l (-)
  Interference figure Commonly difficult to obtain due to fine grain size
  Colour /   pleochroism Colourless to pale green with weak pleochroism. Brown staining from oxidized Fe may occur.
  Zoning 

  Form  Habit Flaky (antigorite), fibrous or asbestiform (rolled-up sheets: chrysotile), very fine-grained, parallel-stacked scaly aggregates (“pseudo-fibres’: lizardite)
  Surface Subhedral, anhedral
  Cleavage {001} perfect; only visible in antigorite
  Twinning Commonly not identifiable
  Extinction Straight to {001}

  Reaction textures  Hydration of mafic minerals: Mesh texture and banded serpentine (pseudomorphs after olivine mimicking fracture patterns), ‘bastite’ (pseudomorphs after pyroxene).
  Alteration /   decomposition Serpentine is a typical alteration product itself

  Occurence     Ign 
  Met Metamorphosed, hydrated ultramafic rocks. Chrysotile and lizardite are stable from sub-metamorphic temperatures to amphibolite-facies conditions
  Sed 
  Hyd  Hydrous alteration of ultramafic and mafic rocks
  Other 

  Distinctive   properties               Habit, very low Δn, occurrence; characteristic replacement textures from olivine and pyroxene breakdown.
  Additional   comments Distinction between the different serpentine varieties may be difficult using optical means only. Interlocking crystals of distinct size are typical for antigorite, while a fibrous habit indicates chrysotile. Lizardite tends to be rather fine-grained, forming the pseudomorphic textures.