Jadeite

Monoclinic  Images    
   
   
  Formula NaAlSi2O6
  Optic class & sign Biaxial positive or negative
  Optical orientation a near X, b = Y, c at a moderate to high angle to Z
  Optical plane (010)
  Relief Moderate to high
  Refractive indices nx = 1.640 -1.681
ny = 1.645 -1.684
nz = 1.652 -1.692
n increases with increasing Fe3+, Ca, Mg; endmember jadeite has minimum n
  Birefringence   (max.)0.006  - 0.021
   Endmember jadeite has minimum Δn
  Optic Angle 2Vx = 84 - 90°
2Vz = 90 - 60°
  Sign of elongation Length-slow, l (+)
  Interference figure Large 2V isogyres, few isochromes; strong optic axis dispersion
  Colour /   pleochroism Colourless to pale green; pleochroic if coloured. Colour indicates presence of other pyroxene components (such as aegirine)
  Zoning 

  Form  Habit Granular, stubby, fibrous
  Surface Anhedral to euhedral
  Cleavage 2 sets {110} distinct, at 93 and 87° (seen in sections ⊥ c). In prismatic sections, the traces of the two principal cleavage sets are parallel
  Twinning Uncommon; simple or multiple on {100} and {001} is reported
  Extinction Inclined; cɅZ = 30 - 55° in (010) sections; c ∥ cleavage traces and prism faces. Symmetrical extinction to {110} cleavage and {110} faces in sections ⊥ c. Extinction angles may be difficult to determine due to strong dispersion.

  Reaction textures  Retrograde albite coronas on jadeite
  Alteration /   decomposition Analcime, albite

  Occurence     Ign 
  Met Jadeitite; Na-rich rocks at high and ultrahigh pressure conditions
  Sed 
  Hyd 
  Other 

  Distinctive   properties               Low to moderate Δn, relief, extinction angles in maximum birefringence sections, characteristic pyroxene cleavage observed in sections ⊥ c; paragenesis.
  Additional   comments Jadeite is commonly optically positive, but 2Vz may exceed 90°, which then makes it negative. Compositional zoning is common with either omphacitic cores, or omphacitic rims.