Spinel

Cubic  Images    
   
   
  Formula (Mg,Fe,Mn,Zn)(Al,Fe3+,Cr)2O4
  Optic class & sign Isotropic 
  Relief High to very high
  Refractive Index    1.719 -2.05
Mg-Fe-Al-dominated spinels 1.719 - 2.05 (spinel-pleonaste-hercynite series, picotite); spinel endmember 1.719, hercynite endmember 1.835; gahnite 1.805, galaxite 1.920; substitution of Fe3+ and Cr for Al increases n
  Birefringence Zero; weak anomalous birefringence observed in gahnite
  Colour Highly variable in colour tone and intensity: colourless (Mg-spinel) or weakly to strongly coloured in green, brown, blue, red, yellow; hercynite is dark green; Cr-rich spinels are brown; Fe-, Mn- or Cr-rich spinels may be near-opaque in standard-thickness thin sections
  Zoning 

  Form  Habit Equant; octahedra if euhedral
  Surface Euhedral to anhedral
  Cleavage None; parting on {111} may occur
  Twinning Common on {111}, simple or multiple ("spinel law"); can only be recognised in thin section from characteristic crystal outlines

  Reaction textures  
  Alteration /   decomposition Relatively stable

  Occurence     Ign Mg-Fe-Cr spinel in peridotite, alkremite, mafic rocks and magnetite seams of mafic intrusions,; gahnite in granitic pegmatite and VMS deposits
  Met Mg-rich spinel in impure marble, silica-undersaturated calcsilicate and metapelitic rocks; hercynitic spinel in medium- to high-grade aluminous metapelites (hercynite + quartz only in granulite facies), also in metabasites; gahnite in skarns, metamorphosed massive sulphide ores, Zn-bearing metapelites and metabasites; galaxite in Mn-rich metamorphic rocks
  Sed Detrital in sands (heavy mineral fraction)
  Hyd Gahnite in metasomatic veins; galaxite in Mn-rich vein deposits
  Other 

  Distinctive   properties               High n, isotropic, typically intense colour (except Mg-spinel), crystal habit
  Additional   comments Note: The magnetite and chromite series of the spinel group are not considered in the descriptions above