Rhodochrosite

Trigonal  Images    

   
   
  Formula MnCO3
  Optic class & sign Uniaxial negative
  Relief Moderate to high
  Refractive indices no = 1.816
ne = 1.597
n and Δn data for endmember composition; n increases with increasing Fe and Zn, decreases with increasing Ca or Mg.
  Birefringence   (max.)0.219
   
  Sign of elongation Hardly applicable due to very high Δn
  Interference figure Well-defined isogyre cross, isochromes over multiple colour orders
  Colour /   pleochroism Colourless to pale pink, weakly pleochroic (O>E) if coloured.
  Zoning Colour zoning may be present

  Form  Habit Granular, botroidal
  Surface Anhedral, rarely euhedral
  Cleavage 3 sets, rhombohedral {1011}
  Twinning Lamellar and contact on {1021}, rare
  Extinction Symmetrical to cleavage traces in sections subparallel to c. Mottled extinction due to abundant miniscule breakouts at cleavage intersections.

  Reaction textures  
  Alteration /   decomposition Mn-oxides and -hydroxides

  Occurence     Ign Carbonatites, rare in pegmatite
  Met Mn-rich metamorphic rocks
  Sed Authigenic and secondary in Mn-rich sediments
  Hyd Low- to moderate-temperature hydrothermal veins
  Other 

  Distinctive   properties               High-order white interference colours, rhombohedral cleavage, and relief change between nO and nE (as for all other rhombohedral carbonates). Rhodochrosite is pink-pleochroic (if coloured); alteration; mineral association.
Rhodochrosite, smithsonite and siderite are distinguished from the other rhombohedral carbonates by their distinctly higher n (both nO and nE; no negative relief in any position).
  Additional   comments n-Δn chart: Cal - calcite, Dol - dolomite, Kut - kutnahorite, Mgs - magnesite, Rds - rhodochrosite, Sid - siderite, Smt - smithsonite