Mellite - The only 'Organic Mineral' in the World

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 MELLITE


Mellite Crystal Form - Pen Sketch 


Introduction

    Mellite is a rare secondary mineral in Lignite deposits, a very peculiar organic mineral. It is chemically an Aluminium salt of Mellitic Acid (Aluminium benzene hexacarboxylate hydrate). It is named after its honey-coloured appearance, which crystallises in the tetragonal system as individual crystals or massive forms. 

Etymology

    It is named after the Greek word for 'Honey' (μέλι, meli)

Crystallography 

  • Crystal Class - Tetragonal Class
  • Crystal System - Ditetragonal dipyramidial class (H/M Symbol - 4/m 2/m 2/m)
  • Crystal Form - Dipyramidial crystals, or massive in some varieties. 
  • Axial Ratios - a:c = 1:1.05887
  • Unit cell parameters - a = 21.91, c = 23.2 ; Z = 16; V = 11

Physical Properties

  • Colour - Brown, Yellowish Brown, Golden, White, Brownish White
  • Lustre - Vitreous to Greasy
  • Streak - White
  • Hardness - 2 to 2.5     
  • Specific Gravity - 1.64
  • Diaphenity - Transparent to Translucent
  • Tenacity - Sectile
  • Cleavage - Poor/Indistinct,   Indistinct on {023}
  • Fracture - Conchoidal
  • Habit - Crystalline, Massive (coarse-grained)

Mellite Crystal
(Source - Wikipedia)


Optical Properties
  • Optical Classification - Uniaxial (-)
  • Refractive Indices - nω = 1.539; nε = 1.511
  • Birefringence - 0.028  
  • Dichroism - none
  • Pleochroism - Weak (O - Yellowish brown; E - Yellow)
  • Surface Relief - Low     
  • Fluorescence - Pale yellow to Blue (under LW & SW UV)    
  • Other Properties - Pyroelectric                  

Chemical Properties

  • Chemical Formula - Al2[C2(COO)6].16H2O 
  • Molecular Weight - 534.47 gm
  • Solubility - Insoluble in water, but soluble in HNO3.
Geological Setting & Occurrence 
     
     Type Locality - Artern, Kyffhäuser District, Thuringia, Germany

     Mellite is associated with lignite deposits. It is believed that it is formed from plant remains in Al-rich clays. It was first discovered in Germany in 1789 and is then also found in Hungary, Australia, the Czech Republic, France, Italy and Russia. 

Images of Mellite Crystals

Mellite Crystal from Hungary, 44 mm twin crystals
(Source - Mindat Database)
(Copyrights - Tinor Horvath)


A cluster of Mellite Crystals, each measuring up to 12mm, from Hungary
(Source - Mindat Database)
(Copyright - Kirstalle and Crys)




Bright Blue Shortwave Ultraviolet Fluorescence of Mellite Crystals from Hungary
(Source - Mindat Database)
(Copyrights - Christopher O'Neil)


For more Images, visit the Mindat Gallery of Mellite at Mindat Mellite Gallery

References 
  • Dana's book of Mineralogy
  • Wikipedia
  • webmineral.com
  • Mindat Database

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