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CategoryOrganic minerals
(repeating unit)
Strunz classification10.AC.05
Crystal systemTetragonal
Crystal classDitetragonal dipyramidal (4/mmm)
H-M symbol: (4/m 2/m 2/m)
Space groupI41/acd
Unit cella = 15.53 Å, c = 23.19 Å; Z = 8
ColorHoney-yellow, deep red, pale shades of red, brown, gray, white;
Crystal habitElongated bipyramidal prismatic; as nodules and coatings, fine-grained massive
Cleavagepoor/indistinct on {023}
TenacitySlightly sectile
Mohs scale hardness2-​2 12
LusterVitreous, resinous, greasy
DiaphaneityTransparent to translucent
Specific gravity1.64
Optical propertiesUniaxial (-) may be anomalously biaxial
Refractive indexnω = 1.539 nε = 1.511
Birefringenceδ = 0.028
PleochroismWeak; O = yellowish brown; E = yellow
Ultraviolet fluorescencePale yellow to blue (LW & SW UV)
Other characteristicsPyroelectric

Mellite, also called honeystone, is an unusual mineral being also an organic chemical. Chemically identified as an aluminium salt of mellitic acid; that is, aluminium benzene hexacarboxylate hydrate, with the chemical formula Al2C6(COO)6·16H2O.[3]

It is a translucent honey-coloured crystal which can be polished and faceted to form striking gemstones. It crystallizes in the tetragonal system and occurs both in good crystals and as formless masses. It is soft with a Mohs hardness of 2 to 2.5 and has a low specific gravity of 1.6.[1][3]

It was discovered originally in 1789 at Artern in Thuringia, Germany. It has subsequently also been found in Russia, Austria, the Czech Republic, and Hungary. It was named from the Greek μέλι meli "honey",[4] in allusion to its color.[2]

It is found associated with lignite and is assumed to be formed from plant material with aluminium derived from clay.[1]

See also[edit]