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Category Organic compounds
(repeating unit)
Strunz classification 10.AC.05
Crystal symmetry Tetragonal (4/m 2/m 2/m) ditetragonal dipyramidal
Unit cell a = 15.53 Å, c = 23.19 Å; Z = 8
Color Honey-yellow, deep red, pale shades of red, brown, gray, white;
Crystal habit Elongated bipyramidal prismatic; as nodules and coatings, fine-grained massive
Crystal system Tetragonal
Cleavage poor/indistinct on {023}
Fracture conchoidal
Tenacity Slightly sectile
Mohs scale hardness 2-2½
Luster Vitreous, resinous, greasy
Streak White
Diaphaneity Transparent to translucent
Specific gravity 1.64
Optical properties Uniaxial (-) may be anomalously biaxial
Refractive index nω = 1.539 nε = 1.511
Birefringence δ = 0.028
Pleochroism Weak; O = yellowish brown; E = yellow
Ultraviolet fluorescence Pale yellow to blue (LW & SW UV)
Other characteristics Pyroelectric
References [1][2][3]

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 in Germany it has subsequently also been found in Russia, Austria, the Czech Republic, and Hungary. It was named from the Greek μέλ˘ι, "melis" for honey, 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]