Mellite

Not to be confused with Melilite.

Mellite
General
Category	Organic minerals
Formula (repeating unit)	Al2[C6(COO)6]·16H2O
IMA symbol	Mel[1]
Strunz classification	10.AC.05
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	I41/acd
Unit cell	a = 15.53 Å, c = 23.19 Å; Z = 8
Identification
Color	Honey-yellow, deep red, pale shades of red, brown, gray, white;
Crystal habit	Elongated bipyramidal prismatic; as nodules and coatings, fine-grained massive
Cleavage	poor/indistinct on {023}
Fracture	conchoidal
Tenacity	Slightly sectile
Mohs scale hardness	2–2+1⁄2
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	[2][3][4]

Mellite, also called honeystone, is an unusual mineral being also an organic chemical. It is chemically identified as an aluminium salt of mellitic acid, and specifically as aluminium benzenehexacarboxylate hexadecahydrate, with the chemical formula Al₂C₆(COO)₆·16H₂O.[4]

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.[2][4]

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",[5] in allusion to its color.[3]

It is found associated with lignite and is assumed to be formed from plant material with aluminium derived from clay.[2]

Cut and polished mellite gemstone

Structure

The crystal structure of mellite has been determined by neutron diffraction and consists of slightly distorted Al(H₂O)₆^3+ octahedra linked by hydrogen bonds to [C₆(COO)₆]^6− mellitate anions and water of crystallization.[6]

• 
  Ball-and-stick model of the asymmetric unit of mellite
• 
  Space-filling model of the asymmetric unit
• 
  Packing of 3×3×3 unit cells

See also

• Mellitic anhydride

References

1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
2. http://rruff.geo.arizona.edu/doclib/hom/mellite.pdf Handbook of Mineralogy
3. "Mellite".
4. http://webmineral.com/data/Mellite.shtml Webmineral data
5. μέλι. Liddell, Henry George; Scott, Robert; A Greek–English Lexicon at the Perseus Project.
6. Robl, Christian; Kuhs, Werner F. (1991). "A neutron diffraction study on hydrogen bonding in the mineral mellite (Al₂[C₆(COO)₆] · 16H₂O) at 15 K". J. Solid State Chem. 92 (1): 101–109. Bibcode:1991JSSCh..92..101R. doi:10.1016/0022-4596(91)90246-E.