{{Short description|Organic mineral}} {{distinguish|Melilite}} {{Infobox mineral | name = Mellite | boxwidth = | boxbgcolor =#7c5745 | boxtextcolor = #fff | image = Mellite-177555.jpg | imagesize = 260px | alt = | caption = | category = Organic minerals | formula = Al<sub>2</sub>[C<sub>6</sub>(COO)<sub>6</sub>]·16H<sub>2</sub>O | IMAsymbol = Mel<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 10.AC.05 | dana = | system = Tetragonal | class = Ditetragonal dipyramidal (4/mmm) <br/>H-M symbol: (4/m 2/m 2/m) | symmetry = ''I4''<sub>1</sub>/acd | unit cell = a = 15.53&nbsp;Å, c = 23.19&nbsp;Å; Z&nbsp;=&nbsp;8 | color = Honey-yellow, deep red, pale shades of red, brown, gray, white; | colour = | habit = Elongated bipyramidal prismatic; as nodules and coatings, fine-grained massive | twinning = | cleavage = poor/indistinct on {023} | fracture = conchoidal | tenacity = Slightly sectile | mohs = 2–{{frac|2|1|2}} | luster = Vitreous, resinous, greasy | streak = White | diaphaneity = Transparent to translucent | gravity = 1.64 | density = | polish = | opticalprop = Uniaxial (−) may be anomalously biaxial | refractive = n<sub>ω</sub> = 1.539 n<sub>ε</sub> = 1.511 | birefringence = δ = 0.028 | pleochroism = Weak; O = yellowish brown; E = yellow | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = Pale yellow to blue (LW & SW UV) | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = Pyroelectric | prop1 = | prop1text = | references = <ref name=Handbook/><ref name=Mindat/><ref name=Webmineral/> }}

'''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 Al<sub>2</sub>C<sub>6</sub>(COO)<sub>6</sub>·16H<sub>2</sub>O.<ref name=Webmineral>http://webmineral.com/data/Mellite.shtml Webmineral data</ref>

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.<ref name=Handbook>http://rruff.geo.arizona.edu/doclib/hom/mellite.pdf Handbook of Mineralogy</ref><ref name=Webmineral/>

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 {{lang|grc|μέλι}} ''meli'' "honey",<ref>{{LSJ|me/li^|μέλι|ref}}.</ref> in allusion to its color.<ref name=Mindat>{{Cite web|url=http://www.mindat.org/min-2638.html|title = Mellite}}</ref>

It is found associated with lignite and is assumed to be formed from plant material with aluminium derived from clay.<ref name=Handbook/>

thumb|center|Cut and polished mellite gemstone

==Structure== The crystal structure of mellite has been determined by neutron diffraction and consists of slightly distorted Al(H<sub>2</sub>O)<sub>6</sub><sup>3+</sup> octahedra linked by hydrogen bonds to [C<sub>6</sub>(COO)<sub>6</sub>]<sup>6−</sup> mellitate anions and water of crystallization.<ref>{{ cite journal | first1 = Christian | last1 = Robl | first2 = Werner F. | last2 = Kuhs | title = A neutron diffraction study on hydrogen bonding in the mineral mellite (Al<sub>2</sub>[C<sub>6</sub>(COO)<sub>6</sub>] · 16H<sub>2</sub>O) at 15 K | journal = J. Solid State Chem. | year = 1991 | volume = 92 | issue = 1 | pages = 101–109 | doi = 10.1016/0022-4596(91)90246-E | bibcode = 1991JSSCh..92..101R }}</ref>

<gallery> File:Mellite-asymmetric-unit-from-xtal-3D-bs-17.png|Ball-and-stick model of the asymmetric unit of mellite File:Mellite-asymmetric-unit-from-xtal-3D-sf.png|Space-filling model of the asymmetric unit File:Mellite-3x3x3-unit-cells-from-xtal-3D-sf.png|Packing of 3×3×3 unit cells </gallery>

==See also== * Mellitic anhydride

==References== {{Commons category| Mellite}} {{Reflist}}

Category:Aluminium minerals Category:Organic minerals Category:Tetragonal minerals Category:Minerals in space group 142 Category:Luminescent minerals Category:Minerals described in 1789

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