Mellite

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Mellite
Mellite-20130.jpg
Mellite, found in: Csordakúti Mine, Hungary
General and classification
other names

Honey stone

chemical formula Al 2 C 6 (COO) 6 · 16H 2 O
Mineral class
(and possibly department) 		Organic compounds
System no. to Strunz
and to Dana 		10.AC.05 ( 8th edition : IX / A.02)
02.50.01.01
Crystallographic Data
Crystal system tetragonal
Crystal class ; symbol ditetragonal-dipyramidal; 4 / m  2 / m  2 / m
Space group I 4 1 / acd (No. 142)Template: room group / 142
Lattice parameters a  = 15.55  Å ; c  = 23.21 Å
Formula units Z  = 8
Physical Properties
Mohs hardness 2 to 2.5
Density (g / cm 3 ) measured: 1.64; calculated: 1.65
Cleavage indistinct
Break ; Tenacity shell-like
colour colorless, white, honey yellow, deep red, brown
Line color White
transparency transparent to translucent
shine Resin gloss, glass gloss
Crystal optics
Refractive indices n ω  = 1.539
n ε  = 1.511
Birefringence δ = 0.028
Optical character uniaxial negative
Pleochroism weak: ω = yellowish brown; ε = yellow

Mellite , also known as honey stone , is a rarely occurring mineral from the mineral class of " organic compounds ". It crystallizes in the tetragonal crystal system with the chemical composition Al 2 C 6 (COO) 6 · 16H 2 O, so it is chemically the aluminum salt of mellitic acid (also aluminum mellitate or aluminum mellitate ).

Mellit usually develops dipyramid crystals in the centimeter range with resin to glass glossy surfaces, but also grainy aggregates . Pure mellite crystals are colorless and transparent. In the case of multicrystalline formation or lattice construction defects, however, it can appear white due to multiple light refraction and take on a honey-yellow, deep red or brown color due to foreign admixtures, whereby the transparency decreases accordingly.

With a Mohs hardness of 2 to 2.5, mellite is one of the soft minerals that, like the reference mineral gypsum (Mohs hardness 2), can just be scratched with a fingernail.

Etymology and history

Mellit was first discovered in the "Auguste lignite opencast mine" near Artern an der Unstrut in Thuringia.

A first brief description of the mineral was provided by Christian August Siegfried Hoffmann in 1789 , who, using Abraham Gottlob Werner's mineral system and the synonym honey stone, wrote the following note: “A still rather unknown fossil whose fatherland is Thuringia. It is honey-yellow in color and is found crystallized in double four-sided pyramids.

A more accurate indication of the locality, as well as various properties of the mineral supplies Dietrich Ludwig Gustav Karsten , who is also the 1789 with the examination of the mineral collection Nathanael Gottfried Leske some mistakenly under the plaster samples einsortierte, is perfectly octahedral grown and honey-yellow crystals. After knowledge of Werner's new mineral system with the honey stone listed there and his own investigations, Karsten recognizes it as identical to it. In his notes he describes the mineral under the keyword honey stone according to Werner's mineral system (95th genus) also as bitumen melliadites , which is found between the bearings of the bituminous wood from Artern. Werner took on the detailed description by Karsten in his book "Oryktognosie or manual for lovers of mineralogy" in 1792.

The name Mellit, which is still valid today, goes back to the mineral systematics written in Latin by Johann Friedrich Gmelin in 1793 , who referred to the mineral as Mellites (from the originally Greek word μέλι [meli] for honey ) and refers to the descriptions of Werner and Karsten relates. Richard Kirwan changed this designation in 1796 in his work "Elements of Mineralogy" to Mellilite , which was finally shortened to Mellit ( Mellite ) in 1801 by René-Just Haüy .

classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the mellite belonged to the mineral class of "organic compounds" and there to the department of "salts of organic acids", where together with julienite and the other members it was abelsonite , calclacite , dashkovaite , Earlandite , formicaite , hoganite , cafehydrocyanite and paceite formed the independent "Mellit-Julienit-Gruppe".

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), assigns mellite to the class of "organic compounds" and there to the department of "salts of organic acids". However, this section is further subdivided according to the salt-forming acid, so that the mineral can be found according to its composition in the sub-section "Benzene salts", where it only forms the unnamed group 10.AC.05 together with Pigotite .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the mellite to the class of "organic minerals" and there in the department of the same name. Here he is to be found as the only member of the unnamed group 50.02.01 within the subdivision of " Salts of organic acids (mellitates, citrates, cyanates and acetates) ".

Crystal structure

Mellite crystallizes tetragonally in the space group I 4 1 / acd (space group no. 142) with the lattice parameters a  = 15.55  Å and c  = 23.21 Å and 8 formula units per unit cell . Template: room group / 142

properties

Physical Properties

Under UV light , some Mellites show a light yellow or blue fluorescence .

Chemical properties

Mellite is soluble in nitric acid and potassium hydroxide , but insoluble in water and ethanol . When heated to around 300 ° C, it gradually decomposes, with the hexahydrate of mellitic acid being formed as an intermediate.

Education and Locations

Honey yellow mellitic crystal from Luschitz, Bílina, Ústí region, Bohemia, Czech Republic
(size: 2.2 × 1.7 × 1.7 cm)

Mellite is a rare secondary mineral and is found ingrown, in small groups or druses in lignite , more rarely in hard coal and sandstone .

So far, the mineral has only been found in a few locations (number of registered locations at mindat.org around 10). In addition to its type locality Artern an der Unstrut in Thuringia , it also appeared in Germany in the former open- cast lignite mine near Goitzsche in Saxony-Anhalt .

The “Csordakúti Mine” near Bicske and the coal mines near Tatabánya in Hungary , where crystals up to 4 cm in size emerged, are worth mentioning due to the extraordinary mellite finds . From Valchov (Moravia) in the Czech Republic , on the other hand, more granular aggregates come from, but beautiful crystal finds are also known from Czech Bohemia .

In Austria, Mellit has so far only been found at Lanz - Stelzling in the market town of Kötschach-Mauthen in Carinthia . Other locations are near Paris in France and near Bogorodizk ( Bogorodetsk ) in the Russian Oblast Tula ( Tulskaja ).

Manufacture of mellitic acid

The mellitic acid (benzene hexacarboxylic acid) C 12 H 6 O 12 can be deposited from the honey stone and can also be prepared in an alkaline solution by treating coal with super-manganese potassium ; it forms colorless needles, is easily soluble in water and alcohol, tastes good and has a strong acidic reaction, and when heated with excess quicklime it breaks down into carbonic acid and benzene.

Use as a gem stone

Emerald- cut mellite

Occasionally it is cut into gemstones by experienced hobby cutters for collectors and enthusiasts .

See also

literature

Web links

Commons : Mellit  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d e Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  721 .
  2. ↑ a b c Mellite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 64  kB ; accessed on August 1, 2017]).
  3. ↑ a b c Mellite at mindat.org (English)
  4. ^ CAS Hoffmann: "Bergmännisches Journal", Volume 2, Volume 1, Verlag der Grazische Buchhandlung, April 1789, pp. 369–398 ( PDF 539 kB )
  5. ↑ DLG Karsten: The mineral cabinet left behind by Mr Nathanael Gottfried Leske, systematically arranged and described , Verlag der IG Müllerschen Buchhandlung, Leipzig 1789, Volume 1, pp. 334–335 ( PDF 284.9 kB )
  6. ↑ a b Thomas Witzke : Discovery of Mellit, honey stone
  7. ^ JF Gmelin: Caroli a Linné Systema naturae per regna tria naturae, secundum classes, ordines, genera, species, cum characteribus, differentiis . 13th edition. tape 3 , 1793, pp. 282 ( strahlen.org [PDF; 120 kB ; Retrieved May 1, 2017] Lipsiae (Vol. 1-3, 1788-1793)).
  8. ↑ Entry on Mellit. In: Römpp Online . Georg Thieme Verlag, accessed on August 22, 2011.
  9. ↑ Mindat - number of locations
  10. ↑ Retro library, Meyers encyclopedia - Mellith .