Melilith group

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Melilith
Melilith.jpg
Melilith in the thin section of an olivine melilithite from Hegau, crossed polarizers
General and classification
chemical formula (Ca, Na) 2 (Mg, Al) [4] [Si 2 O 7 ]
Mineral class
(and possibly department) 		Silicates and Germanates
System no. to Strunz
and to Dana 		9.BB.10 ( 8th edition : VIII / C.02)
04.55.01.03
Crystallographic Data
Crystal system tetragonal
Crystal class ; symbol tetragonal-scalenohedral; 4 2 m
Space group P 4 2 1 m (No. 113)Template: room group / 113
Lattice parameters a  = 7.83  Å ; c  = 5.00 Å
Formula units Z  = 2
Physical Properties
Mohs hardness 5 to 5.5
Density (g / cm 3 ) 2.9 to 3
Cleavage clear after {001}, indistinct after {100}
Break ; Tenacity uneven
colour colorless, white, gray, yellow, greenish brown
Line color White
transparency transparent to translucent
shine Glass shine, greasy shine on fresh breakage
Crystal optics
Refractive indices n ω  = 1.632 to 1.669
n ε  = 1.626 to 1.658
Birefringence δ = 0.006 to 0.011
Optical character uniaxial negative

The Melilithgruppe (short: the Melilith ) is a rather rarely occurring group of minerals from the mineral class of " silicates and germanates ". They crystallize in the tetragonal crystal system with the general formula A 2 B (T 2 O 7 ) , where A can stand for Ca, Na and Ba, B can stand for Mg, Al, Fe, Be, B and Zn, and T for Si , Al and B can stand. The mineral, classically known as "Melilite", has the chemical composition (Ca, Na) 2 (Mg, Al) [4] [Si 2 O 7 ] and is a product of a gapless series of mixed crystals with the end members Gehlenit Ca 2 Al [AlSiO 7 ] and Åkermanit Ca 2 Mg (Si 2 O 7 ). The minerals of the Melilith group mostly develop only small crystals in the millimeter range with a tabular or short to long columnar habit , but also granular to massive mineral aggregates .

Etymology and history

Melilith was first discovered near Capo di Bove in the Alban Mountains in Italy. The first description was in 1796 by Jean-Claude Delamétherie , who named the mineral in relation to its often honey-yellow color after the Greek words meli for "honey" and lithos for "stone".

classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the Melilith belonged to the mineral class of "silicates and Germanates" and there to the department of " group silicates (sorosilicates)", where he named the "Melilith group" with the System no. VIII / C.02 and the other members Åkermanite , Andrémeyerite , Barylite , Gehlenite , Gugiaite , Hardystonite , Jeffreyite , Kaliobarylite , Meliphan and Okayamalite .

The 9th edition of Strunz's mineral systematics , which has been in force since 2001 and is used by the International Mineralogical Association (IMA), also assigns Melilith to the class of "silicates and germanates" and there in the department of "group silicates (sorosilicates)". However, this section is further subdivided according to the type of silicate group formation, the possible presence of further anions and the coordination of the cations , so that the mineral according to its composition and structure in the subdivision “Si 2 O 7 groups, without non-tetrahedral anions; Cations in tetrahedral [4] and greater coordination ”can be found, where it is also named after the“ Melilith group ”with the system no. 9.BB.10 and the other members Åkermanite, Barylite, Cebollite , Gehlenite, Gugiaite, Hardystonite, Jeffreyite and Okayamalite.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Melilith to the class of "silicates and germanates", but there in the more finely divided division of "group silicates: Si 2 O 7 groups, generally without additional anions" . Here, too, he is the namesake of the "Melilith Group" with the system no. 55.04.01 and the other members Åkermanit, Gehlenit and Okayamalith can be found within the subdivision of the “ Group silicates: Si 2 O 7 groups, generally without additional anions and with cations in [8] and lower coordination ”.

Crystal structure

Crystal structure of the Melilite group: the blue tetrahedra represent the anions T 2 O 7 , the orange tetrahedra represent the positions of the cations B, the yellow circles the position of the cations A.

Melilith crystallizes tetragonally in the space group P 4 2 1 m (space group no. 113) with the lattice parameters a  = 7.83  Å and c  = 5.00 Å as well as two formula units per unit cell . Template: room group / 113

In the crystal structure, the anions T 2 O 7 form planes with tetrahedral gaps which are occupied by the cations B. The cations A occupy positions above and below these levels and connect them with one another in the third dimension (see structure diagram on the right).

properties

Pure Melilith is colorless. Usually, however, it appears as a result of foreign admixtures in a white to gray, yellow or greenish-brown color.

Melilith is difficult to melt in front of the soldering tube . However, acids destroy it, whereby it " gelatinizes ".

Melilith crystals with a "peg structure".

Under the microscope, Melilith appears in thin section mostly in the form of elongated, rectangular crystals. Since Melilite is very susceptible to transformation, zeolite fibers are often observed in the crystals, which grow vertically from the two base surfaces into the interior of the crystals, the so-called "stake structure". Anomalous (blue or brown) interference colors often appear under crossed polarizers.

Education and Locations

Melilite often forms as a mixture in ultra-basic , calcium-rich volcanic rocks , and very often occurs in paragenesis with perovskite . In Germany, such volcanic rocks can be found in the Hegau , the Urach volcanic area , the Kaiserstuhl , in the Eifel , in some volcanic vents in the northern Hessian Depression and in the Heldburger Gangschar .

In addition, Melilith can also be formed metamorphically, for example from impure limestone influenced by regional or contact metamorphosis.

So far (as of 2010) Melilith has been found at 130 sites around the world, including in Algeria , Antarctica , Bolivia , Brazil , China , Germany , France , Greenland , Israel , Italy , Yemen , Canada , Kazakhstan and the Democratic Republic Republic of the Congo , Madagascar , Morocco , Mexico , Namibia , Norway , Austria , Romania , Russia , Sweden , Switzerland , Tanzania , the Czech Republic , the USA , the United Kingdom (Great Britain) and the West Bank (Palestinian Territories).

Melilith is also a common component of so-called CAIs ( calcium-aluminum-rich inclusions ) in meteorites, which are among the oldest materials in our solar system.

In blast furnace slag , Melilith regularly forms a major part of the mixture.

Importance in petrography

The presence of Melilith in igneous rocks is important for the classification because igneous rocks which contain levels of Melilith exceeds 10% and in which both the amount of Melilith which the Foide exceeds no longer using the Streckeisen chart classified and named, but own a Group of "rocks containing melilite " (volcanic: Melilithite , plutonic: Melilitholite ). If less than 10% Melilite is present, the name "Melilite" is placed in front of the name determined using the route iron diagram (e.g. Melilite nephelinite).

It should also be noted that Melilite is counted among the mafic minerals , although externally it is a "light" mineral, which is important when determining the color index of a rock.

See also

literature

Web links

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

Individual evidence

  1. ↑ a b c d David Barthelmy: Melilite Mineral Data. In: webmineral.com. Accessed April 14, 2019 .
  2. ↑ a b c American-Mineralogist-Crystal-Structure-Database - Melilite. In: rruff.geo.arizona.edu. Accessed April 14, 2019 .
  3. ↑ a b c d Melilite. In: mindat.org. Hudson Institute of Mineralogy, accessed April 14, 2019 .
  4. ^ A b 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.  568-569 (English).
  5. ↑ Hans Pichler, Cornelia Schmitt-riegraf: Rock -forming minerals in thin sections . 2nd Edition. Enke, Stuttgart 1993, ISBN 3-8274-1260-9 , pp. 59-61 .
  6. ^ Albert Schreiner: Hegau and western Lake Constance . In: Collection of Geological Guides . 3. Edition. tape 62 . Borntraeger, Berlin 2008, ISBN 978-3-443-15083-9 , pp. 63-66, 69 .
  7. ↑ Wolfgang Roser, Jürgen Mauch: The Swabian volcano . GO Druck-Media-Verlag, Kirchheim unter Teck 2003, ISBN 3-925589-29-5 , p. 78 .
  8. ↑ Wolfhard Wimmenauer: Geological map of Baden-Württemberg: Explanations for the sheet Kaiserstuhl . 5th, completely revised edition. State Office for Geology, Raw Materials and Mining, Freiburg 2003, DNB  971928606 , p. 97-98 .
  9. ^ Josef Frechen: Siebengebirge on the Rhine, Laacher volcanic area, maar area of ​​the West Eifel . In: Collection of Geological Guides . 2nd Edition. tape 56 . Borntraeger, Berlin 1971, ISBN 3-443-15010-1 , p. 69, 78 .
  10. ↑ Wedepohl, KH: The tertiary basaltic volcanism of the Hessian depression north of the Vogelsberg . In: The opening . Special volume 28, 1978, p. 162-163 .
  11. ↑ HG Huckenholz, C.-D- Werner: The tertiary volcanic rocks of the Heldburger Gangschar (Bavarian-Thuringian grave field) . In: European Journal of Mineralogy . tape 2 , supplement 2, 1990, p. 1-42 .
  12. ↑ Walter Ehrenreich Tröger: Optical determination of the rock-forming minerals . 2nd Edition. Part 2. Schweitzerbart, Stuttgart 1969, DNB  458442976 , p. 130 .
  13. ↑ Find location list for Melilith at the Mineralienatlas and at Mindat
  14. ↑ Th. Posch, H. Mutschke, Mario Trieloff, Th. Henning: Infrared spectroscopy of calcium-aluminum-rich inclusions (CAIs): Analog material of protoplanetary dust? In: The Astrophysical Journal . tape 656 , 2007, p. 615–620 (English, available online at iopscience.iop.org [PDF; 346 kB ; accessed on April 14, 2019]).
  15. ↑ Rudolf Jubelt, Peter Schreiter: Rock determination book . Dausien, Hanau 1972, ISBN 3-7684-6244-7 , p. 95 .
  16. ↑ RW LeMaitre (Ed.): Igneous Rocks - A Classification and Glossary of Terms . 2nd Edition. Cambridge University Press, Cambridge 2004, ISBN 0-521-61948-3 , pp. 11 .