Andrémeyerite

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Andrémeyerite
General and classification
other names

IMA 1972-005

chemical formula BaFe 2 [4] [Si 2 O 7 ]
Mineral class
(and possibly department) 		Silicates and Germanates
System no. to Strunz
and to Dana 		9.BB.20 ( 8th edition : VIII / C.02)
56.02.06c.06
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group P 2 1 / c (No. 14)Template: room group / 14
Lattice parameters a  = 7.49  Å ; b  = 13.78 Å; c  = 7.08 Å
β  = 118.2 °
Formula units Z  = 4
Twinning multiple twins after {100}
Physical Properties
Mohs hardness 5.5
Density (g / cm 3 ) measured: 4.15; calculated: 4.14
Cleavage completely according to {100} and {010}
colour light emerald green
Line color Please complete!
transparency transparent to translucent
shine Please complete!
Crystal optics
Refractive indices n α  = 1.740
n β  = 1.740
n γ  = 1.760
Birefringence δ = 0.020
Optical character biaxial positive
Pleochroism X = light bluish green
Y = Z = colorless with a faint, brownish tint

Andrémeyerite is a very rarely occurring mineral from the mineral class of "silicates and germanates" with the chemical composition BaFe 2 [4] [Si 2 O 7 ] and is therefore chemically a barium - iron - silicate . Structurally, the mineral belongs to the group silicates .

Andrémeyerite crystallizes in the monoclinic crystal system and develops mostly transparent to translucent, pseudo-orthorhombic crystals and multiple twins of less than 0.2 millimeters in size in a light emerald green color.

Etymology and history

Andrémeyerite was first discovered on the Nyiragongo volcano north of the city of Goma in the Democratic Republic of the Congo and described in 1973 by Thure Georg Sahama (1910–1983), J. Siivola and P. Rehtijärvi, who named the mineral after the Belgian geologist André Marie Meyer (1890 –1965) named.

Type material of the mineral is stored at the University of Helsinki in Finland.

classification

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the andremeyerite belonged to the mineral class of "silicates and germanates" and there to the department of " group silicates (sorosilicates)", where together with Åkermanite , barylite , gehlenite , gugiaite , Hardystonit , Jeffreyit , Kaliobarylith , melilite , Meliphan and Okayamalith the "melilite group" with the system number. VIII / C.02 formed.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns Andrémeyerite 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 the only member of the unnamed group 9.BB.20 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns andremeyerite 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 it is together with Barytolamprophyllit , Delindeit , Ericssonite , Lamprophyllit , Orthoericssonit , Eveslogit and Nabalamprophyllit in the "Seidozerit-Lamprophyllit group (Lamprophyllit subgroup)" with the system number. 56.02.06c within the subsection of " Group silicates: Si 2 O 7 groups and O, OH, F and H2O with cations in [4] and / or> [4] coordination ".

Crystal structure

Andrémeyerite crystallizes monoclinically in the space group P 2 1 / c (space group no. 14) with the lattice parameters a  = 7.49  Å ; b  = 13.78 Å; c  = 7.08 Å and β = 118.2 ° as well as 4 formula units per unit cell . Template: room group / 14

Education and Locations

Andrémeyerite forms in vesicles from melilite - leucite - nepheline - foidites . In addition to those already mentioned, other accompanying minerals include apatite , idolite , kirschsteinite , clinopyroxene , magnetite and troilite .

So far (as of 2012) Andrémeyerite could only be detected at its type locality , the Nyiragongo volcano in the Democratic Republic of the Congo.

See also

literature

  • Ilmari Haapala: Th. G. Sahama's (1910–1983) volcanological and mineralogical studies in Africa: Part I. Expeditions to the Virunga Volcanic Field and petrologicalmineralogical studies on the Nyiragongo volcano . In: Bulletin of the Geological Society of Finland . tape 83 , 2011, p. 41–55 ( geologinenseura.fi [PDF; 755 kB ; accessed on January 17, 2017]).
  • Elio Cannillo, Fiorenzo Mazzi, Giuseppe Rossi: Crystal structure of andremeyerite BaFe (Fe, Mn, Mg) Si 2 O 7 . In: American Mineralogist . tape 73 , 1988, pp. 608-612 ( minsocam.org [PDF; 488 kB ; accessed on January 17, 2017]).
  • Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 733 (first edition: 1891).

Web links

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.  570 .
  2. ↑ Webmineral - Andrémeyerite
  3. ↑ a b c d e f Andrémeyerite . 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; 66  kB ; accessed on January 17, 2017]).
  4. ↑ a b c d e Mindat - Andrémeyerite