Krieselit

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Krieselit
Krieselite.jpg
Krieselit from Tsumeb , Namibia
General and classification
other names

IMA 2000-043a

chemical formula
  • Al 2 GeO 4 (F, OH) 2
  • (Al, Ga) 2 (GeO 4 ) (OH) 2
Mineral class
(and possibly department) 		Silicates and Germanates
System no. to Strunz
and to Dana 		9.AF.35 ( 8th edition : VIII / B.02)
52.03.01.02
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Space group Pnma (No. 62)Template: room group / 62
Lattice parameters a  = 4.809  Å ; b  = 9.111 Å; c  = 8.536 Å
Formula units Z  = 4
Physical Properties
Mohs hardness 5.5 to 6.5

(VHN 50–100 g = 473–566 kg / mm²)

Density (g / cm 3 ) 4.069 (calculated)
Cleavage no
Break ; Tenacity irregular; not specified
colour beige to white
Line color White
transparency translucent
shine Greasy shine
Crystal optics
Refractive index n  = 1.737 (calculated)

Krieselite is a very rare island silicate from the mineral class of " silicates and germanates ". It crystallizes in the orthorhombic crystal system with the chemical composition Al 2 GeO 4 (F, OH) 2 and is therefore chemically seen an aluminum - germanate with additional fluorine ions (F - ) and hydroxide ions (OH - ). The fluorine and hydroxide ions indicated in round brackets can represent each other in the formula ( substitution , diadochy), but are always in the same proportion to the other components of the mineral.

Krieselit develops fibrous aggregates up to 50 µm in length that come together to form crusts. Individual fibers represent wedge-shaped aggregates of leaflets according to {110}. There are also hemispherical aggregates up to 200 μm in diameter.

Etymology and history

The German mineral collector Markus Ecker from Spiesen , who handed over the mineral purchased from a mineral dealer in 1994, is considered to be the discoverer of the krieselite . Corresponding studies led to the determination of the presence of a new mineral, which was recognized by the International Mineralogical Association (IMA) in 2000 and described as krieselite by a German research team with Jochen Schlüter , Thorsten Geisler , Dieter Pohl and Thomas Stephan in 2010 . The mineral was named after the German chemist Friedrich Wilhelm Kriesel, who was chief chemist and head of the laboratory at the Tsumeb Mine around 1920. At the same time as Otto Hermann August Pufahl (1855–1924), Kriesel discovered the elements germanium and gallium in germanite, which was first found in the Tsumeb mine .

Type material of the mineral (holotype) is kept in the Mineralogical Museum of the University of Hamburg in Germany (location MMHH, catalog no. TS 385).

classification

In the meanwhile outdated, but still common 8th edition of the mineral classification according to Strunz , the krieselite belongs to the mineral class of "silicates and germanates" and there to the department of " island silicates with non-tetrahedral anions (Neso-subsilicates)", where it belongs together with the eponymous topaz and the other members andalusite , Boromullit , Kanonait , kyanite , mullite , sillimanite and Yoderit the "Topas group" with the system no. VIII / B.02 forms.

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 classifies Krieselite in the category of "island silicates (nesosilicates)". However, this is further subdivided according to the crystal structure , so that the mineral is classified in the sub-section “Island silicates with additional anions; Cations can be found in [4] er, [5] er and / or only [6] er coordination ”, where the“ topaz group ”with the system no. 9.AF.35 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Krieselite to the class of "silicates" and there to the department of "island silicates: SiO 4 groups and O, OH, F and H 2 O". Here it is only together with the eponymous Topaz in the "Topas group" with the system no. 52.03.01 to be found in the subsection of " Island silicates: SiO 4 groups and O, OH, F and H 2 O with cations only in [6] coordination ".

Chemism

Krieselite forms the germanium-dominant analogue to the silicon-dominated topaz and also represents the fluorine-dominant analogue to the synthetic hydroxyl-dominated Al 2 GeO 4 (OH) 2 .

It has the measured composition (Al 1.860 Ga 0.102 As 3+ 0.036 Zn 0.020 Mg 0.016 Fe 3+ 0.012 Na 0.009 Sb 3+ 0.005 Ti 0.003 Cu 0.001 ) Σ = 2.064 (Ge 0.844 Al 0.143 Si 0.013 ) Σ = 1.000 O4 (F. 1.103 OH 0.897 ) Σ = 2.000 . This formula can be written simply as Al 2 GeO 4 (F, OH) 2 . Krieselit is another representative of the minerals containing germanium in Tsumeb; Of the 23 germanium minerals currently known (2016), fifteen have also been found in Tsumeb, all but two of which have their type locality here.

Crystal structure

Krieselite crystallizes in the orthorhombic crystal system in the space group Pnma (space group no. 62) with the lattice parameters a  = 4.809  Å ; b  = 9.111 Å and c  = 8.536 Å as well as four formula units per unit cell . Template: room group / 62

Krieselite is, just like synthetic Al 2 GeO 4 (OH) 2 , isotype (isostructural) with topaz. In analogy to the structure of the topaz, the structure of the kriselite can be seen as a close-packed arrangement of anions (O 2− , OH - , F - ) with Al 3+ on one third of the octahedron positions and Si 4+ on one twelfth of the tetrahedron positions to be discribed. It consists of [GeO 4 ] 4− groups connecting octahedral chains of Al [O 4 (F, OH) 2 ] –7 octahedra in a zigzag shape parallel to the c-axis [001]. Four of the six the Al 3+ ion surrounding anions belong to the [GeO 4 ] 4- tetrahedra, whereas the two remaining anions either F - - OH or - represent groups. In the synthetic Al 2 GeO 4 (OH) 2 the Si of the tetrahedral positions of the topaz is replaced by Ge 4+ and, to a lesser extent , Al 3+ .

properties

morphology

Drawing of a wedge-shaped aggregate of Krieselite crystals from the Tsumeb Mine

Krieselite forms hemispherical aggregates up to 0.2 mm in diameter as well as crusts of fibers that are up to 50 μm in length and up to 5 μm in thickness. The fibers, for their part, are packs of fine leaves that come together to form wedge-shaped structures. The leaflets are arranged in parallel (110). The impression of a wedge is created due to the decreasing length of the leaflets along the fiber axis [1 1 0]. The resulting wedge surfaces are inclined to an insignificant degree against the leaflets and cannot be indexed. The long and thin surface of the individual leaflets is each formed by {001} and creates the impression of a surface on the side of the wedge - which is in accordance with the observed parallel optical extinction.

physical and chemical properties

The crystals of the kriselite are beige to white, the line color of the mineral is indicated as white. The surfaces of the translucent crystals have a greasy sheen .

The mineral has no cleavage , but breaks unevenly like sillimanite or andalusite . With a Mohs hardness of 5.5 to 6.5, kriselite is one of the medium-hard to hard minerals that can be scratched with a steel file sometimes a little easier and sometimes a little heavier than the reference mineral orthoclase . The calculated density of the mineral is 4.069 g / cm³.

Education and Locations

So far (as of 2016) the mineral could only be found at its type locality . The type locality is the world-famous Cu-Pb-Zn-Ag-Ge-Cd deposit of the "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb , Oshikoto region , Namibia .

Krieselit is a typical secondary mineral and formed in the oxidation zone in dolomite stones seated hydrothermal polymetallic ore deposit- Tsumeb. It sits in cavities in the tennantite - chalcosine - galena - germanite - ore . Accompanying minerals are quartz , wulfenite , angelsite and graphite , whereby intergrowths between kriselite, wulfenite and graphite have been observed. In another find, krieselite is accompanied by Schneiderhöhnite and stottite .

For the first stage investigated (type stage), neither the exact location nor the year of discovery are known. The levels at which Krieselit later identified, are from the 29th floor of the Tsumeb mine and were salvaged here 1,972th

use

With a GeO 2 content of around 38%, kriselite would be a rich germanium ore. However, with only a few known levels, it is one of the rarest secondary minerals of the Tsumeb Mine and is therefore only of interest to mineral collectors.

See also

literature

  • Jochen Schlueter, Thorsten Geisler, Dieter Pohl, Thomas Stephan (2010): Krieselite, Al 2 GeO 4 (F, OH) 2 : A new mineral from the Tsumeb mine, Namibia, representing the Ge analogue of topaz. In: New Yearbook for Mineralogy, Abhandlungen , Volume 187 (Issue 1), pp. 33–40.
  • Krieselit , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 191 kB ).

Web links

Commons : Krieselite  - Collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o p q r s t u Jochen Schlüter, Thorsten Geisler, Dieter Pohl, Thomas Stephan (2010): Krieselite, Al 2 GeO 4 (F, OH) 2 : A new mineral from the Tsumeb mine, Namibia, representing the Ge analogue of topaz. In: New Yearbook for Mineralogy, Abhandlungen , Volume 187 (Issue 1), pp. 33–40.
  2. ↑ a b c d e Krieselit , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 191 kB ) .
  3. ↑ a b Mindat - Krieselit
  4. ^ Friedrich Wilhelm Kriesel (1922): Gallium in the Germanite from Tsumeb. In: Metall und Erz , Volume 20, pp. 257-259.
  5. ^ Friedrich Wilhelm Kriesel (1924): About the analysis of the new germanium-gallium mineral "germanite". In: Chemikerzeitung , Volume 48, p. 961.
  6. ^ Type mineral catalog Germany - storage of the holotype stage Krieselit
  7. ^ Paul H. Ribbe, GV Gibbs (1971): The crystal structure of topaz and its relation to physical properties. In: American Mineralogist , Volume 56, pp. 24-30.
  8. ↑ B. Marler, B. Wunder (1998): Crystal structure of dialuminium dihydroxogermanate, Al 2 GeO 4 (OH) 2 . In: Zeitschrift für Kristallographie - New Crystal Structures , Volume 213, p. 3.
  9. ↑ Mindat - Number of localities for Krieselit
  10. ↑ Find location list for Krieselite at the Mineralienatlas and at Mindat