Otjisumeit

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Otjisumeit
Otjisumeite.jpg
White, prismatic otjisumeit crystals
General and classification
other names
  • IMA 1978-080
  • Mineral TI
chemical formula
  • PbGe 4 O 9
  • Pb [Ge 4 O 9 ]
Mineral class
(and possibly department)
Silicates and Germanates
System no. to Strunz
and to Dana
9.JA.15 ( 8th edition : IV / C.08)
07.10.03.01
Similar minerals Willemite
Crystallographic Data
Crystal system triclinic
Crystal class ; symbol triklin-pedial; 1 or triclinic pinacoidal; 1
Space group P 1 (No. 1) or P 1 (No. 2)Template: room group / 1Template: room group / 2
Lattice parameters a  = 6.945  Å ; b  = 6.958 Å; c  = 9.279 Å,
α  = 102.94 °; β  = 103.05 °; γ  = 114.77 °
Formula units Z  = 2
Physical Properties
Mohs hardness ≈ 3
Density (g / cm 3 ) 5.77 (calculated)
Cleavage indistinct after {001}
Break ; Tenacity not specified; not specified
colour colorless-clear, white
Line color White
transparency translucent to translucent
shine Greasy gloss to semi-diamond gloss
Crystal optics
Refractive indices n α  = 1.920
n β  = [1.922] (calculated)
n γ  = 1.943
Refractive index n  = 1.93 (measured); 1.90 (calculated)
Birefringence δ = 0.023
Optical character biaxial positive
Axis angle 2V = 20 °
Other properties
Chemical behavior easily soluble in hot ENT 3

Otjisumeit is a very rarely occurring mineral from the mineral class of " silicates and germanates " (formerly oxides and hydroxides ). It crystallizes in the triclinic crystal system with the chemical formula PbGe 4 O 9 , so it is chemically a lead germanate.

Otjisumeit forms idiomorphic, pseudo-hexagonal crystals stretched more or less along the c-axis [001] up to a maximum of 1 mm in length and less than 0.1 mm in diameter, which have a long columnar to needle-like habit or are also fibrous. They are usually fused to form radial-radial aggregates . The mineral was found - together with chalcosine , siderite and schaurteit - in small cavities in corroded germanite - renierite - tennantite ore in the Tsumeb Mine , Namibia.

Etymology and history

Downtown Tsumeb. The Herero expression "Otjisume" for Tsumeb provided the name for the mineral Otjisumeit.

Bruno H. Geier (1902–1987), the former chief mineralogist of the Tsumeb Corporation , who noticed the mineral among other specimens from Tsumeb as early as the mid-1970s, is considered the discoverer of Otjisumeit . Corresponding investigations on material which the mineral collector Walter Richard Kahn had made available led to the determination of the presence of a new mineral, which was recognized in 1978 by the International Mineralogical Association (IMA) under the number "IMA 1978-080" and in 1981 by a German -US American research team with Paul Keller , Heinz Hess and Pete J. Dunn in the German science magazine "New Yearbook for Mineralogy, MONTHS" as Otjisumeit was described.

The mineral was named after the location Tsumeb, which in the Herero language is "Otjisume", which means "place of frogs" or "place of green algae".

Type material of the mineral is in the archive of the University of Stuttgart in the "Mineralogical Collection of Professor Keller" (register no. TM-78.80-TI- (B76) at location 0/824-s27 / 2, holotype) as well as in the Smithsonian Institution belonging to the National Museum of Natural History , Washington, DC (Register Nos. 145746, 149053, Cotyp).

classification

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the Otjisumeit belonged to the mineral class of " oxides and hydroxides " and there to the department of "oxides with the molar ratio of metal: oxygen = 2: 3", where it together with barbarity , Batiferrit , Haggertyit , Hawthorneit , Hibonit , Lindqvistit , magnetoplumbite , Nežilovit , Plumboferrit , Yimengit and Zenzénit the magnetoplumbite group having the system no. IV / C.08 .

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 the Otjisumeit to the class of “silicates and Germanates” and there in the “Germanates” department. Here he can be found as the only member of the unnamed group 9.JA.15 within the previously unnamed subdivision “A”.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Otjisumeit to the class of "oxides and hydroxides", like the outdated Strunz'sche systematics, but there, however, to the category of "multiple oxides". Here he is the only member of the unnamed group 10/07/03 within the subdivision of " Multiple Oxides ".

Chemism

Microprobe analyzes on Otjisumeit led to contents of 35.5% PbO and 64.7% GeO 2 . On the basis of 9 oxygen atoms per formula unit, this resulted in the empirical formula Pb 2+ 1.03 Ge 4+ 3.99 O 9 , which was idealized to PbGe 4 O 9 and contents of 34.8% PbO and 65.2% GeO 2 required.

There is a synthetic compound with the identical composition PbGe 4 O 9 , but which has a hexagonal structure, making it a polymorph of the Otjisumeits. Otjisumeit may represent the low-temperature modification of this hexagonal synthetic phase.

Crystal structure

Otjisumeit crystallizes triclinically in the space group P 1 (space group no. 1) or P 1 (no. 2) with the lattice parameters a  = 6.945  Å ; b  = 6.958 Å; c  = 9.279 Å; α = 102.94 °; β = 103.05 ° and γ = 114.77 ° as well as two formula units per unit cell . Template: room group / 1Template: room group / 2

The structure of the Otjisumeits is still unclear. The synthetic hexagonal polymorph of the Otjisumeits is isotypic to hexagonal benitoite , i. H. it crystallizes with the same structure as benitoite.

properties

morphology

Otjisumeit is found in idiomorphic crystals stretched more or less along the c-axis [001], which are a maximum of 1.0 mm long and have a diameter of less than 0.3 mm. The crystal habit of the pseudo-hexagonal crystals is long columnar to needle-like, often they are also fibrous. It is characterized by an intergrowth to form sheaf- and tuft-shaped to radial-radial aggregates. The pseudo-hexagonal shapes of the Otjisumeit crystals could not be indexed because they were too small for an optical measurement.

physical and chemical properties

Otjisumeit crystals are colorless or white, but their line color is always white. The surfaces of the crystals, which are water-clear to translucent, depending on their color, show a distinct semi-diamond-like or greasy sheen .

The crystals of the Otjisumeits show an indistinct cleavage according to {001}. The mineral has a Mohs hardness of ≈ 3 and is therefore one of the medium-hard minerals that, like the reference mineral calcite, can be scratched with a copper coin. Measured values ​​for the density of the Otjisumeits do not exist, the calculated density for the mineral is 5.77 g / cm³.

Otjisumeit is readily soluble in hot nitric acid HNO 3 .

Education and Locations

Otjisumeit occurs as a typical secondary formation in the heavily corroded germanium ore of a complex Cu-Pb-Zn deposit in carbonate rocks. Lead and germanium come from the decomposition of primary germanium ores and sulfidic ore minerals such as germanite , renierite , tennantite and galena . Accompanying minerals in the decomposition cavities include germanite, renierite, tennantite and galena as well as chalcosine , schaurteit, siderite, calcite, gypsum and quartz . Originally only two stages with Otjisumeit were known, the exact location of which in the Tsumeb Mine could not be located. The age sequence of the minerals identified on one of the two levels is ore → siderite → otjisumeit → schaurteit. The other sample did not show any appearance. Here is the age sequence: ore → chalcocene → siderite → otjisumeit. Otjisumeit was later identified in the first description of calvertite , gallobeudantite and galloplumbogummite and thus belongs to the paragenesis of the types of these three minerals.

As a very rare mineral formation, Otjisumeit could only be described from one point of discovery so far (as of 2016). Its type locality is the world-famous Cu-Pb-Zn-Ag-Ge-Cd deposit of the "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb , Oshikoto Region , Namibia . The exact location within the "Tsumeb Mine" is not known.

use

Otjisumeit is only of interest to mineral collectors due to its rarity.

See also

literature

  • Paul Keller, Heinz Hess, Pete J. Dunn: Bartelkeit, PbFe 2+ Ge 3 O 8 , a new germanium mineral from Tsumeb, Namibia . In: Chemistry of the Earth . tape 40 , 1981, ISSN  0009-2819 , pp. 201-206 .
  • Otjisumeit. In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America. 2001 ( PDF, 69 kB )

Web links

Commons : Otjisumeite  - 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 Paul Keller, Heinz Hess, Pete J. Dunn: Otjisumeit, PbGe 4 O 9 , a new mineral from Tsumeb, Namibia . In: New yearbook for mineralogy, monthly books . 1981 (issue 2), 1981, p. 49-55 .
  2. ^ A b Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  716 .
  3. a b Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 275 + 322 .
  4. Paul Keller: Tsumeb / Namibia - one of the most spectacular mineral discovery sites on earth . In: Lapis . 9 (issue 7/8), 1984, p. 13-63 .
  5. ↑ Type mineral catalog Germany - storage of the holotype stage Otjisumeit
  6. Catalog of Type Mineral Specimens - O. (PDF 37 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, p. 6 , accessed May 1, 2020 .
  7. a b Otjisumeit. In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America. 2001 ( PDF, 69 kB )
  8. Mindat - Number of localities for Otjisumeit
  9. location list for the Otjisumeit Mineralienatlas and Mindat
  10. Wolfgang Bartelke: The ore deposit of Tsumeb / South West Africa and its minerals . In: The opening . tape 27 , 1976, p. 393-439 .