Gebhardit

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Gebhardit
General and classification
other names

IMA 1979-071

chemical formula
  • Pb 8 [O | Cl 6 | (As 3+ 2 O 5 ) 2 ]
  • Pb 8 (As 3+ 2 O 5 ) 2 OCl 6
Mineral class
(and possibly department) 		Oxides and hydroxides
System no. to Strunz
and to Dana 		4.JB.50 ( 8th edition : IV / J.06)
02/26/05/01
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  = 6.72  Å ; b  = 11.20 Å; c  = 34.19 Å
β  = 85.2 °
Formula units Z  = 4
Frequent crystal faces {100}, {001}
Physical Properties
Mohs hardness ≈ 3 (VHN = 80-100 kp / mm 2 )
Density (g / cm 3 ) 6.0 (calculated)
Cleavage excellent after {001}, clearly after {010}
Break ; Tenacity not specified; not specified
colour brown
Line color White
transparency transparent
shine Diamond luster
Crystal optics
Refractive indices n α  = 2.08
n β  = not defined
n γ  = 2.12
Birefringence δ = 0.04
Optical character biaxial negative
Axis angle 2V = 34
Pleochroism weak from X = Y = pale brown to Z = brown
Other properties
Chemical behavior Dissolution in cold, dilute HCl and HNO 3 to form tiny As 2 O 3 octahedra

Gebhardite is a very rarely occurring mineral from the mineral class of " oxides and hydroxides " with the chemical formula Pb 8 [O | Cl 6 | (As 3+ 2 O 5 ) 2 ]. This makes the mineral a lead - arsenite with additional oxygen and chlorine ions.

Gebhardite crystallizes in the monoclinic crystal system and develops predominantly long prismatic crystals and pile-shaped aggregates with a distinct diamond luster.

Etymology and history

Gebhardite was discovered on what was then the only step in the world with grown Reinerite crystals from Tsumeb. The discoverer is Georg Gebhard (* 1945) from Reichshof-Oberwehnrat, who found the stage in 1977 and in 1979 handed it over to the authors of the type publication for analysis. The mineral was examined by a team of researchers at the University of Bochum led by Olaf Medenbach , W. Gebert and Kurt Abraham . After it was recognized by the International Mineralogical Association (IMA) in 1979, it was officially first described in 1983. The authors named the mineral after the chemist and mineral collector Georg Gebhard.

The holotype stage is on permanent loan in the permanent collection of the École nationale supérieure des mines de Paris ( Mines ParisTech ), Paris , France . Further type material is deposited in the National Museum of Natural History , Washington, DC , United States (catalog no. 147360).

classification

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the Gebhardite belonged to the division of "Arsenite (with As 3+ )", where together with Fetiasite, Paulmooreit, Schneiderhöhnit and Vajdakit it formed the group of "Arsenite with [As 2 O 5 ] 4− groups ”with the system no. IV / J.06 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), assigns Gebhardite to the newly defined division of "Arsenites, Antimonites, Bismutites, Sulphites, Selenites and Tellurites". This is further subdivided according to the possible presence of crystal water and / or additional anions , so that the mineral can be found according to its composition in the subsection “Arsenite, Antimonide, Bismutide, without additional anions, without H 2 O”, where it can be found as the only member is the unnamed group 4.JB.50 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Gebhardite to the class of "phosphates, arsenates and vanadates" and there to the category of "basic or halogen-containing antimonites, arsenites and phosphites". Here he is the only member of the unnamed group 46.02.05 within the sub-section “ Basic or halogenated antimonites, arsenites and phosphites with various formulas ”.

Crystal structure

Gebhardite crystallizes in the monoclinic crystal system in the space group P 2 1 / c (space group no. 14) with the lattice parameters a  = 6.72  Å ; b  = 11.20 Å; c  = 34.19 Å and β = 85.2 ° as well as four formula units per unit cell . Template: room group / 14

In the structure of Gebhardit, the Pb atoms coordinate with Cl and O to form polyhedra , which are formed from 6 to 8 atoms. The trivalent As coordinates with three oxygen atoms to form a tetrahedral pyramid , the tip of which is occupied by As. Two of these “tetrahedra” each form a group [As 2 O 5 ] −4 . An oxygen atom is only bonded to Pb with a closest distance of 2.7 Å. The structure of Gebhardit consists of eight Pb (Cl, O) 6-8 polyhedra, which form more or less parallel to (100) parallel, corrugated sheets which are linked to form a framework of dimers of As 3+ O 3 pyramids . The dimers sit in the [010] tunnels of this framework.

properties

morphology

Gebhardite forms fibrous groups up to 5 mm in size from long prismatic crystals on which the surface shapes {100} and {001} could be identified. The crystals can grow together parallel or subparallel to divergent-rayed and in the latter case form pile-shaped aggregates. The aggregates resemble Millerite aggregates in their appearance. Curved crystals were also occasionally found.

physical and chemical properties

The crystals of Gebhardite are brown, the color of the streak of the mineral is described as white. The transparent crystals have a pronounced diamond luster, which is also reflected in the comparatively high refraction of light with a refractive index of 2.08 to 2.12. The Mohs hardness of the mineral is about 3 and thus corresponds to that of the reference mineral calcite , the calculated density is 6.0 g / cm³.

Gebhardit dissolves in cold dilute HCl and / or HNO 3 easily, forming tiny As 2 O 3 - octahedron .

Education and Locations

Gebhardite forms secondary and occurred in the lower oxidation zone of the hydrothermal polymetallic ore deposit Tsumeb, which is located in dolomite stones. The exact place of origin of the Gebhardite grade within the Tsumeb deposit is not known. The piece measures approx. 5 × 5 × 3 cm and consists of deeply weathered, holey-looking white carbonate and silicate masses on which an unusual secondary mineral paragenesis has grown. Gebhardite is one of the last oxidation products formed. Accompanying minerals at the type locality are Reinerite in up to 1.5 cm in size, white to light green, e.g. Some crystals appearing loosened, mimetite in small gray, tufted and warty aggregates, needle-like smithsonite , willemite in porcelain-like, leafy-rayed structures, hematite in z. T. pseudocuboctahedral crystals, Fraipontite in the form of white spherulites and crusts grown in quartz and grown on quartz and hematite, as well as quartz . Particularly interesting is the occurrence of the lead-chlorine arsenate mimetesite in direct contact with the lead-chlorine arsenite Gebhardite, which indicates tightly limited pH / Eh conditions for the stability of this paragenesis.

As a very rare mineral formation, Gebhardite could only be proven 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 .

use

With a PbO content of around 75% by weight, Gebhardite would be a rich lead ore. Due to its extreme rarity, however, the mineral is only of interest to collectors.

See also

literature

  • Olaf Medenbach, W. Gebert, Kurt Abraham (1983): Gebhardit, Pb 8 OCl 6 (As 2 O 5 ) 2 , a new arsenite from Tsumeb, Southwest Africa / Namibia. In: New Yearbook Mineralogie Monatshefte , Volume 1983 (Issue 10), 445–450.
  • Gebhardite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 70.2 kB )

Web links

Individual evidence

  1. ↑ a b c d e 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.  268 .
  2. ↑ a b c Gebhardite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 70.2 kB )
  3. ↑ a b c d e f g h i j k l m n Olaf Medenbach, W. Gebert, Kurt Abraham (1983): Gebhardit, Pb 8 OCl 6 (As 2 O 5 ) 2 , a new arsenite from Tsumeb, southwest Africa / Namibia. In: New Yearbook Mineralogie Monatshefte , Volume 1983 (Issue 10), 445–450.
  4. ↑ Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 259 .
  5. ↑ R. Klaska, W. Gebert (1990): polytypes and structure of Gebhardit - Pb 8 OCl 6 (As 2 O 5 ) 2 . In: Zeitschrift für Kristallographie , Vol. 159, 75–76 ( PDF, 112kB ).
  6. ↑ Juraj Majzlan, Petr Drahota, Michal Filippi (2014): Parageneses and Crystal Chemistry of Arsenic Minerals. In: Reviews in Mineralogy & Geochemistry , Volume 79, pp. 134-135.
  7. ↑ Find location list for Gebhardite at the Mineralienatlas and at Mindat
  8. ↑ Mindat - Number of localities for Gebhardit