Gallobeudantite

from Wikipedia, the free encyclopedia
Gallobeudantite
General and classification
other names

IMA 1994-021

chemical formula
  • Pb (Ga, Al, Fe) 3 [(OH) 6 | SO 4 | AsO 4 ]
  • PbGa 3 [(AsO 4 ), (SO 4 )] 2 (OH) 6
  • PbGa 3 (AsO 4 ), (SO 4 ) 2 (OH) 6
Mineral class
(and possibly department) 		Phosphates, arsenates and vanadates
System no. to Strunz
and to Dana 		8.BL.05 ( 8th edition : VII / B.35)
43.04.01.10
Similar minerals Beudantite
Crystallographic Data
Crystal system trigonal
Crystal class ; symbol ditrigonal-pyramidal; 3 m
Space group R 3 m (No. 160)Template: room group / 160
Lattice parameters a  = 7.225  Å ; c  = 17.03 Å
Formula units Z  = 3
Frequent crystal faces {10 1 1} and {0001}
Physical Properties
Mohs hardness 4th
Density (g / cm 3 ) 4.61 (calculated)
Cleavage after {001} ?; brittle
Break ; Tenacity even to shell-like
colour pale yellow, slightly greenish, cream colored
Line color white to pale yellow
transparency transparent
shine Glass gloss
Crystal optics
Refractive indices n ω  = 1.763
n ε  = 1.750
Birefringence δ = 0.013
Optical character uniaxial negative

Gallobeudantite is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the trigonal crystal system with the chemical composition Pb (Ga, Al, Fe) 3 [(OH) 6 | SO 4 | AsO 4 ] and is therefore chemically a lead - gallium - sulfate - arsenate with additional hydroxide ions (OH - ) .

Gallobeudantite develops isolated rhombohedral crystals up to 1 mm in size, modified by {0001} , which have intense chemical zoning and are pale yellow, greenish or cream colored. Occasionally, disc-shaped crystals come together to form sub-parallel aggregates .

Etymology and history

The type specimen of the Gallobeudantite, which was probably recovered in the Tsumeb Mine around 1960, was in the private collection of Mark N. Feinglos, where it was held for a long time to be fleischerite . As a result of investigations for extensive natural resources gallium - substitution in minerals of the jarosite - alunite family also crystals were examined this stage, which turned out to appropriate analysis as a new mineral. The mineral was recognized by the International Mineralogical Association (IMA) in 1994 and described as gallobeudantite in 1996 by a US-Canadian research team including John Leslie Jambor , DeAlton R. Owens , Joel D. Grice and Mark N. Feinglos . It was named after its crystal-chemical and structural relationship with beudantite and the dominance of gallium in the position that is usually occupied by Fe 3+ in minerals of the jarosite type .

Type material of the mineral (holotype) is kept in the Canadian Museum of Nature , Ottawa in Canada (catalog no. 81518), as well as in the Geological Survey of Canada , Ottawa.

classification

In the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the gallobeudantite belonged to the division of "anhydrous phosphates, with foreign anions F, Cl, O, O", where it was used together with Beudantite , Corkit , Hidalgoit , Hinsdalit , Kemmlitzite , Orpheit , Schlossmacherite , Svanbergite and Woodhouseit form the Beudantite group VII / B.35 .

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 gallobeudantite to the category of “phosphates etc. with additional anions; without H 2 O “. However, this is further subdivided according to the relative size of the cations involved and the molar ratio of the additional anions to the phosphate, arsenate or vanadate complex (RO 4 ), so that the mineral can be classified in the sub-section “With medium-sized and large cations; (OH, etc.): RO 4  "can be found, where it is used together with Beudantit, Corkit, hidalgoite, Hinsdalite, Kemmlitzit, Orpheit, Svanbergit, 1: = 3 Weilerit and woodhouseite the" Beudantitgruppe "with the system number. 8.BL.05 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns gallobeudantite to the class of "phosphates, arsenates and vanadates" and there to the category of "composite phosphates etc. (anhydrous composite anions with hydroxyl or halogen)". Here is in the "Beudantit group" with the system no. 43.04.01 and the other members Beudantit, Corkit, Hidalgoit, Orpheit, Hinsdalit, Svanbergit, Kemmlitzit, Woodhouseit and Weilerit can be found within the subdivision Compound Phosphates etc., (anhydrous compound anions with hydroxyl or halogen) .

Chemism

Gallobeudantite has the measured chemical composition Pb 1.04 (Ga 1.49 Fe 3+ 0.82 Al 0.62 Zn 0.10 Ge 0.02 ) Σ = 3.05 [(AsO 4 ) 1.14 (SO 4 ) 0.86 ] Σ = 2.00 (OH) 5.94 . This formula can be written in simplified form as PbGa 3 (AsO 4 , SO 4 ) 2 (OH) 6 with AsO 4 : SO 4 = 1: 1, which equates to 29.63% PbO; 37.32% Ga 2 O 3 ; 15.25% As 2 O 5 ; Requires 10.63% SO 3 and 7.17% H 2 O. Gallobeudantite is thus another representative of the minerals containing gallium; Of the eight gallium minerals currently known (2016), six have been found in Tsumeb, and they all have their type locality here. For five of them (Gallobeudantit, Galloplumbogummit , Krieselit , Söhngeit and Tsumgallit ) Tsumeb is the only place in the world to date.

Crystal structure

Gallobeudantite crystallizes in the trigonal crystal system in the space group R 3 m (space group no. 160) , with the lattice parameters a  = 7.225  Å and c  = 17.03 Å and three formula units per unit cell . Template: room group / 160

From a structural standpoint crystal Gallobeudantit belongs in the alunite About group and is isostructural with Corkit , wherein the gallium in the crystal structure of iron and arsenic the phosphorus replaced. As in Corkit and Beudantit, the Pb-O polyhedron is an icosahedron , the six Pb-O bond lengths of 2.82 Å being greater than the six Pb-OH bond lengths of 2.78 Å. The gallium polyhedron is a slightly distorted octahedron that forms (001) layers of polyhedra with common corners. The two tetrahedron positions [AsO 4 ] and [SO 4 ] are aligned parallel [001]. The positions are well ordered, as demonstrated by the average bond lengths, which, at 1.70 Å for As-O and 1.55 Å for SO, are in agreement with the bond lengths of other arsenates and sulfates. It is the degree of order of these two tetrahedral positions which determines the lowering of the symmetry from R 3 m to R 3 m . In the new nomenclature of the alunite supergroup, however, the necessity of investigations into the crystal structure refinement is emphasized in order to actually demonstrate a greater degree of order of the tetrahedron positions in gallobeudantite.

properties

morphology

Gallobeudantite forms crystals up to 200 µm in size, which show the rhombohedron {10 1 1} as the supporting form , which is occasionally modified by the surfaces of the basic pinacoid {0001}. The size of the gallobeudantite crystals sitting on scorodite from a second find, however, reaches up to 1 mm. These are disc-shaped crystals, which probably have a very flat rhombohedron as a supporting shape.

physical and chemical properties

The crystals of gallobeudantite are pale yellow, slightly greenish or cream-colored, the line color of the mineral is described as white to pale yellow. The surfaces of the transparent crystals have a glass-like sheen . The mineral has no cleavability , breaks but because of its brittleness like glass or quartz , wherein the fractured surfaces are flat to conchoidal. The plane break can also be an indication of the good cleavage according to {0001}, which other representatives of the Beudantite group show.

With a Mohs hardness of 4, gallobeudantite is one of the medium-hard minerals that, like the reference mineral fluorite, can be easily scratched with a pocket knife. The calculated density of the mineral is 4.61 g / cm³.

Modifications and varieties

Gallobeudantite is the gallium-dominant analogue of the Fe 3+ -dominated beudantite. The crystals show an intensive chemical zoning and contain, among other things, zones from Gallium-rich Beudantite and Hidalgoite. Further zones consist of the Gallium- rich analogues of segnitite , corkite, kintoreit and arsenocrandallite, which have not yet been described . The latter contains up to 12.7 wt.% GeO 2 , which corresponds to about 75% of the possible germanium substitution if one uses a formula with Ca (Ga, Al, Fe) 2 Ge 4+ (AsO 4 ) 2 (OH ) 6 assumes. Most of the <100 µm crystals in their outer zone consist of a Ca-Ga-AsO 4 -beudantite-type analogue.

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 . In addition, only two stages with two different parageneses have been described of this mineral .

Gallobeudantit comes from the oxidation zone in dolomite stones seated hydrothermal polymetallic ore deposit- Tsumeb, the exact location is still unknown. The 3 × 3 × 1.5 cm large type specimen consists of blackish solid copper ore, which is a germanite-like ore with up to 3 mm large renierite inclusions and a 1 mm thick vein from Otjisumeit. The gallobeudantite is found in cavities in the ore , on the surface of the step there are finely distributed and locally hematite and goethite, a few Stolzite crystals, and spongy silver containing mercury. The complete suite of accompanying minerals to the mineral therefore consists of stolzite , silver containing mercury , otjisumeit , beudantite containing gallium, hidalgoite containing gallium, hematite , goethite , renierite, gallite , tennantite containing gallium and chalcosine .

In a second stage , discus-shaped gallobeudantite crystals with a yellowish color are 1 mm in size on a 5 mm scorodite crystal. In addition, the scorodite crystal contained 9.5% Ga 2 O 3 , making it an extremely gallium-rich variety of this mineral.

See also

literature

  • John Leslie Jambor , DeAlton R. Owens, Joel D. Grice, Mark N. Feinglos (1996): Gallobeudantite, PbGa 3 [(AsO 4 ), (SO 4 )] 2 (OH) 6 , a new mineral species from Tsumeb, Namibia, and associated new gallium analogues of the alunite-jarosite family. In: Canadian Mineralogist , Volume 34, pp. 1305-1315 ( PDF, 1.85 MB ).
  • Gallobeudantite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 63 kB )

Web links

Individual evidence

  1. ^ 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.  462 .
  2. ↑ a b c d e f g h i j k l m n o p q John Leslie Jambor , DeAlton R. Owens, Joel D. Grice, Mark N. Feinglos (1996): Gallobeudantite, PbGa 3 [(AsO 4 ), (SO 4 )] 2 (OH) 6 , a new mineral species from Tsumeb, Namibia, and associated new gallium analogues of the alunite-jarosite family. In: Canadian Mineralogist , Volume 34, pp. 1305-1315 ( PDF, 1.85 MB ).
  3. ↑ a b c d e Gallobeudantite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 63 kB )
  4. ^ John Leslie Jambor (1999): Nomenclature of the Alunite supergroup. In: Canadian Mineralogist , Volume 37, pp. 1323-1341 ( PDF, 1.69 MB ).
  5. ↑ a b Peter Bayliss, Uwe Kolitsch, Ernest H. Nickel, Allan Pring (2010): Alunite supergroup: recommended nomenclature. In: Mineralogical Magazine , Volume 74, pp. 919-927 ( PDF, 210 kB ).
  6. ↑ a b c Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 276 and 167 .
  7. ↑ Mindat - Number of localities for Gallobeudantite
  8. ↑ Find location list for gallobeudantite in the Mineralienatlas and in Mindat