Ekatite

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Ekatite
General and classification
other names

IMA 1998-024

chemical formula (Fe 3+ , Fe 2+ , Zn) 12 (OH) 6 [AsO 3 ] 6 [AsO 3 , HOSiO 3 ] 2
Mineral class
(and possibly department) 		Oxides and hydroxides
System no. to Strunz
and to Dana 		4.JB.75 ( 8th edition : IV / J.04)
04/43/10/02
Crystallographic Data
Crystal system hexagonal
Crystal class ; symbol dihexagonal-pyramidal; 6 mm
Space group P 6 3 mc (No. 186)Template: room group / 186
Lattice parameters a  = 12.773  Å ; c  = 5.051 Å
Formula units Z  = 1
Physical Properties
Mohs hardness ≈ 3
Density (g / cm 3 ) 4.061 (calculated)
Cleavage no
Break ; Tenacity brittle; not specified
colour brownish black
Line color brown
transparency translucent
shine Glass luster, diamond luster
Crystal optics
Refractive indices n ω  = ≈ 1.99
n ε  = ≈ 2.08
Refractive index n  = 2.02 (measured); 2.013 (calculated)
Birefringence δ = 0.09
Optical character uniaxial positive
Pleochroism strong from ω = dark brownish black to ε = medium brown, both with a greenish tinge

Ekatite is a very rarely occurring mineral from the mineral class of " oxides and hydroxides ". It crystallizes in the hexagonal crystal system with the chemical formula (Fe 3+ , Fe 2+ , Zn) 12 (OH) 6 [AsO 3 ] 6 [AsO 3 , HOSiO 3 ] 2 , so it is chemically an iron - zinc - arsenite - Silicate with additional hydroxide ions .

Ekatite forms cluster and sheaf-shaped aggregates of striped, delicate, needle-like crystals. The mineral was found in a matrix of etched quartz and chalcosine in the Tsumeb Mine , Namibia.

Etymology and history

The mineral collectors Michael Grieser , Mannheim , and Herbert Nägele , Windhoek, drew attention to the presumably new mineral and made research material available . Corresponding investigations led to the determination of the presence of a new mineral, which was recognized in 1998 by the International Mineralogical Association (IMA) under the number "IMA 1998-024" and in 2001 by Paul Keller from the University of Stuttgart in the science magazine "European Journal of Mineralogy" as Ekatite has been described. The mineral was named after the mining engineer and former owner of the lithium pegmatite of the "Rubicon Mine" in Namibia Dieter Ekat (1935-1996).

Type material of the mineral is in the archive of the University of Stuttgart in the "Mineralogical Collection of Professor Keller" (register no. TM-98.24-Gro19 at location 0/824-s27 / 2, holotype).

classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the ekatite belonged to the mineral class of "oxides and hydroxides" and there to the department of "arsenites (with As 3+ )", where together with armangite , cafarsite , magnussonite , Tooeleit and Zimbabweit formed the unnamed group IV / J.04 .

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 the ekatite to the class of "oxides and hydroxides", but in the newly defined division of "arsenites, antimonites, Bismutite, Sulphite, Selenite and Tellurite ”. 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 sub-section “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.75 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Ekatite to the class of "phosphates, arsenates and vanadates" and there in the department of " phosphates ". Here it can only be found together with Kraisslit in the unnamed group 43.04.10 within the subdivision of “ Compound phosphates etc., (anhydrous compound anions with hydroxyl or halogen) ”.

Chemism

Wavelength-dispersive element analyzes and microprobe analyzes on ekatite yielded mean values ​​of 27.26% Fe 2 O 3 ; 21.19% FeO; 3.80% ZnO; 42.56% As 2 O 3 ; 2.12% SiO 2 and 3.42% H 2 O. On the basis of 20 cations, the empirical formula (Fe 3+ 6.02 Fe 2+ 5.20 Zn 0.82 ) Σ = 12.04 was calculated (OH) 6.07 (AsO 3 ) 6.06 [(AsO 3 ) 1.52 (HOSiO 3 ) 0.62 ] Σ = 2.14 , resulting in (Fe 3+ , Fe 2+ , Zn) 12 ( OH) 6 [AsO 3 ] 6 [AsO 3 , HOSiO 3 ] 2 was idealized. This composition requires contents of 27.45% Fe 2 O 3 ; 21.41% FeO; 3.73% ZnO; 41.94% As 2 O 3 ; 2.07% SiO 2 and 3.40% H 2 O.

Crystal structure

Ekatite crystallizes hexagonally in the space group P 6 3 mc (space group no.186) with the lattice parameters a  = 12.773  Å and c  = 5.051 Å as well as one formula unit per unit cell . Template: room group / 186

The crystal structure of the ekatite contains dimers of (Fe 3+ , Fe 2+ , Zn) O 4 (OH) 2 - octahedra with common surfaces, which are connected to each other by common edges so that endless double chains are created. Furthermore, they are linked via common corners in such a way that a three-dimensional framework is created which contains two types of channels parallel to the c-axis [001]. The larger of these channels is hexagonal and includes six AsO 3 groups, which are arranged so that the lone electron pairs (English: lone pairs) of As 3+ point in the direction of its center. Trigonale AsO 3 - pyramids and HOSiO 3 - tetrahedron take in a random distribution, a smaller channels of triangular cross section, wherein either the lone pairs or (OH) groups in the direction of c-axis [001] oriented.

Ekatite represents a new structure type, but has structural affinities with phosphoellenbergerite , ellenbergerite and holtedahlite , various tellurites , selenites and phosphites of the transition metals as well as the synthetic compound Zn 7 (OH) 3 (SO 4 ) (VO 4 ) 3 . Tightly packed zigzag double chains of M (1) O 4 (OH) 2 octahedral dimers are typical of all structures related to ekatite.

properties

morphology

Ekatite forms tufts and sheaves, e.g. Sometimes even radial aggregates of delicate, needle-like, typically striped crystals. These are stretched along the c-axis [001] and show indistinct (hk0) shapes, but no end faces. The crystals reach lengths of up to 2 mm, but only have a diameter of less than 0.2 mm. Sheaf-shaped ekatite aggregates with needle-like crystals up to 1.5 cm in length became known later.

physical and chemical properties

Ekatite crystals are brownish-black in color, but their streak color is always brown. The surfaces of the translucent crystals show a strong glass-like sheen , with the high values ​​for light refraction indicating a diamond sheen . Ekatite has a high light refraction and a very high birefringence (δ = 0.09). Its strong pleochroism from ω = dark brownish-black to ε = medium brown, both with a greenish tinge, is also very characteristic. Although the colors are not typical for it, an Fe 3+ -Fe 2+ charge transfer between the octahedra with common surfaces is held responsible for the pleochroism .

Education and Locations

Ekatite occurs as a typical secondary formation in corroded ores in the oxidation zone of a complex Cu-Pb-Zn deposit in carbonate rocks. Iron, zinc and arsenic come from the decomposition of primary sulfidic ore minerals such as sphalerite and tennantite . The ekatite crystals have grown on or in a matrix of etched quartz and chalcosine . No other accompanying minerals were identified.

As a very rare mineral formation, Ekatite could only be described from one source 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. But the steps described in the type Publication were both from the same dealer, after which they have the deep soles are to be found the deposit. This agrees with the occurrence of other minerals containing As 3+ from the “Second Oxidation Zone” of the “Tsumeb Mine” such as Leiteit , Schneiderhöhnite , Ludlockite , Claudetite and Arsenolite .

use

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

See also

literature

  • Paul Keller: Ekatite, (Fe 3+ , Fe 2+ , Zn) 12 (OH) 6 [AsO 3 ] 6 [AsO 3 , HOSiO 3 ] 2 , a new mineral from Tsumeb, Namibia, and its crystal structure . In: European Journal of Mineralogy . tape 13 , 2001, p. 767-777 , doi : 10.1127 / 0935-1221 / 2001 / 0013-0769 ( rruff.info [PDF; 829 kB ]).
  • Paul Keller, Falk Lissner, Thomas Schleid: (Fe 3+ , Fe 2+ ) 6 (OH, O) 3 [AsO 3 ] 3 [AsO 3 , HOSiO 3 ]: Ekatite, an iron (III / II) hydroxide containing hydrogen silicate -Oxo arsenate (III) . In: Zeitschrift Kristallographie, Suppl. Volume 16 , 1999, p. 84 .

Web links

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o p q Paul Keller: Ekatite, (Fe 3+ , Fe 2+ , Zn) 12 (OH) 6 [AsO 3 ] 6 [AsO 3 , HOSiO 3 ] 2 , a new mineral from Tsumeb, Namibia, and its crystal structure . In: European Journal of Mineralogy . tape 13 , 2001, p. 767-777 , doi : 10.1127 / 0935-1221 / 2001 / 0013-0769 ( rruff.info [PDF; 829 kB ]).
  2. ↑ a b c Joseph A. Mandarino: New Minerals . In: The Canadian Mineralogist . tape 40 , 2002, p. 251 , doi : 10.2113 / gscanmin.40.1.247 ( rruff.info [PDF; 108 kB ]).
  3. ^ Type mineral catalog Germany - storage of the holotype stage Ekatite
  4. ↑ Catalog of Type Mineral Specimens - E. (PDF 40 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, accessed August 29, 2019 .
  5. ^ Ludi von Bezing, Rainer Bode, Steffen Jahn: Namibia . 1st edition. Doris Bode Verlag, Haltern 2007, ISBN 978-3-925094-88-0 , p. 526 .
  6. ↑ Mindat - Number of localities for Ekatite
  7. ↑ Find location list for Ekatite at the Mineralienatlas and at Mindat
  8. ↑ Paul Keller: Tsumeb / Namibia - one of the most spectacular mineral discovery sites on earth . In: Lapis . 9 (issue 7/8), 1984, p. 13-63 .
  9. ↑ Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 1-328 .
  10. ^ Gerhard Söhnge: Tsumeb: a historical scetch (Scientific research in South West Africa (5th series)) . 1st edition. Publisher of the SWA Scientific Society, Windhoek 1967, p. 1-92 .