Zemannite

from Wikipedia, the free encyclopedia
Zemannite
Zemannite-77637.jpg
Zemannite from the Moctezuma Mine (Bambolla Mine), Moctezuma (Sonora) , Mexico
General and classification
other names

IMA 1968-009

chemical formula Mg 0.5 ZnFe 3+ [TeO 3 ] 3 · 4.5 H 2 O
Mineral class
(and possibly department) 		Oxides and hydroxides (including V 4 + / 5 + vanadates, arsenites, sulfites, selenites, tellurites and iodates)
System no. to Strunz
and to Dana 		4.JM.05 ( 8th edition : IV / K.10)
03/34/02/01
Similar minerals Keystoneite , Kinichilit , Francisite , Quetzalcoatlite
Crystallographic Data
Crystal system hexagonal
Crystal class ; symbol hexagonal dipyramidal; 6 / m
Space group P 6 3 / m (No. 176)Template: room group / 176
Lattice parameters a  = 9.41  Å ; c  = 7.64 Å
Formula units Z  = 2
Frequent crystal faces {10 1 0}, {10 1 1}
Twinning not observed
Physical Properties
Mohs hardness "soft"
Density (g / cm 3 ) 4.05 to 4.36
Cleavage Well
Break ; Tenacity very brittle
colour light brown to dark brown, reddish brown, orange
Line color White
transparency translucent to opaque
shine Diamond luster
Crystal optics
Refractive indices n ω  = 1.850
n ε  = 1.930
Birefringence δ = 0.0800
Optical character uniaxial positive
Pleochroism red-brown to yellow-brown

Zemannite is a very rare mineral from the mineral class of " oxides and hydroxides " with the chemical composition Mg 0.5 ZnFe 3+ [TeO 3 ] 3 · 4.5H 2 O and is therefore a water-containing magnesium - zinc - iron - tellurate .

Zemannite crystallizes in the hexagonal crystal system and usually forms small prismatic crystals that are brown in color.

Etymology and history

Zemannite was first mentioned in 1961, but still without a name, as a possible new mineral from Moctezuma ( Sonora , Mexico ). It was initially a pure zinc - tellurate (IV) held with unclear was whether it is a salt of tellurous acid or Metatellursäure acted. Due to problems in determining the exact chemical formula, the notification of the new mineral to the International Mineralogical Association (IMA) was delayed .

It was not until 1967 that Eckhart Matzat succeeded in determining the chemical composition more precisely on the basis of data from crystal structure analyzes , which from then on was specified as (Na, H) 2 (Zn, Fe 3+ , Mn, Mg) 2 [TeO 3 ] 3 · nH 2 O has been. Two years later, together with SJ Williams, he published the mineral for the first time under the name Zemannite , in recognition of the scientific merits of the Austrian mineralogist Josef Zemann (* 1923), especially in the field of tellurium-containing minerals. In the same year, it was recognized by the IMA as a new mineral called zemannite and the revised chemical formula.

Later investigations showed that zemannite , like the related kinichilite , only contains traces of sodium and mainly magnesium . The formula was then corrected again and corresponds to the current form: Mg 0.5 ZnFe 3+ [TeO 3 ] 3 · 4.5H 2 O.

The type locality is Mina La Bomballa in Moctezuma, Sonora, Mexico.

classification

Already in the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the zemannite belonged to the mineral class of the "oxides and hydroxides (V 4 + / 5 + vanadates, arsenites, sulfites, selenites, tellurites and iodates)" Department of " Sulphites , Selenites and Tellurites ", where together with Francisite , Keystoneit , Kinichilit and Quetzalcoatlite it formed the unnamed group IV / K.10 .

The valid since 2001 and by the International Mineralogical Association used (IMA) 9th edition of Strunz'schen systematic mineral also assigns the zemannite in the class of "oxides and hydroxides," there, however, in the extended section of the "Arsenites, Antimonite , Bismuthite , Sulfites, selenites and tellurites ”. This is further subdivided according to the possible presence of further anions and water of crystallization , so that the mineral can be classified in the sub-section “Tellurite without additional anions; with H 2 O ”, where only together with Keystoneit and Kinichilit the“ Zemannite group ”named after him with the system no. 4.JM.05 forms.

In contrast to Strunz's systematics, the systematics of minerals according to Dana , which is mainly used in the English-speaking world, assigns zemannite to the class of “ sulfates , chromates and molybdates ” and there in the category of “selenites, tellurites and sulfites”. Here he is also the namesake of the "Zemannite group" with the system no. 03/34/02 and the other members Kinichilit and Keystoneit can be found within the subdivision of " Selenite - Tellurite - Sulphite ".

Crystal structure

Crystallographic Data
Crystal structure of zemannite
Crystal structure of zemannite
Crystal system hexagonal
Room group (no.) P 6 3 / m (No. 176)Template: room group / 176
Lattice parameters a  = 9.41  Å
c  = 7.64 Å
Formula units Z  = 2

Zemannite crystallizes in the hexagonal crystal system in the space group P 6 3 / m (space group no. 176) with the lattice parameters a  = 9.41  Å and c  = 7.64 Å as well as two formula units per unit cell . Template: room group / 176

The Te 4+ cations form complex [TeO 3 ] 2− anions with three oxygen atoms . The oxygen atoms form the base of a trigonal pyramid with the tellurium cation at the top. This arrangement can also be viewed as a tetrahedron , with the free, non-bonding electron pair of the Te 4+ cation pointing into the only unoccupied corner of the tetrahedron (see also VSEPR model ).

The Zn 2+ and Fe 3+ cations share the same position in the crystal structure, whereby this is statistically 60% occupied by iron and 40% by zinc, but the values ​​can also assume different proportions for different crystals. If the structure also contains traces of Mn 2+ ions, these are also in the zinc / iron position. The zinc / iron cations are surrounded by six oxygen atoms in the form of distorted octahedra , the oxygen atoms are also part of the [TeO 3 ] 2− anions. The [(Fe, Zn) O] 6 octahedra and [TeO 3 ] 2− anions thereby form a three-dimensional network with large channels parallel to the crystallographic c axis. Therefore the structure of the zemannite is often described as " zeolite-like ".

The Mg 2+ cations are also octahedral surrounded by six water molecules each. The [Mg (H 2 O) 6 ] 2+ - complexes are located in the above described channels of the network and are used to balance the charge. The positions of the Mg 2+ cations in the crystal structure are only half occupied, which is also indicated in the chemical formula of zemannite (“Mg 0.5 ”).

morphology

Hexagonal combination prism and pyramid.png
Hexagonal combination prism pyramid Basis.png


Zemannite crystal forms

Zemannite forms needle-shaped to prismatic single crystals that usually only reach a length of a few millimeters. Since the crystals of the secondary mineral can usually grow undisturbed on rock surfaces, they are mostly idiomorphic and reflect the symmetry of the crystal class 6 / m (hexagonal-dipyramidal). Zemannite crystals are characterized by a straight hexagonal prism, which is often closed by hexagonal pyramids over the two base surfaces of the prisms. The pyramid surfaces are only hinted at with some crystals, the tip of the pyramids is missing. Models of the ideal crystal shape and the shape with incompletely developed pyramidal faces are shown in the figure on the right.

Education and Locations

Zemannite is a secondary mineral selected from the weathering of solid tellurium or telluridhaltiger minerals, for example Sylvanit or Calaverit arises. The elemental tellurium or telluride anions (Te 2- or Te 2 2- ) are about to Te 4+ - cations oxidized with the oxide ions stable Oxotellurat (IV) ions ( tellurite ions ) [TeO 3 ] 2− form. In addition to the elements mentioned in the formula, zemannite can also contain traces of manganese and of alkali and alkaline earth metals .

Accompanying minerals ( parageneses ) of zemannite are often the two naturally occurring modifications of tellurium dioxide (TeO 2 ), tellurite and paratellurite , and other Te 4+ -containing minerals as well as native tellurium.

In addition to the Mina La Bambolla ( Mina Moctezuma ), the type locality near Moctezuma, the Kawazu mine near Shimoda (Japan) is another place where Zemannite was found.

use

Due to its rarity and the often microscopic crystals, zemannite has no technical significance and is only traded as a collector's item.

See also

Literature and Sources

  • RV Gaines: The Moctezuma tellurium deposit . In: Mineralogical Record . tape 1 , no. 2 , 1970, p. 40-43 .
  • Joseph Anthony Mandarino , E. Matzat, SJ Wiliams: Zemannite, a zinc tellurite from Moctezuma, Sonora, Mexico . In: Canadian Mineralogist . tape 14 , 1976, p. 387–390 ( rruff.info [PDF; 280 kB ; accessed on October 3, 2017]).
  • E. Matzat: The crystal structure of an unnamed zeo Tellurminerals, {(Zn, Fe) 2 [TeO 3 ] 3 } Na x H 2-x  • n H 2 O . In: Tschermaks Mineralogische und Petrologische Mitteilungen . tape XII , 1967, p. 108-117 .
  • R. Miletich: Crystal chemistry of the microporous tellurite minerals zemannite and kinchilite, Mg 0.5 [Me 2+ Fe 3+ (TeO 3 ) 3 ] • 4.5 H 2 O, (Me 2+ = Zn; Mn) . In: European Journal of Mineralogy . tape 7 , 1995, p. 509-523 .
  • Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .

Web links

Commons : Zemannite  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d e f R. Miletich: Crystal chemistry of the microporous tellurite minerals zemannite and kinchilite, Mg 0.5 [Me 2+ Fe 3+ (TeO 3 ) 3 ] • 4.5 H 2 O, (Me 2+ = Zn; Mn) . In: European Journal of Mineralogy . tape 7 , 1995, p. 509-523 .
  2. ↑ a b Zemannite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 66  kB ; accessed on October 3, 2017]).
  3. ^ Joseph Anthony Mandarino , JS Williams: Five new minerals from Moctezuma, Sonora, Mexico . tape  133 , 1961, pp. 2017 .
  4. ↑ E. Matzat: The crystal structure of an unnamed zeo Tellurminerals, {(Zn, Fe) 2 [TeO 3 ] 3 } Na x H 2-x  • n H 2 O . In: Tschermaks Mineralogische und Petrologische Mitteilungen . tape XII , 1967, p. 108-117 .
  5. ^ Joseph Anthony Mandarino , E. Matzat, SJ Wiliams: Zemannite, a new tellurite mineral from Moctezuma, Sonora, Mexico . In: Canadian Mineralogist . tape  10 , 1969, p. 139–140 ( rruff.info [PDF; 156 kB ; accessed on October 3, 2017] abstract).
  6. ^ H. Hori, E. Koyama, K. Nagashima: Kinchilite, an new mineral from the Kawazu mine, Shimoda city, Japan . In: Mineralogical Journal . tape 13 , 1981, pp. 333-337 .
This version was added to the list of articles worth reading on April 7, 2008 .