Zemannite

• IMA 1968-009

Zemannit is a very rarely occurring mineral from the mineral class of " oxides and hydroxides " with the chemical composition Mg0, 5ZnFe3 [ TeO3 ] 3 · 4.5 H2O. It crystallizes in the hexagonal crystal system, and generally is small prismatic crystals having a brown color.

Etymology and history

Zemannit was the first time in 1961, but still without a name, mentioned as a possible new mineral from Moctezuma ( Sonora, Mexico). It was initially thought of as a zinc - tellurate (IV ), where it was not clear whether it was a salt of the acid or the tellurous Metatellursäure. Because of the problems in determining the exact chemical formula, the release of the new Minerals to the International Mineralogical Association (IMA ) was delayed.

In 1967 succeeded Eckhart Matzat on the basis of data from crystal structure analysis a more accurate determination of the chemical composition from now on as (Na, H) 2 (Zn, Fe3 , Mn, Mg ) 2 [ TeO3 ] was 3 x given n H2O. Two years later he published together with SJ Williams, the mineral first time under the name Zemannit, in recognition of the scientific achievements of the Austrian mineralogist Zemann Josef (* 1923), particularly in the area of ​​tellurium minerals. In the same year it was recognized as a new mineral named Zemannit and revised chemical formula of the IMA.

Later studies showed that Zemannit, as well as the related Kinichilit, contains only traces of sodium and magnesium mainly, the formula was then corrected again, and corresponds to the present form: Mg0, 5ZnFe3 [ TeO3 ] 3 · 4.5 H2O.

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

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the Zemannit belonged to the mineral class of " oxides and hydroxides " and then to the Department of " sulphites, Selenite and Tellurite " where he along with Francisit, Keystoneit, Kinichilit and Quetzalcoatlit an independent group formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Zemannit also in the class of " oxides and hydroxides (V [5,6 ] vanadates, arsenides, antimonides, Bismuthide, Suldide, selenides, tellurides, iodides ) ", but there in the advanced division of the " a Antimonite, Bismuthite, sulfites, Selenite and Tellurite ". This department is also further divided according to the possible presence of other anions and crystal water, so that the mineral according to its composition in the subsection " Tellurite without additional anions, with H2O" is to find where it only together with Keystoneit and Kinichilit after him named " Zemannitgruppe " with the system no. 4.JM.05 forms.

In contrast to the Strunz'schen classifications predominantly common in English-speaking classification of minerals according to Dana assigns the Zemannit in the class of " sulfates, chromates and molybdates " and there in the department of " Selenite, Tellurite and sulphites " one. He is also named as the " Zemannitgruppe " with the system no. " Sulfites Selenite - - Tellurite " to find 34.03.02 and the other members Kinichilit and Keystoneit within the subdivision of.

Education and Locations

Zemannit is a secondary mineral that arises from the weathering of native tellurium or telluridhaltiger minerals, for example sylvanite or Calaverit. The elemental tellurium or telluride anions ( Te2 or TE22 ) are oxidized to Te4 cations, which together with oxide ions stable Oxotellurat (IV ) ions ( tellurite ions) [ TeO3 ] 2 - form. Zemannit, in addition to the elements mentioned in the formula, also contain traces of manganese as well as alkali and alkaline earth metals.

Accompanying minerals ( mineral assemblages ) of Zemannit are often the two modifications occurring in the nature of Tellurdioxids ( TeO2 ), tellurite and paratellurite, and other Te4 minerals containing tellurium and dignified.

In addition to the La Bambolla Mine ( Moctezuma mine ) the type locality Moctezuma the Kawazu mine is in Shimoda (Japan) another locality of Zemannit.

Morphology

Zemannit forms acicular to prismatic crystals, usually reach a length of only a few millimeters. Since the crystals of the secondary mineral usually undisturbed can grow on rock surfaces, they are usually idiomorphic trained and reflect the symmetry of the crystal class 6 / m ( hexagonal dipyramidal ) resist. Characteristic crystals Zemannit is a straight hexagonal prism that is often cut off from the hexagonal pyramids on both base surfaces of the prisms. However, the pyramid faces are at some crystals only indicated, the apex of the pyramids is missing. Models of the ideal crystal form and the form with incompletely developed pyramid faces are shown in the right figure.

Crystal structure

Zemannit crystallizes in the hexagonal crystal system, space group P63 / m with lattice parameters a = 9.41 Å and c = 7.64 Å and two formula units per unit cell.

The Te4 cations form with three oxygen atoms complex [ TeO3 ] 2 - anions. The oxygen atoms form the basal plane of a trigonal pyramid, at the head of the Tellurkation is. This arrangement can also be viewed as a tetrahedron, with the free, non- bonding electron pair of the Te4 cation in the only unoccupied corner of the tetrahedron shows (see also VSEPR model).

The Zn2 - and Fe3 cations share the same position in the crystal structure, which is statistically occupied to 60% iron and 40% zinc, the values ​​can, however, take other relationships in different crystals. Contains the structure traces of Mn2 ions, these are also located on the zinc / iron position. The zinc / iron cations are surrounded by six oxygen atoms in the form of distorted octahedrons, the oxygen atoms are at the same time part of the [ TeO3 ] 2 - anions. The [ (Fe, Zn) O] 6 octahedra and [ TeO3 ] 2 - anions, thereby forming a three dimensional network with wide channels parallel to the crystallographic c axis. Therefore, the structure of the Zemannits is often described as " zeolithartig ".

The Mg2 cations are also surrounded by six water molecules octahedrally. The [Mg (H2O ) 6] 2 complexes are located in the above-mentioned channels of the network and are used to balance the charge. The positions of the Mg2 cations in the crystal structure are occupied only half of what is already implies in the chemical formula of Zemannit ( " Mg0, 5").

Use

Due to the rarity and the often microscopic crystals Zemannit has no technical meaning and is exclusively traded as collector's item.