Braunstein (mineral group)

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Braunstein is the collective name for manganese - minerals and synthetically produced manganese oxides having an approximate composition of MnO 1.7 to MnO 2 . Brown stones come in very different forms and with different properties. Because of their poor crystallinity, the exact structure of many brown stones is unknown. The name Braunstein comes from the Middle Ages. It is due to the brown color obtained when glazing pottery with manganese oxides.

Occurrence

Brown stones are widespread and are mined in many countries. The majority of the reserves, however, consist of ores with low to medium manganese content (defined as <44% Mn), which must be concentrated before use. South Africa , which has the largest known reserves, Brazil , Australia and Gabon are currently (2005) the main exporters of high manganese ores. In the former USSR ( Ukraine , Georgia ), India and Ghana , the high-grade deposits are largely exhausted. These countries, which used to be among the most important producing countries, are therefore only of minor importance as exporters of ores with a low Mn content. The deposits in China and the USA consist mainly of ores with a low Mn content. With the exception of Greece, there are no significant deposits in Europe . In Germany , manganese ores used to be mined near Gießen , Waldalgesheim and in the southern Harz near Ilfeld .

Modifications and structures of brownstone

The basic building block for the crystal structure of brownstones is the MnO 6 - octahedron in which the central manganese atom is surrounded by six oxygen atoms. By regularly linking these octahedra via edges and / or corners, as with the tetrahedron linking of silicates , different structures can be formed. A distinction is made between the chain structures ( '' ino- manganates ''), network or tunnel structures ( '' Tekto-manganates '') and layer structures ( '' phyllo manganates ''). In these structures, tetravalent manganese can also be replaced by trivalent or bivalent manganese. In addition, foreign cations as well as water and hydroxide ions can be incorporated. This results in the complex structure and diverse appearance of brown stones.

α-MnO 2

Grubig trained cryptomelan from Morocco

α-Brownstone with a tetragonal crystal structure consists of double chains of edge- sharing MnO 6 octahedra. The double chains are connected to neighboring double chains via common octahedron corners. The double chains are oriented perpendicular to each other and thus result in a three-dimensional framework with a (2x2) tunnel structure, which is stabilized by the incorporation of foreign cations during crystallization. Because of the charge neutrality, defects or manganese ions with an oxidation number lower than +4 must then be present in the lattice. In addition, water can be stored. The composition of α-manganese dioxide can be described as X N (Mn 3+ , Mn 4+ ) 8 O 16 x H 2 O. α-Mangestone can be produced synthetically. The manganese omelans, manganese minerals with a related structure and embedded foreign cations, are also often counted as α-brownstone. In Kryptomelan K + ions incorporated in manjiroite Na + ions, in Coronadit Pb 2+ ions and hollandite Ba 2+ ions. Psilomelan is related to Hollandit and contains Ba 2+ ions and water for stabilization .

β-MnO 2

Crystalline pyrolusite aggregate from New Mexico, USA

β-brownstone with a tetragonal crystal structure consists of slightly distorted MnO 6 octahedra, which are linked by common edges to form long chains. Its lattice structure corresponds to the rutile type (TiO 2 ), that is, in a corrugated hexagonal close packing of the O 2− anions every second octahedral gap is occupied by an Mn 4+ cation. The chains are linked to one another via the corners of the octahedra (that is, shared oxygen atoms). This results in a (1x1) tunnel structure with square cavities between octahedra. The tunnels are not big enough to store foreign ions. β-Mangestone can be produced synthetically and occurs naturally as the mineral pyrolusite , which is relatively well crystallized and practically anhydrous. In anhydrous form, it is a stoichiometric manganese dioxide with the empirical formula MnO 2 . The name pyrolusite is derived from the Greek words πυρο “pyro” (fire) and “louein” (to wash), as this mineral was used in ancient times to discolor green glasses.

R-MnO 2 (Ramsdellite)

Ramsdellite in Arizona, USA

In Ramsdellite , with an orthorhombic crystal structure , two MnO 6 octahedral chains are linked to double chains via common edges. These double chains are connected to the neighboring chains via octahedral corners (that is, shared oxygen atoms). The result is a (1x2) tunnel structure with rectangular cavities. The tunnels are usually empty, but they can contain small amounts of Na + and Ca 2+ ions and water. Ramsdellite has the same structure as goethite (FeO (OH)) and gibbsite (Al (OH) 3 ). In contrast to pyrolusite, the mineral ramsdellite occurs only very rarely in nature. It is named after the American mineralogist Lewis Stephen Ramsdell.

γ-MnO 2

γ-brownstone is actually not a modification of its own, but an intergrowth of pyrolusite and ramsdellite - that is, on a microscopic scale areas with a pyrolusite and ramsdellite structure alternate. The structure is even more complicated, since Mn 3+ ions can be built into the lattice instead of Mn 4+ ions , cations can be faulty and H 2 O or OH - ions can be built into the structure. Therefore, γ-MnO 2 is often described by the formula Mn (IV) 1-x Mn (III) x O 2-x OH x . γ-Brownstone can be produced synthetically and occurs naturally as the mineral nsutite . This is named after the large deposits of this mineral near Nsuta in Ghana .

δ-MnO 2

Birnessite / Ranciéit pseudomorphism, presumably after Serandite . The olive-green "hedgehogs" are made of aegirine and some small, colorless droplets of opal can be seen on the white microcline crystals

δ-brownstone consists of layers of corner- sharing MnO 6 octahedra. Since naturally occurring δ-brownstones crystallize poorly, their structure is largely unknown. The naturally occurring minerals birnessite [Na, Ca, Mn (II)] Mn 7 O 14 2.8H 2 O] and vernadite MnO 2 xH 2 O belong to this group. Vernadites typically contain small additions of foreign ions (K + , Mg 2+ , Ca 2+ , Ba 2+ and Fe 2+ ) and 15-25% water. Birnessite is the most common naturally occurring mineral with a layer structure of MnO 6 octahedra. The layer spacing is approximately 7.3 Å along the c-axis. Cations and water are embedded between the layers in birnessite. The mineral is named after the location Birness ( Aberdeenshire ) in Scotland . δ-brown stones can also be produced synthetically.

ε-MnO 2

ε-manganese dioxide consists of a hexagonal close packing of O 2− anions. In this half of the octahedral holes are occupied by Mn 4+ cations in a random distribution.

use

Manganese raw material

Brown stones are the most important raw material for the production of metallic manganese and manganese compounds such as manganese chloride , manganese carbonate , manganese sulfate and potassium permanganate . In addition, brown stones are also used directly for the production of ferromanganese and other manganese alloys .

Cells and batteries

Brownstone or manganese dioxide are among the most important cathode materials for batteries . The reason lies in the combination of physical and electrochemical properties with good environmental compatibility and a relatively low price. The electrochemical activity of brown stones in batteries depends on various factors, for example the structure, the specific surface area , the porosity and the purity of the material . Brown stones are used in both zinc-carbon and alkaline-manganese batteries and in anhydrous form in lithium batteries . In aqueous systems, γ-manganese dioxide has the highest electrochemical activity and is therefore used almost exclusively. In the past, high-quality or processed natural brown stone ( NMD , the abbreviation of the English name natural manganese dioxide ) was often used. Due to increased demands on the activity and purity of the material, only synthetic brown stones are used today. This can be chemically produced brownstone ( CMD , the abbreviation of the English name chemical manganese dioxide ) or electrolyte brownstone ( EMD , the abbreviation of the English name electrolytic manganese dioxide ) produced by electrochemical MnO 2 deposition from Mn 2+ solutions .

Ferrites

Ferrites are electrically non-conductive ferromagnetic materials with the general formula M [Fe 2 O 4 ], where M stands for a divalent metal cation. Natural and synthetic manganese dioxide are used as the starting material for the production of soft magnetic ferrites (with M = Zn 2+ and Mn 2+ ).

Oxidizing agent

Brown stones are good oxidizing agents and are used for inorganic and organic reactions in the laboratory and on an industrial scale. Oxidations with brown stones are technically important in the dye and fragrance industry. The most important example is the oxidation of aniline to hydroquinone . Manganese chloride and elemental chlorine are formed by the action of hot hydrochloric acid on manganese dioxide . This reaction (also called the Weldon process ) was previously used to produce chlorine.

Glass and ceramics industry

Brown stones have been used to discolor glass since ancient times ("glassmaker's soap"). In the 16th century, the traveling whales or Venetians in particular procured the brown stones they needed for the glass factories in Venice ( Murano ). They are also used in the production of brown glazes on pottery and in the coloring of clinker bricks . The inexpensive natural brown stones are usually used for these applications.

catalyst

Mangestone is occasionally used as a catalyst , for example in the Walter drive to break down hydrogen peroxide into water vapor and oxygen .

See also

literature

  • Eberhard Preisler: Modern processes in large-scale chemistry: Braunstein . Chemistry in our time 14 (5), pp 137-148 (1980), Wiley-VCH Verlag GmbH, ISSN  0009-2851
  • Qi Feng, Hirofumi Kanoh and Kenta Ooi: Manganese oxide porous crystals . Journal of Materials Chemistry 9, pp. 319-333 (1999), Royal Society of Chemistry Publishing, ISSN  0959-9428 (review article in English)
  • Jeffrey E. Post: Manganese oxide minerals: Crystal structure and economic and environmental significance . Proceedings of the National Academy of Sciences of the United States 96 (7), pp. 3447-3454 (1999), National Academy of Sciences, ISSN  1091-6490 (review article in English)
  • Heinz Cassebaum: The position of the Braunstein investigations by JH Pott (1692–1777) in the history of manganese , Sudhoffs Archiv 63 (1979), pp. 136–153

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. 245 .