Anatase

• Octahedrite
• Octahedral Schörl

Anatase (also octahedrite or octahedral schorl ) is a commonly occurring mineral from the mineral class of " oxides and hydroxides ". It crystallizes in the tetragonal crystal system with the chemical composition of TiO2 and usually develops dipyramidal and tabular crystals of a few millimeters to several centimeters in size, vary in color from black to gray, brown, reddish- brown and blue. The colors are based on contamination with impurities; pure anatase is colorless, but is only very rarely naturally.

• 6.1 as a pigment
• 6.2 as a gemstone

Etymology and history

Anatase was named after the Greek word ἀνάτασις [ anátasis ] for expansion, stretching or even the rise of routes, as most of the found, dipyramidalen crystals in contrast to other -oriented tetragonal minerals show an above-average length in the direction of the Z axis.

For the first time blue anatase 1783 by Jacques Louis de Bournon was mentioned in a letter to Jean -Baptiste de L' Isle Romé as indigo blue tourmaline. In other anatase varieties have been described and named differently by different scientists. So described Horace - Bénédict de Saussure Octaèdrit, Jean -Claude Delamétherie Oisanit. 1801 examined René -Just Haiiy first colorless anatase, which was found in Le Bourg d'Oisans in the French department of Isère, and recognized the possibility that it is in the various previously known minerals to varieties of the same mineral, which he called anatase.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the anatase belonged to the mineral class of " oxides and hydroxides " and then to the Department of " oxides with the molar ratio of metal: oxygen = 1:2 ", where he along with Downeyit 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 anatase also in the class of " oxides and hydroxides " and there in the department of " Metal: Oxygen = 1: 2 and comparable " one. This division, however, is further divided according to the size of the cations involved as well as the crystal structure, so that the mineral according to its composition in the subsection "With medium-sized cations: structure of edge-sharing octahedra " is to find where it is the only member of the unnamed group 4.DD .05 forms.

The most common in English-speaking classification of minerals according to Dana assigns the anatase in the class of " oxides and hydroxides " and there in the department of " oxides ". Here he is the only member of the unnamed group 04:04:04 within the subdivision "simple oxides with a cation charge of 4 ( AO2 ) ".

Modifications and varieties

Anatase, brookite and rutile in addition to a modification of titanium dioxide. From about 915 ° C, it transforms into rutile.

Education and Locations

Anatase normally forms secondarily by conversion of other titanium-containing minerals in hydrothermal veins and fissures in granite, mica schist, gneiss and diorite, but can also occur in volcanic and metamorphic rocks. Accompanying minerals are next to brookite and rutile or titanite, ilmenite, titaniferous magnetite, hematite and quartz. In addition, anatase is in the form of pseudomorphs after titanite and ilmenite, and the mineral transforms itself into rutile, also pseudomorphs of rutile after anatase.

In Germany, anatase Among other findings in the Black Forest (Baden- Württemberg), in the Bavarian Fichtelgebirge and the Swabian-Franconian, Bavarian and Upper Palatinate Forest, in Hesse, in the Harz (Lower Saxony, Saxony -Anhalt, Thuringia ), the Eifel region (North Rhine -Westphalia, Rhineland -Palatinate ), in Taunus, in Saarland, in the Saxon Erzgebirge, in Schleswig -Holstein and the Thuringian Forest.

In Austria, the mineral occurred mainly in the regions Carinthia, Lower Austria, Salzburg, Styria, Tyrol, Upper Austria and Vienna to light and in Switzerland it was found among other things in the cantons of Bern, Glarus, Graubünden, Ticino, Uri and Valais.

Other localities are Andorra, Antarctica, Argentina, Armenia, Ethiopia, several regions in Australia, Belgium, Bolivia, Brazil, Bulgaria, Chile, China, Cote d'Ivoire, Finland, many regions in France, Cambodia, Cameroon, Greece, Greenland, Guyana, India, Indonesia, Iraq, Ireland, many regions in Italy, Japan, several regions in Canada, the channel Island of jersey, Kazakhstan, Madagascar, Malawi, Morocco, Mexico, Mongolia, Namibia, New Zealand, Niger, North Korea, several regions in Norway, Pakistan, Peru, Poland, Portugal, Reunion, Romania, several regions in Russia, Sweden, Slovakia, Spain, South Africa, Sudan, Turkey, Uganda, Ukraine, Uzbekistan, in several regions of the United Kingdom ( UK), and in many regions of the United States of America (USA).

Anatase crystals can be produced artificially by means of CTR method (chemical transport reactions ).

Crystal structure

Anatase crystallized tetragonal in the space group I41/amd with the lattice parameters a = 3.78 Å and c = 9.51 Å and four formula units per unit cell.

Crystal optics

Anatase tetragonal crystal will be described in accordance with the Neumann principle generally as an optically uniaxial material. Some sources describe a biaxial optical behavior that is not consistent with the crystal structure of strongly colored crystals. For different causes are possible:

• Lowering of the symmetry by the incorporation of foreign atoms, analogous to the symmetry reduction in shells
• Symmetry reduction by mechanical stresses that produce a stress-induced birefringence.
• Misinterpretation of microscopic polarization results from the interaction of color and interference figures

Use

As a pigment

Anatase is used as a white pigment in the paint industry. It is produced by the sulphate process. By the rutile higher photocatalytic activity for the decomposition of organic components, for example Polymers results, the range of use is limited. Typical areas of application are photocatalysts, synthetic fibers, food and cosmetics colors E171 and as raw material for industry, eg Special ceramics or glasses. Nanoteiliger anatase is partially used in sunscreen creams. UV radiation having a wavelength less than about 380 nm is absorbed.

As a gemstone

Anatase is rarely used as a gemstone because it is very brittle and prone to breakage due to its good cleavage while grasping and soldering. But he has honed a certain value among collectors.