Cryolite
Cryolite ( Aluminiumtrinatriumhexafluorid, Natriumhexafluoridoaluminat (III ) ) is a rarely occurring mineral from the mineral class of halides. It crystallizes in the monoclinic crystal system with the chemical composition Na3 [ AlF6 ] and developed either pseudo-cubic crystals or massive aggregates in white, brown, gray, brown - black or reddish color.
Special Features
At a temperature of about 560 ° C enters through the cryolite in the cubic crystal system. Because of this property, the mineral is an important geological thermometer to clarify the conditions of formation of rocks. When annealing in an open glass tube evolved hydrogen fluoride ( HF).
Cryolite is completely soluble in concentrated sulfuric acid, hydrochloric acid, in contrast, only partially.
Etymology and history
Was first found and described cryolite 1799 in Ivigtut in Greenland by Peder Christian Abildgaard ( 1740-1801 ). He named the mineral because of its characteristic appearance after the Greek words κρύος [ kryos ] " Frost, Ice " ( the German word " (ice) crust ", via Latin " crusta " and Old High German " hroso " for " crust, ice cream " yet find; related to Tocharian A " Krost " for " cold" ) and λίθος [lit ʰ os] "stone" ( etymology unclear), so composed "Ice stone ".
Classification
Under the old classification of minerals according to Strunz ( 8th edition ) of cryolite is one of the anhydrous Doppelhalogeniden. These mineral class is, however, restructured since the 9th edition of Strunz 's Mineral classification and the classification of complex halides is based now more on the crystal structure. Cryolite is found therefore in the subsection "Island Aluminofluoride ( Neso - Aluminofluoride ) ".
Education and Locations
Cryolite forms as gangue particularly in zinnführenden granite pegmatites and in fluoritreichen, topaz rhyolites containing.
The main mineable locality Ivigtut ( Kitaa ) in Greenland is now exhausted. Mined cryolite was there 1865-1987 In addition, the mineral was discovered in the following locations. :
- In northern and southern regions of Brazil;
- Saxony in Germany;
- Quebec in Canada;
- At Semipalatinsk in Kazakhstan;
- Khomas and Kunene in Namibia;
- Bauchiplateau in Nigeria;
- Oppland in Norway;
- The regions of Eastern Siberia, North-West Russia and the Urals in Russia;
- Aragon in Spain;
- Bohemia in the Czech Republic;
- Zhytomyr Oblast in Ukraine; as well as
- Several regions in the United States.
Synthetic production
The synthesis is carried out Hexafluoridokieselsäure and sodium aluminate.
Crystal structure
Cryolite crystallizes in the monoclinic crystal system, space group P21 / a with lattice parameters a = 5.4024 Å; b = 5.5959 Å, c = 7.7564 Å and β = 90.278 °, and two formula units per unit cell. This leads to a lattice distortion.
Use
A large-scale application of cryolite is the fused-salt electrolysis for the production of aluminum ( Hall - Heroult process). Where the characteristic of the relatively low melting point of cryolite (1012 ° C) is used. Aluminum oxide ( corundum), the starting material of the electrolysis, have a melting temperature of 2050 ° C. For the fused-salt electrolysis, the eutectic mixture is used. It consists of 10.5% Al2O3 and Na3 [ AlF6 ]. The melting point of the eutectic is then up to 960 ° C. It is this relatively low operating temperature allows the large-scale application of the fused salt electrolysis.
Cryolite is further used in the foundry industry. The mold material during casting can be mixed with cryolite. This mixing, however, can adversely affect the surface quality of the workpiece.
Synthetic cryolite is produced in the shell of welding electrodes as a flux use.
Cryolite is also used as a grinding active substance in resin bonded abrasives and coated abrasives in. Due to the high temperatures encountered during grinding at the top of the abrasive grain, the cryolite melts. In this case, the molten cryolite corrodes the abraded steel tension, thereby preventing clogging of the abrasive article.
The problem is the occurrence of cryolite in the zinc phosphating of for example aluminum automobile bodies or galvanized surfaces. Aluminum is here precipitated in the phosphating as cryolite and must be filtered out again.
Cryolite optically highly reflective surface is used for the production. It is in this case in thin layers alternating with another substance, such as zinc selenide, evaporated in vacuo ( multilayer mirrors in laser technology ).
In Kryolithglas used for the production of ocular prostheses, it provides for a milky white turbidity of the lens.
Precautions
The mineral or chemical compound is considered toxic and hazardous to the environment. Harmful in particular any inhalation and ingestion of Kryolithteilchen, the acute discomfort in the respiratory tract with functional dyspnea ( difficult breathing activity ) and eventually emphysema ( overinflation of the alveoli ) is able to perform. Furthermore, loss of appetite, nausea, vomiting and constipation result.
In the longer term cryolite has a toxic effect on bone, teeth and kidneys.