Why does cryolite lower a melting point

Cryolite

Cryolite
Cryolite with siderite, galena and chalcopyrite from Jvigtut, Greenland
chemical formula

N / A3[AlF6]

Mineral grade Halides
3.CB.15 (8th edition: III / B.03) according to Strunz
06/11/01/01 to Dana
Crystal system monoclinic
Crystal class; Symbol after Hermann-Mauguin monoclinic prismatic $ \ 2 / m $
colour white, brown, gray, brown-black, reddish
Line color White
Mohs hardness 2,5
Density (g / cm3) 2.96 to 2.98 [1]
shine moist glass luster, greasy luster, pearlescent luster
transparency transparent to translucent
fracture uneven
Cleavage no
Habitus pseudocubic crystals, massive aggregates
Twinning after {110}
Crystal optics
Refractive index α = 1.339 to 1.339; β = 1.339 to 1.339; γ = 1.340 to 1.340 [1]
Birefringence
(optical character)
δ = 0.001 [1] ; biaxial positive
Optical axis angle 2V = 43 °
Pleochroism none
Other properties
Melting point 1012 ° C
Similar minerals Anhydrite, fluorite, halite

Cryolite (Aluminum trisodium hexafluoride, Sodium hexafluoridoaluminate (III)) is a rather seldom occurring mineral from the mineral class of halides. It crystallizes in the monoclinic crystal system with the chemical composition Na3[AlF6] [2] and develops either pseudocubic crystals or massive aggregates of white, brown, gray, brown-black, or reddish color.

Special properties

At a temperature of around 560 ° C, cryolite changes into the cubic crystal system. Due to this property, the mineral is an important geological thermometer for clarifying the formation conditions of rocks. When glowing in an open glass tube, hydrogen fluoride (HF) is generated.[3]

Cryolite is completely soluble in concentrated sulfuric acid, but only partially in hydrochloric acid.[3]

Etymology and history

Cryolite mine in Ivittuut (Greenland) in the summer of 1940

Cryolite was first found and described in Ivigtut in Greenland in 1799 by Peder Christian Abildgaard (1740-1801). He named the mineral because of its characteristic appearance after the Greek words κρύος [krúos] "Frost, Eis" (in the German word "(ice) crust" over the Latin "crusta" and Old High German "hroso" for "crust, ice") related to Tocharic A “krost” for “cold”) and λίθος [lítʰos] “stone” (etymology unclear), thus composed “ice stone”.

classification

According to the old systematics of minerals according to Strunz (8th edition), the cryolite belongs to the anhydrous double halides. However, this mineral class has been restructured since the 9th edition of Strunz's mineral system and the classification of the complex halides is now based more on the crystal structure. Cryolite can therefore be found in the subsection "Island aluminofluorides (Neso-aluminofluorides)".

Education and Locations

As a gangue mineral, cryolite is mainly formed in tin-bearing granite pegmatites and in fluorite-rich, topaz-containing rhyolites.

The most important mining site Ivigtut (Kitaa) in Greenland has now been exhausted. Cryolite was mined there between 1865 and 1987. In addition, the mineral was discovered in the following places:

  • In northern and southern regions of Brazil;
  • Saxony in Germany;
  • Québec in Canada;
  • at Semei in Kazakhstan;
  • Khomas and Kunene in Namibia;
  • Abdominal plateau in Nigeria;
  • Oppland in Norway;
  • the regions of Eastern Siberia, Northwest Russia and Urals in Russia;
  • Aragon in Spain;
  • Bohemia in the Czech Republic;
  • Zhytomyr Oblast in Ukraine; as
  • several regions in the US.[4]

Synthetic manufacture

The synthesis takes place from hexafluoridosilicic acid and sodium aluminate.

$ \ mathrm {H_2SiF_6 + 6 \ NH_3 + 2 \ H_2O \ longrightarrow 6 \ NH_4F + SiO_2} $
$ \ mathrm {6 \ NH_4F + 3 \ NaOH + Al (OH) _3 \ longrightarrow Na_3 [AlF_6] + 6 \ NH_3 + 6 \ H_2O} $

Crystal structure

Unit cell of cryolite

Cryolite crystallizes in the monoclinic crystal system in the space groupP21/ a with the grid parametersa = 5.4024 Å; b = 5.5959 Å, c = 7.7564 Å and β = 90.278 ° as well as two formula units per unit cell.[5] This leads to a grid distortion.[6]

use

A large-scale application of cryolite is melt-flow electrolysis for the production of aluminum (Hall-Héroult process). There the property of the relatively low melting point of cryolite (1012 ° C) is used. Aluminum oxide (corundum), the starting material for electrolysis, has a melting temperature of 2050 ° C. The eutectic mixture is used for the melt electrolysis. It consists of 10.5% Al2O3 and well3[AlF6]. The melting point of the eutectic is then 960 ° C. It is only this relatively low working temperature that enables the large-scale application of fused-salt electrolysis.

Cryolite is also used in the foundry industry. Cryolite can be added to the molding material during casting. However, this admixture can negatively affect the surface quality of the workpiece.

Synthetically produced cryolite is used as a flux in the shell of welding electrodes.

Cryolite is also used as an abrasive substance in synthetic resin-bonded abrasives and in abrasives on substrates. Because of the high temperatures that occur at the tip of the abrasive grain during grinding, the cryolite melts. The melted cryolite corrodes the ground steel chip and prevents the grinding tool from clogging.

The occurrence of cryolite during zinc phosphating of, for example, aluminum automobile bodies or hot-dip galvanized surfaces is problematic. Aluminum is precipitated as cryolite in the phosphating bath and has to be filtered out again.

Cryolite is also used to produce optically highly reflective surfaces. It is vaporized in thin layers alternating with another substance, for example zinc selenide, in a vacuum (multi-layer mirror in laser technology).

In the cryolite glass, which is used to manufacture eye prostheses, it causes the glass to become milky-white.

Precautions

The mineral or chemical compound is classified as toxic and dangerous for the environment. Inhalation and ingestion of cryolite particles are particularly harmful to health, as they can acutely lead to complaints in the respiratory tract with functional dyspnea (difficult breathing) and finally pulmonary emphysema (overinflation of the alveoli). Furthermore, loss of appetite, nausea, vomiting and constipation are the consequences.[7]

In the long term, cryolite has a toxic effect on bones, teeth and kidneys.

Individual evidence

  1. 1,01,11,2MinDat - Cryolite
  2. ↑ Stefan Weiß: The great lapis mineral directory. 5th edition. Christian Weise Verlag, Munich 2008, ISBN 3-921656-17-6.
  3. 3,03,1Klockmann's textbook on mineralogy (1976), p.491 (see literature)
  4. ↑ MinDat - Localities for Cryolite (Eng.)
  5. ↑ American Mineralogist Crystal Structure Database - Cryolite (AlF6N / A3)
  6. ↑ U. Müller: Inorganic structural chemistry. 6th edition. Vieweg + Teubner, 2008, ISBN 978-3-8348-0626-0 p. 298
  7. 7,07,1Entry to Sodium hexafluoroaluminate in the GESTIS substance database of the IFA, accessed on October 29, 2012 (JavaScript required)Template: GESTIS / without ZVG
  8. ^ Entry in the European chemical Substances Information SystemIT IS (CAS no. 15096-52-3 must be entered there manually)

literature

  • Petr Korbel, Milan Novák: Minerals encyclopedia. Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0, p. 67.
  • Paul Ramdohr, Hugo Strunz: Klockmann's textbook on mineralogy. 16th edition. Ferdinand Enke Verlag, 1978, ISBN 3-432-82986-8, pp. 490,491.
  • Riedel, E. (2002). Inorganic chemistry. de Gruyter, Berlin.
  • Holleman-Wiberg (1995). Inorganic Chemistry Textbook. de Gruyter, Berlin.
  • American Mineralogist; January 2006; v. 91; no. 1; p. 97-103; DOI: 10.2138 / 2006.1772

Web links

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