Cryolite

from Wikipedia, the free encyclopedia
Cryolite
Cryolite-Siderite-mun05-150a.jpg
Cryolite with siderite from Ivigtut, Greenland (size: 7.4 cm × 3.6 cm × 2.1 cm)
General and classification
chemical formula
  • α-Na 2 [8] Na [6] [AlF 6 ]
  • Na 3 [AlF 6 ]
Mineral class
(and possibly department) 		Halides
System no. to Strunz
and to Dana 		3.CB.15 ( 8th edition : III / B.03)
06/11/01/01
Similar minerals Anhydrite, fluorite, halite
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group P 2 1 / n (No. 14, position 2)Template: room group / 14.2
Lattice parameters a  = 5.40  Å ; b  = 5.60 Å; c  = 7.76 Å
β  = 90.3 °
Formula units Z  = 2
Twinning often after {110}, penetrating, repetitive, and polysynthetic twins
Physical Properties
Mohs hardness 2.5
Density (g / cm 3 ) measured: 2.97; calculated: 2,973 (2)
Cleavage no; Secretions according to {001} and {110}
Break ; Tenacity uneven; brittle
colour white, brown, gray, brown-black, reddish
Line color White
transparency transparent to translucent
shine moist glass luster, fat luster, pearlescent luster
Crystal optics
Refractive indices n α  = 1.339
n β  = 1.339
n γ  = 1.340
Birefringence δ = 0.001
Optical character biaxial positive
Axis angle 2V = 43 °
Pleochroism none
Other properties
Special features weak thermoluminescence; yellow fluorescence under UV light

Cryolite ( aluminum trisodium hexafluoride , sodium hexafluoroaluminate (III) ) is a rather seldom occurring mineral from the mineral class of halides . It crystallizes in the monoclinic crystal system with the simplified chemical composition Na 3 [AlF 6 ] and develops either pseudo- cubic crystals or massive aggregates in white, brown, gray, brown-black or reddish color.

Etymology and history

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

Cryolite was first found and described in 1799 in Ivittuut in Greenland by Peder Christian Abildgaard (1740-1801). He named the mineral because of its characteristic appearance after the Greek words κρύος [krýos] "frost, ice" and λίθος [lítʰos] "stone", which means "ice-stone".

classification

Already in the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the cryolite belonged to the mineral class of "halides" and there to the division of "double halides with [BF 4 ] 1− , [SiF 6 ] 2− and [AlF 6 ] 3− ", where together with elpasolite he created the" cryolite-elpasolite group "with the system no. III / B.03 and the other members Bøgvadit , Calcjarlit , Colquiriit , Jarlit , Jørgensenit , Kryolithionit and Simmonsit .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns cryolite to the “halides” class, but in the “complex halides” department. This is further subdivided according to the type of crystal structure, so that the mineral can be found according to its structure in the sub-section "Island aluminofluoride (Neso-aluminofluoride)", where the "cryolite group" with the system no. 3.CB.15 and the other members elpasolite and simmonsite.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns cryolite to the class of "halides" and there to the department of "complex halides - aluminum fluorides". Here, too, it is named in the "cryolite group" with the system no. 06/11/01 to be found in the subsection “ Complex halides - aluminum fluorides with various formulas ”.

Crystal structure

Unit cell of cryolite at room temperature, projected onto plane a – c

Cryolite crystallizes in the monoclinic crystal system in the space group P 2 1 / n (space group no. 14, position 2) with the lattice parameters a  = 5.40  Å ; b  = 5.60 Å, c  = 7.76 Å and β = 90.3 ° as well as two formula units per unit cell . This leads to a grid distortion. Template: room group / 14.2

The crystal structure of cryolite is composed of a prism-shaped parallelepiped with [AlF 6 ] 3- - octahedra sitting insular fashion at the corners and in the center of the prism. The Na + ions occupy the centers of the base and prism surfaces as well as the centers of the prism edges.

properties

At a temperature of around 560 ° C, cryolite changes into the cubic crystal system . Because of 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.

Cryolite melts very easily and is completely soluble in concentrated sulfuric acid, but only partially in hydrochloric acid .

The mineral has weak thermoluminescent properties and glows yellow fluorescent under UV light , the fluorescence being more intense the shorter the UV light is.

Education and Locations

Cryolite forms as gangue especially in zinnführenden granite - pegmatites and in fluoritreichen , topaz containing rhyolites .

The most important mining site Ivittuut in Greenland is now exhausted. Cryolite was mined there between 1865 and 1987. The mineral was also discovered in the following places:

Synthetic manufacture

The synthesis takes place from hexafluoridosilicic acid , sodium and aluminum hydroxide.

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 melt electrolysis . It consists of 18.5% Al 2 O 3 and 81.5% Na 3 [AlF 6 ]. 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. Due to the high temperatures that occur at the tip of the abrasive grain during grinding, the cryolite melts. Here corroded the molten cryolite abraded steel tension and thus prevents clogging of the grinding wheel.

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

safety instructions
Surname

Sodium hexafluoroaluminate, cryolite

CAS number

15096-52-3

GHS labeling of hazardous substances
08 - Dangerous to health 07 - Warning 09 - Dangerous for the environment

danger

H and P phrases H: 332-372-411
P: 260

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

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

literature

Web links

Commons : Cryolite  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  161 .
  2. ↑ a b Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .
  3. ^ Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  330-332 .
  4. ↑ a b c d e f Cryolite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 97  kB ; accessed on October 6, 2017]).
  5. ↑ a b c d e Mindat - Cryolite (English)
  6. ↑ U. Müller: Inorganische Strukturchemie . 6th edition. Vieweg + Teubner, 2008, ISBN 978-3-8348-0626-0 , p. 298 .
  7. ↑ a b Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp.  491 (first edition: 1891).
  8. ↑ Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p.  357-358 .
  9. ↑ Find location list for cryolite in the Mineralienatlas and Mindat
  10. ^ AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin 2007, ISBN 978-3-11-017770-1 , p. 1140.
  11. ↑ a b c Entry on sodium hexafluoroaluminate in the GESTIS substance database of the IFA , accessed on July 29, 2017 (JavaScript required)