Ammonium fluorosilicate

Ammonium fluorosilicate has three major polymorphs: α-(NH₄)₂[SiF₆] form is cubic (space group Fm3m, No. 225) and corresponds to the mineral cryptohalite. The β form is trigonal (scalenohedral) and occurs in nature as mineral bararite. A third (γ) form was discovered in 2001 and identified with the hexagonal 6mm symmetry. In all three configurations, the [SiF₆]²⁻ octahedra are arranged in layers. In the α form, these layers are perpendicular to [111] directions. In the β- and γ- forms, the layers are perpendicular to the c-axis.{{cite journal|author =Boldyreva, E. V.|author-link1=Elena Boldyreva |author2=Shakhtshneider, T. P. |author3=Sowa, H. |author4=Ahsbas, H. |year=2007|title=Effect of hydrostatic pressure up to 6 GPa on the crystal structures of ammonium and sodium hexafluorosilicates, (NH₄)₂[SiF₆] and Na₂[SiF₆]; a phase transition in (NH₄)₂[SiF₆] at 0.2–0.3 GPa|journal=Zeitschrift für Kristallographie|pages=23–29|doi=10.1524/zkri.2007.222.1.23|volume=222|s2cid=97174719 }} (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.Klein, C. and Dutrow, B. (2008) The 23rd Edition of the Manual of Mineral Science. John Wiley & Sons, Hoboken, NJ.) The silicon atoms of α-(NH₄)₂[SiF₆] (alpha), have cubic close packing (CCP). The γ form has hexagonal close packing and the β-(NH₄)₂[SiF₆] has primitive hexagonal packing.To learn about the primitive hexagonal structure, see [http://www.polymorf.net/matter4.htm Primitive hexagonal packing] {{webarchive|url=https://web.archive.org/web/20090526142134/http://www.polymorf.net/matter4.htm |date=2009-05-26 }}. In all three phases, 12 fluorine atoms neighbor the (NH₄)⁺.

Although bararite was claimed to be metastable at room temperature,{{cite journal|doi=10.1063/1.1727071|title=Structure of Cubic Ammonium Fluosilicate: Neutron-Diffraction and Neutron-Inelastic-Scattering Studies|year=1966|last1=Schlemper|first1=Elmer O.|journal=The Journal of Chemical Physics|volume=44|pages=2499–2505|issue=6|bibcode=1966JChPh..44.2499S }} it does not appear one polymorph has ever turned into another. Still, bararite is fragile enough that grinding it for spectroscopy will produce a little cryptohalite.Oxton, I. A., Knop, O., and Falk, M. (1975) [https://archive.today/20121216022602/http://article.pubs.nrc-cnrc.gc.ca/ppv/RPViewDoc?issn=1480-3291&volume=53&issue=22&startPage=3394 "Infrared Spectra of the Ammonium Ion in Crystals"]. II. The Ammonium Ion in Trigonal Environments, with a Consideration of Hydrogen Bonding. Canadian Journal of Chemistry, 53, 3394–3400. Even so, ammonium fluorosilicate assumes a trigonal form at pressures of 0.2 to 0.3 GPa. The reaction is irreversible. If it is not bararite, the phase is at least very closely related.

The hydrogen bonding in (NH₄)₂[SiF₆] allows this salt to change phases in ways that normal salts cannot. Interactions between cations and anions are especially important in how ammonium salts change phase. (To learn more about the β-structure, see Bararite.)

This chemical makes rare appearances in nature.Barnes, J. and Lapham, D. (1971) "Rare Minerals Found in Pennsylvania". Pennsylvania Geology, 2, 5, 6–8. It is found as a sublimation product of fumaroles and coal fires. As a mineral, it is either called cryptohalite or bararite, the two being two polymorphs of the compound.Anthony, J. W., Bideaux, R. A., Bladh, K. W., and Nichols, M. C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson.

• [http://www.handbookofmineralogy.org/pdfs/BARARITE.pdf link to bararite] {{Webarchive|url=https://web.archive.org/web/20160401193238/http://www.handbookofmineralogy.org/pdfs/Bararite.pdf |date=2016-04-01 }}
• [http://www.handbookofmineralogy.org/pdfs/CRYPTOHALITE.pdf link to cryptohalite] {{Webarchive|url=https://web.archive.org/web/20211202010021/http://www.handbookofmineralogy.org/pdfs/Cryptohalite.pdf |date=2021-12-02 }}

Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, including hydrogen fluoride, silicon tetrafluoride, and nitrogen oxides. It will corrode aluminium. In water, ammonium fluorosilicate dissolves to form an acid solution.

Inhaling dust can lead to pulmonary irritation, possibly death. Ingestion may also prove fatal. Irritation of the eyes comes from contact with the dust, as well as irritation or ulceration of the skin.

Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.[http://cameochemicals.noaa.gov/chemical/5407 Ammonium fuorosilicate], CAMEO Chemicals, NOAA It is also used to help neutralize washing machine water as laundry sour.

{{commons category|Diammonium hexafluorosilicate}}

• Hexafluorosilicic acid
• Sodium fluorosilicate

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• [https://web.archive.org/web/20090526142134/http://www.polymorf.net/matter4.htm Primitive hexagonal packing]

{{fluorine compounds}}

{{Ammonium salts}}

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Category:Ammonium compounds

Category:Hexafluorosilicates