Ammonium fluorosilicate (also known as ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (NH4)2SiF6. It is a toxic chemical, like all salts of fluorosilicic acid.[Wiberg, E., Wiberg, N., and Holleman, A. F. (2001) Inorganic chemistry. Academic Press, San Diego.] It is made of white crystals,
Structure
Ammonium fluorosilicate has three major polymorphs: α-(NH
4)
2SiF
6 form is cubic (
space group Fm
3m, 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 SiF62− VSEPR theory are arranged in layers. In the α form, these layers are perpendicular to ] (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-(NH
4)
2SiF
6 (alpha), have
Sphere packing (CCP). The γ form has
Sphere packing and the β-(NH
4)
2SiF
6 has primitive hexagonal packing.
[To learn about the primitive hexagonal structure, see Primitive hexagonal packing .] In all three phases, 12
fluorine atoms neighbor the (NH
4)
+.
Although bararite was claimed to be Metastability at room temperature,[Oxton, I. A., Knop, O., and Falk, M. (1975) "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 in (NH4)2SiF6 allows this salt to change phases in ways that normal salts cannot. Interactions between and are especially important in how Ammonium change phase.
Natural occurrence
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
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.
]
Chemical properties and health hazards
Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, including hydrogen fluoride, silicon tetrafluoride, and
nitrogen oxides. It will
Corrosion 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.
Uses
Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.
[ Ammonium fuorosilicate, CAMEO Chemicals, NOAA] It is also used to help neutralize washing machine water as
laundry sour.
See also
-
Hexafluorosilicic acid
-
Sodium fluorosilicate
External links
