Ammonium fluorosilicate (also known as ammonium hexafluorosilicate, ammonium fluosilicate or ammonium silicofluoride) has the formula (NH
4)
2SiF
6. 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,
[ which have at least three polymorphs][ and appears in nature as rare minerals cryptohalite or ]bararite
Bararite is a natural form of ammonium fluorosilicate (also known as hexafluorosilicate or fluosilicate). It has chemical formula (NH4)2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is name ...
.
Structure
Ammonium fluorosilicate has three major polymorphs: α-(NH4)2 6">iF6form is cubic
Cubic may refer to:
Science and mathematics
* Cube (algebra), "cubic" measurement
* Cube, a three-dimensional solid object bounded by six square faces, facets or sides, with three meeting at each vertex
** Cubic crystal system, a crystal system w ...
( 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 6">iF6sup>2− octahedra are arranged in layers. In the α form, these layers are perpendicular to ">11directions. In the β- and γ- forms, the layers are perpendicular to the c-axis.] (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 6">iF6(alpha), have cubic close packing
In geometry, close-packing of equal spheres is a dense arrangement of congruent spheres in an infinite, regular arrangement (or lattice). Carl Friedrich Gauss proved that the highest average density – that is, the greatest fraction of space occu ...
(CCP). The γ form has hexagonal close packing and the β-(NH4)2 6">iF6has primitive hexagonal packing. In all three phases, 12 fluorine
Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...
atoms neighbor the (NH4)+.[
Although bararite was claimed to be metastable at ]room temperature
Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
, it does not appear one polymorph
Polymorphism, polymorphic, polymorph, polymorphous, or polymorphy may refer to:
Computing
* Polymorphism (computer science), the ability in programming to present the same programming interface for differing underlying forms
* Ad hoc polymorphi ...
has ever turned into another.[ Still, bararite is fragile enough that grinding it for ]spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
will produce a little cryptohalite.[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 ]hydrogen bond
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...
ing in (NH4)2 6">iF6allows this salt to change phases in ways that normal salts cannot. Interactions between cation
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
s and anions are especially important in how ammonium salts change phase.[ (To learn more about the β-structure, see ]Bararite
Bararite is a natural form of ammonium fluorosilicate (also known as hexafluorosilicate or fluosilicate). It has chemical formula (NH4)2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is name ...
.)
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 fumaroles and coal fires. As a mineral, it is either called cryptohalite or bararite
Bararite is a natural form of ammonium fluorosilicate (also known as hexafluorosilicate or fluosilicate). It has chemical formula (NH4)2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is name ...
, 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.]
link to bararite
link to cryptohalite
/ref>
Chemical properties and health hazards
Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, including hydrogen fluoride
Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...
, 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.][
]
Uses
Ammonium fluorosilicate finds use as a disinfectant, and it is useful in etching glass, metal casting, and electroplating.[Ammonium fuorosilicate](_blank)
CAMEO Chemicals, NOAA It is also used to help neutralize washing machine water as laundry sour.
See also
* Hexafluorosilicic acid
* Sodium fluorosilicate
References
External links
Primitive hexagonal packing
{{Authority control
Ammonium compounds
Hexafluorosilicates