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 Hexafluorosilicic acid is an inorganic compound with the chemical formula . Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless. Hexafluo ...

.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.

Structure

Ammonium fluorosilicate has three major polymorphs: α-(NH₄)₂ 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 In mathematics, physics and chemistry, a space group is the symmetry group of a repeating pattern in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of the pattern that ...

Fm3m, No. 225) and corresponds to the mineral cryptohalite. The β form is

trigonal In crystallography, the hexagonal crystal family is one of the six crystal family, crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the tr ...

(scalenohedral) and occurs in nature as mineral bararite. A third (γ) form was discovered in 2001 and identified with the

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is d ...

6mm symmetry. In all three configurations, the iF₆sup>2−

octahedra In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...

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 iF₆(alpha), have cubic close packing (CCP). The γ form has

hexagonal 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 occ ...

and the β-(NH₄)₂ iF₆has primitive hexagonal packing. In all three phases, 12

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

atoms neighbor the (NH₄)⁺. Although bararite was claimed to be

metastable In chemistry and physics, metastability is an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball is onl ...

room temperature Room temperature, colloquially, denotes the range of air temperatures most people find comfortable indoors while dressed in typical clothing. Comfortable temperatures can be extended beyond this range depending on humidity, air circulation, and ...

, it does not appear one polymorph 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 electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...

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 Grading in education is the application of standardized measurements to evaluate different levels of student achievement in a course. Grades can be expressed as letters (usually A to F), as a range (for example, 1 to 6), percentages, or as num ...

. 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 (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

ing in (NH₄)₂ 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

anion 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 conven ...

s are especially important in how ammonium salts change phase. (To learn more about the β-structure, see Bararite.)

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

fumarole A fumarole (or fumerole) is a vent in the surface of the Earth or another rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

s 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.
link to bararite

link to cryptohalite

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 chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...

silicon tetrafluoride Silicon tetrafluoride or tetrafluorosilane is a chemical compound with the formula Si F4. This colorless gas is notable for having a narrow liquid range: its boiling point is only 4 °C above its melting point. It was first prepared in 1771 ...

, and

nitrogen oxides In atmospheric chemistry, is shorthand for nitric oxide () and nitrogen dioxide (), the nitrogen oxides that are most relevant for air pollution. These gases contribute to the formation of smog and acid rain, as well as affecting tr ...

. It will

corrode Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

. 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.

References

External links

Primitive hexagonal packing
{{Authority control Ammonium compounds Hexafluorosilicates