Ammonium fluorosilicate
Encyclopedia
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. It is made of white crystals, which have at least three polymorphs
and appears in nature as rare minerals cryptohalite or bararite
.
: α-(NH4)2SiF6 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. (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.) The silicon atoms of α-(NH4)2SiF6 (alpha), have cubic close(st) packing
(CCP). The γ form has hexagonal close(st) packing
and the β-(NH4)2SiF6 has primitive hexagonal packing. In all three phases, 12 fluorine
atoms neighbor the (NH4)+.
Although bararite was claimed to be metastable
at room temperature
, 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. 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
ing in (NH4)2SiF6 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
.)
and coal fires. As a mineral, it is called cryptohalite or bararite
.
, silicon tetrafluoride
, and nitrogen oxides. It will corrode
aluminium
. In water, ammonium fluorosilicate dissolves to form a basic 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.
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
and appears in nature as rare minerals cryptohalite or bararite
Bararite
Bararite is a natural form of ammonium fluorosilicate . It has chemical formula 2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is named after the place where it was first described, Barari, India...
.
Structure
Ammonium fluorosilicate has three major polymorphsPolymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
: α-(NH4)2SiF6 form is cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
(space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
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
Hexagonal crystal system
In crystallography, the hexagonal crystal system is one of the 7 crystal systems, the hexagonal lattice system is one of the 7 lattice systems, and the hexagonal crystal family is one of the 6 crystal families...
6mm symmetry. In all three configurations, the (SiF6)2- octahedra
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
are arranged in layers. In the α form, these layers are perpendicular to [111
Miller index
Miller indices form a notation system in crystallography for planes and directions in crystal lattices.In particular, a family of lattice planes is determined by three integers h, k, and ℓ, the Miller indices. They are written , and each index denotes a plane orthogonal to a direction in the...
] directions. 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.) The silicon atoms of α-(NH4)2SiF6 (alpha), have cubic close(st) packing
Sphere packing
In geometry, a sphere packing is an arrangement of non-overlapping spheres within a containing space. The spheres considered are usually all of identical size, and the space is usually three-dimensional Euclidean space...
(CCP). The γ form has hexagonal close(st) packing
Sphere packing
In geometry, a sphere packing is an arrangement of non-overlapping spheres within a containing space. The spheres considered are usually all of identical size, and the space is usually three-dimensional Euclidean space...
and the β-(NH4)2SiF6 has primitive hexagonal packing. In all three phases, 12 fluorine
Fluorine
Fluorine is the chemical element with atomic number 9, represented by the symbol F. It is the lightest element of the halogen column of the periodic table and has a single stable isotope, fluorine-19. At standard pressure and temperature, fluorine is a pale yellow gas composed of diatomic...
atoms neighbor the (NH4)+.
Although bararite was claimed to be metastable
Metastability
Metastability describes the extended duration of certain equilibria acquired by complex systems when leaving their most stable state after an external action....
at room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
, it does not appear one polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
has ever turned into another. Still, bararite is fragile enough that grinding it for spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
will produce a little cryptohalite. Even so, ammonium fluorosilicate assumes a trigonal form at pressures of 0.2 to 0.3 GPa
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
. The reaction is irreversible. If it is not bararite, the phase is at least very closely related.
The hydrogen bond
Hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
ing in (NH4)2SiF6 allows this salt to change phases in ways that normal salts cannot. Interactions between cations and anions are especially important in how ammonium salts
Ammonium
The ammonium cation is a positively charged polyatomic cation with the chemical formula NH. It is formed by the protonation of ammonia...
change phase. (To learn more about the β-structure, see Bararite
Bararite
Bararite is a natural form of ammonium fluorosilicate . It has chemical formula 2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is named after the place where it was first described, Barari, India...
.)
Natural occurrence
This chemical makes rare appearances in nature. It is found as a sublimation product of fumarolesFumarole
A fumarole is an opening in a planet's crust, often in the neighborhood of volcanoes, which emits steam and gases such as carbon dioxide, sulfur dioxide, hydrochloric acid, and hydrogen sulfide. The steam is created when superheated water turns to steam as its pressure drops when it emerges from...
and coal fires. As a mineral, it is called cryptohalite or bararite
Bararite
Bararite is a natural form of ammonium fluorosilicate . It has chemical formula 2SiF6 and trigonal crystal structure. This mineral was once classified as part of cryptohalite. Bararite is named after the place where it was first described, Barari, India...
.
Chemical properties and health hazards
Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, including hydrogen fluorideHydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
, silicon tetrafluoride
Silicon tetrafluoride
Silicon tetrafluoride or Tetrafluorosilane is the chemical compound with the formula SiF4. This tetrahedral molecule is notable for having a remarkably narrow liquid range...
, and nitrogen oxides. It will corrode
Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...
aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
. In water, ammonium fluorosilicate dissolves to form a basic 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.See also
- Hexafluorosilicic acid
- Sodium fluorosilicateSodium fluorosilicateSodium fluorosilicate is a compound with formula Na2SiF6....