Ammonium fluorosilicate has the formula 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.
Structure
Ammonium fluorosilicate has three major polymorphs: α-2 form is cubic and corresponds to the mineral cryptohalite. The β form is trigonal and occurs in nature as mineral bararite. A third form was discovered in 2001 and identified with the hexagonal 6mm symmetry. In all three configurations, the 2−octahedra are arranged in layers. In the α form, these layers are perpendicular to Miller index| directions. In the β- and γ- forms, the layers are perpendicular to the c-axis. The silicon atoms of α-2, have cubic close packing. The γ form has hexagonal close packing and the β-2 has primitive hexagonal packing. In all three phases, 12 fluorine atoms neighbor the +. 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 bonding in 2 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.
Natural occurrence
This chemical makes rare appearances in nature. 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.
Ammonium fluorosilicate is noncombustible, but it will still release dangerous fumes in a fire, including hydrogen fluoride, silicon tetrafluoride, and nitrogen oxides. It will corrodealuminium. 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.
