Ammonium fluorosilicate

Ammonium fluorosilicate has three major polymorphs: α-(NH4)2[SiF6] 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. (Note: trigonal symmetry is part of the hexagonal group, but not all hexagonal crystals are trigonal.) The silicon atoms of α-(NH4)2[SiF6] (alpha), have cubic close packing (CCP). The γ form has hexagonal close packing and the β-(NH4)2[SiF6] has primitive hexagonal packing. In all three phases, 12 fluorine atoms neighbor the (NH4)+. it does not appear one polymorph has ever turned into another. 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 (NH4)2[SiF6] 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.) ==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. ==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 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. It is also used to help neutralize washing machine water as laundry sour. ==See also==