Cryptohalite

Composition of Cryptohalite

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. A third (γ) form was discovered in 2001 and identified with the hexagonal 6mm symmetry. In all three configurations, the [SiF6] 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)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). 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 (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.)