Crystal Chemistry and Atomic Order in Brucite-related Double-layer Structures

Brucite-related double layer structures are characterized by a regular but distinct stacking of [Me2x+Me3+ (OH)2x+2]+ octahedral layers with ordered cation distribution and usually x = 2 or x = 3, alternating with anionic {A·nH2O}- interlayers. The stacking sequence of the layers is rhombohedral (3R) or hexagonal (2H) parallel to [001]. Evidence of order by X-ray experiments is largely prohibited by the weakness of the superstructure reflections due to pseudosymmetry ruled by structural modules and crystal imperfection. The ordered distribution of cations influences the distribution of anions and the maximum ideal water contents of interlayers.