A crystal-chemical study of natural and synthetic anionic clays.

Abstract : A comparative crystallochemical study was performed on natural and synthetic hydrotalcitelike compounds with similar compositions. The nature of the brucite-like sheet stacking was addressed by means of powder X-ray diffraction. From the resulting electron diffraction patterns it was possible to establish the order-disorder of the cations in the brucite-like sheet. The results show that a natural sample from Snarum is an intergrowth of hydrotalcite (3R1 polytype) and manasseite (2H1 polytype) at a ratio of 77:23 (wt.%). An aluminian serpentine is associated with the hydrotalcite and manasseite minerals. The structure of a synthetic sample, Mg:Al = 2:1, was determined as space group R3¯m. For a few crystals in this sample, the octahedral cation distribution is compatible with the observed supercell (a=a3). A second synthetic sample showed the presence of stacking faults and was described as a random layer sequence of two polytypes (3R and 2H).