A comparative crystallochemical study was performed on natural and synthetic hydrotalcite-like 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 $R\overline{3}m$. For a few crystals in this sample, the octahedral cation distribution is compatible with the observed supercell (a = a′ √3). A second synthetic sample showed the presence of stacking faults and was described as a random layer sequence of two polytypes (3R and 2H).

Two samples of hydrotalcite, probably worked on by Carl Christian Hochstetter, have been discovered in the Museum für Naturkunde Berlin and given neotype status by the IMA CNMNC (proposal 15-J). The two samples from Snarum, Norway, have been reanalysed and give Mg5.84Al2.07Fe0.09(OH)16(CO3)1.08·4H2O and Mg5.75Al2.21Fe0.04(OH)16(CO3)1.12·4H2O. Both samples have a mix of the two polytypes 3R/2H in the ratio of 69.2/30.8 and 69.5/30.5%. A discussion of hydrotalcite and hydrotalcite-like phase occurrences in the literature is also presented.