Minerals of the hydrotalcite group: crystal chemistry and a new perspective on 'old' minerals

The paper summarizes the data on the structures of hydrotalcite group minerals – layered double hydroxides with the general formula M2+ 6 M3+ 2 (OH)16Am– 2/m·4H2O (М2+ = Mg2+, Ni2+; М3+ = Al3+, Fe3+, Cr3+, Mn3+, Co3+; A = CO32–, Cl– and OH–). It is shown that all of them crystallize with the structure of 3R- and 2H-polytypes without the formation of superstructures. The a unit-cell parameter is in the range of 3.05–3.13 Å. The characteristic interlayer distances (d00n) for the members of the group with carbonate and chloride anions are ~7.80 and 8.04 Å, respectively (c = d00n × 2 for 2H and c = d00n × 3 for 3R). Three hydrotalcite group minerals should be reconsidered taking into account new crystallographic data and regularities: takovite and droninoite most likely correspond to minerals of the quintinite group with M2+ : M3+ = 2 : 1, rather than to minerals of the hydrotalcite group, and the data on reevesite indicate that this name could describe two minerals with M2+ : M3+ = 3 : 1 and 2 : 1.