Hydrotalcite-Group Minerals: Crystal Chemistry and a New Look at “Old” Minerals

The data on the structures of the hydrotalcite-group minerals—layered double hydroxides with the general formula $$M_{6}^{{2 + }}M_{2}^{{3 + }}{{({\text{OH}})}_{{16}}}A_{{2/m}}^{{m-}}$$ ⋅4H2O (М2+ = Mg2+, Ni2+; М3+ = Al3+, Fe3+, Cr3+, Mn3+, Co3+; A = $${\text{CO}}_{3}^{{2 - }}$$ , Cl– and OH–)—are summarized. It is shown that all of them crystallize with the structure of 3R- and 2H-polytypes without the formation of superstructures. Their unit-cell parameter a ranges within 3.05–3.13 Å. The characteristic interlayer distances d00n for the group members 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 the 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 the hydrotalcite-group minerals, and the data on reevesite indicate that this name could describe two minerals with M2+ : M3+ = 3 : 1 and 2 : 1.