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 d_00n for the group members with carbonate and chloride anions are ~7.80 and 8.04 Å, respectively (c = d_00n × 2 for 2H and c = d_00n × 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.