Authors: František Kovanda, Květa Jirátová and Radka Kalousková
Abstract: Hydrotalcite, a magnesium-aluminum hydroxycarbonate, is a naturally occuring mineral of chemical composition Mg6Al2(OH)16CO3.4H2O exhibiting a layered crystal structure. It was discovered in Sweden in 1842 and its name was derived from the fact that it can be crushed easily into a white powder, which is similar to the talc. The first studies on the synthesis, stability, solubility and structure determination were carried out by Feitknecht [1,2] almost 70 years ago. Based on its crystal structure, hydrotalcite is included in the waterbearing carbonates with unfamiliar anions (V/E group of the minerals according to the Strunz classification system). The hydrotalcite group of minerals  is presented in Table 1. The Feitknecht’s hypothesis of double-sheet crystal structure considering the structure with intercalated hydroxide layers was later refused because the single-crystal XRD analysis showed that all the cations are localized in the same layer. The hydrotalcite crystal structure is similar to that of brucite, Mg(OH)2, where each Mg2+ cation is octahedrally surrounded by six OH- anions and the different octahedra [Mg(OH)6]4- share edges to form infinite sheets (Fig. 1). The sheets are stacked one on top of the other and are held together by weak interactions via hydrogen bonds. In the hydrotalcite, the Mg2+/Al3+ isomorphous substitution in octahedral sites of the hydroxide sheet results in a net positive charge that is neutralized by the interlayers composed of carbonate anions and water molecules.