Thermal Evolution of Natural Layered Double Hydroxides: Insight from Quintinite, Hydrotalcite, Stichtite, and Iowaite as Reference Samples for CO3- and Cl-Members of the Hydrotalcite Supergroup

In Situ high-temperature powder X-ray diffraction experiments were undertaken for the coarse crystalline natural layered double hydroxides (LDHs) quintinite, hydrotalcite, stichtite, and iowaite in the temperature range 25-1000 degrees C, with thermal analyses of these minerals and their annealed forms carried out in parallel. In the temperature range from 25 degrees C to 170-210 degrees C quintinite, hydrotalcite, and stichtite (carbonate members of the LDH family) demonstrated contraction of the basal d(00n)-value of 0.1-0.3 angstrom, followed by a sharp contraction of 1.0-1.1 angstrom at T > 170-210 degrees C. The high-temperature modified states were stable up to 380-420 degrees C, before decomposing to an amorphous phase. Iowaite (chloride member of the family) was stable up to 320 degrees C and transformed to an amorphous phase at higher temperature. Iowaite experiences continuous contraction of the d(00n)-value of up to 0.5 angstrom in the temperature range 25-200 degrees C, reaching a plateau at a temperature range of 200-320 degrees C. Assessing the reversibility of thermal transformation shows complete reconstruction of the crystal structure of the hydrotalcite and iowaite heated to 300 degrees C. Solid-state nuclear magnetic resonance analysis shows that some Al changes coordination from 6- to 4-fold, synchronously with quintinite transformation to the amorphous phase. All phases transform to periclase and a spinel-type compound upon further heating. Thermal analysis of samples annealed at 125 degrees C shows that carbonate members do not have a tendency to form dehydrated phases, whereas for iowaite, a dehydrated phase having 0.9 apfu lesser water content as in the initial sample has been obtained. Thermal evolution of LDHs is found to depend on the nature of the interaction of interlayer species and water molecules to H atoms of the metal-hydroxide layer.