2

were precipitated under different conditions (pH = 8ÿ12; T = 25ÿ80ëC). All the samples are replete with stacking faults which are not eliminated by post-precipitation hydrothermal treatment (80ÿ180ëC, 18 h). DIFFaX simulations show that the layer stacking sequence of the disordered samples can be generated by a mixture of motifs corresponding to the 3R 1 and 2H 1 polytypes. These specific sequences are selected in preference to others because of the need for hydrogen bonding between the intercalated carbonates and hydroxide sheets. Thermodynamic considerations show that faulted crystals have greater stability than ordered crystals. Stacking faults arising from a mixture of 3R 1 and 2H 1 motifs, while having the sam