Correlation between the d -value and the M 2 + :M 3 + cation ratio in Mg–Al–CO 3 layered double hydroxides

Abstract Twenty-six samples of natural layered double hydroxides (LDHs: hydrotalcite, pyroaurite and quintinite) from various localities worldwide have been studied by means of single-crystal, powder X-ray diffraction and electron microprobe analysis. High crystallinity of samples allowed for the good accuracy of the measured d -values and reliable values of the M 2 + :M 3 + ratios. Based on the obtained data, it has been found out that d -value for LDHs depends upon the M 2 + :M 3 + ratio that can be explained by purely crystal chemical reasons. The ideal d -values for M 2 + :M 3 +  = 2:1 (quintinite group) and 3:1 (hydrotalcite group) are 7.56 and 7.80 A, respectively, whereas intermediate cation ratios correspond to the intermediate d -values. The observed quantitative correlation between d -value and M 2 + :M 3 + cation ratio for carbonate LDHs can be described by the linear function d  = 0.2385 ×  R  + 7.0852 (R = M 2 + :M 3 + ), which allows for the estimation of M 2 + :M 3 + on the basis of d -value and vice versa.