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 M2+:M3+ ratios. Based on the obtained data, it has been found out that d-value for LDHs depends upon the M2+:M3+ ratio that can be explained by purely crystal chemical reasons. The ideal d-values for M2+:M3+ = 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 M2+:M3+ cation ratio for carbonate LDHs can be described by the linear function d = 0.2385 x R + 7.0852 (R = M2+:M3+), which allows for the estimation of M2+:M3+ on the basis of d-value and vice versa. (C) 2016 Elsevier B.V. All rights reserved.