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We present an X-ray diffraction and multi-nuclear ( 2 H and  43 Ca) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H2O interface. Variable-temperature  43 Ca MAS NMR and controlled humidity XRD indicate that Ca 2+  occurs as proximity-restricted outer-sphere hydration complexes between -120 and +25 °C in a two-layer hydrate and at  T  ≤ -50 °C in a 2:1 water/solid paste. Changes in the  43 Ca NMR peak width and position with temperature are more consistent with diffusion-related processes than with dynamics involving metal-surface interactions such as site exchange. The  2 H NMR signal between -50 and +25 °C for a two-layer hydrate of Ca-hectorite is similar to that of Na- and other alkali metal hectorites and represents  2 H 2 O molecules experiencing anisotropic motion describable using the  2 H C 2 /C 3  jump model we proposed previously.  2 H T 1  relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for  2 H 2 O dynamics, yielding GHz-scale rotational reorientation rates compatible with the C 3  component of the C 2 /C 3  hopping model. The apparent activation energy for  2 H 2 O rotational diffusion in the two-layer hydrate is greater for Ca-hectorite than Na-hectorite (25.1 vs. 21.1 kJ/mol), consistent with the greater affinity of Ca 2+  for H 2 O. The results support the general principle that the dynamic mechanisms of proximity-restricted H 2 O are only weakly influenced by the cation in alkali metal and alkaline earth metal smectites and provide critical evidence that the NMR resonances of charge-balancing cations in smectites become increasingly influenced by diffusion-like dynamic processes at low temperatures as the charge density of the unhydrated cation increases.

Structural and dynamical relationships of Ca 2+ and H 2 O in smectite/ 2 H 2 O systems