Neutron diffraction has been used to study the hydration of calcium ions confined to the 8.5-Angstrom pores between flat vermiculite clay platelets. We have measured the first 27 (001) Bragg reflections for hydrated calcium vermiculite of two isotopic compositions, the first containing D2O and the second containing H2O. We have then used difference analysis to establish the structure of the interlayer water molecules. We find that in contrast to calcium,ions in 1 m CaCl2, which are coordinated to 10 water molecules, calcium ions intercalated into the vermiculite clay are coordinated to an average of only 6 water molecules. This hydration is very similar to that of calcium in more concentrated aqueous solutions and also to that of divalent cadmium and mercury ions. Our results may therefore go some way to explaining why these heavy-metal ions can act as poisons for physiological processes.