The crystal structure of a low-silica zeolite with the faujasite structure, Ca LSX (Si/Al = 1), has been determined from powder X-ray and neutron diffraction data. The crystal structure was refined by the Rietveld least-squares method in the cubic space group Fd3 [a = 25.1080(1) Angstrom]. The final R(wp)-factor and the chi(2) for the combined refinement were 10.1% and 1.81, respectively. The calcium ions are located preferentially in site I and site II, and distortions of the zeolite framework are ascribed to the interaction between these calcium ions and framework oxygen atoms. H-2 NMR studies of C6D6 adsorbed in Ca LSX (I molecule per supercage) confirm that deuterated benzene is bound strongly at site II and interacts facially with calcium so that rapid rotation about the six-fold axis of the sorbate is observed in the temperature range 260-330 K. The activation energy for this rotation is only 4.9 kJ mol(-1). By contrast, deuterated benzene in Na LSX exhibits rapid isotropic motion down to 260 K; the diffusion coefficient and activation energy for this system are calculated to be D-298 = 1.9 X 10(-11), m(2) s(-1) and E(a) = 10.6 kJ mol(-1), respectively, from H-2 NMR spin-lattice relaxation measurements.