Young's moduli of zeolite single crystals (natrolite, Na2(Al2Si3O10) . 2H2O, NAT-type) have been determined directly using static three-point bend, compression and indentation experiments for the first time. The values compare well with those previously determined from dynamic studies (Brillouin scattering and ultrasound) of the same zeolite. A strong anisotropy in elasticity is detected, in particular a larger Young's modulus along the crystal z-axis compared to those along the x and y axes, which reflects the anisotropic, fibrous structure of natrolite. The idealised, siliceous natrolite structure has previously been predicted to possess on-axis negative Poisson's ratios (auxetic behaviour), but the measured elastic constants are not consistent with this hypothesis. We use new simulation results via a molecular mechanics approach to show that the presence of extra-framework cations and water molecules in the true aluminosilicate zeolite framework removes the on-axis auxetic behaviour expected for the hypothetical siliceous framework and then discuss our new calculated elastic constants in relation to the experimental values. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.