Mordenite is a zeolite whose approximate composition is (Na2, K2,Ca)4[Al8Si40O96] 28 H2O. Unit cell dimensions, determined by X-ray powder diffractometry for 35 natural samples, fell within the following ranges: a=18.052–18.168, b=20.404–20.527, c=7.501–7.537 A. The indexed powder pattern of a typical sample is reported. Complete wet chemical analyses of 12 samples, partial analyses of three others, and 6 analyses from the literature reveal that mordenites vary only slightly in chemical composition. Si occupies 80 to 85% of the tetrahedra, and the exchangeable cations are mainly Na and Ca, with minor K. The lattice constant b is negatively correlated to the ratio Si/(Si+Al+Fe‴).