IV. Summary: Specimens of the fibrous zeolite mordenite (ptilolite) have been examined by X-ray methods, and the specific gravity and pyroelectric properties determined. Laue and oscillation photographs indicate that the structure possesses orthorhombic symmetry, and the unit cell, with edges a 18·25, b 20·35, c 7·50 Å., contains four molecules of composition (Ca,Na₂,K₂)Al₂Si₁₀O₂₄.7H₂O. The space-group is either D2h17=Cmcm or C2v12=Cmc; the pyroelectric tests are somewhat inconclusive, but indicate that the c-axis [001] is probably polar. The structure is probably based on a framework of linked tetrahedra.