hydrated sodium, potassium, and calcium aluminosilicate mineral (Na₂,K₂,Ca) Al₂Si₁₀O₂₄·7H₂O, in the zeolite family. It is one of the most abundant zeolites in altered volcanic deposits, and it commonly occurs as white, glassy needles filling veins and cavities in igneous rocks. It is also found in marine sediments, as in the Ural Mountains and in dikes where water has attacked and altered volcanic glasses, as on the Isle of Arran in Scotland. Mordenite's molecular structure is a framework containing chains of five-membered rings of linked silicate and aluminate tetrahedra (four oxygen atoms arranged at the points of a triangular pyramid about a central silicon or aluminum atom). Its high ratio of silicon to aluminum atoms makes it more resistant to attack by acids than most other zeolites.