Mordenite is a member of the Zeolite family. It’s a relatively common mineral, though quality specimens are harder to find. Its acicular crystal clusters and cotton ball-like aggregates are what make it popular with collectors, but it is also commonly found as a contrasting matrix host forming non-crystalline crusts, cracked and clay-like masses and soft fibrous/cottony coatings. It’s usually white or colorless, sometimes pale yellow or pink, and exhibits a glassy (crystalline) to silky/earthy (matrix) luster. Some of those hosted minerals include Heulandite, Calcite, Pentagonite, Stilbite, Chalcedony, Quartz and Cavansite.
The crystal system for Mordenite is orthorhombic, forming sprays of radial acicular crystals, and aggregates that can be radiating, fibrous, cottony, columnar and encrusting.
Notable locations for quality Mordenite specimens include Canada, Iceland, India, Italy and the USA (Arizona, Wyoming).