A fibrous or needlelike mineral belonging to the zeolite family of silicates. It crystallizes in the monoclinic system in pseudo-orthorhombic prismatic crystals which are often acicular (see illus.). Most commonly it is found in radiating fibrous aggregates. There is perfect prismatic cleavage, the hardness is 5–5½ on Mohs scale, and the specific gravity is 2.25. The mineral is white or colorless with a vitreous luster that inclines to pearly in fibrous varieties. The chemical composition is Na₂(Al₂Si₃O₁₀) · 2H₂O, but some potassium is usually present substituting for sodium.