A monoclinic mineral, (K,Na,Ca)_1-2(Si,Al)₈O₁₆.6H₂O ; zeolite group; commonly occurs in complex twinned crystals; in basalt amydules, in pelagic red clays, in palagonite tuffs, in alkaline saline lakes from silicic vitric volcanic ash, in alkaline soils, and around hot springs in Roman baths.