A monoclinic mineral, Ca₂Al₄Si₄O₁₆.9H₂O ; zeolite group; pseudotetragonal; vitreous, transparent to translucent; in cavities in leucitic tephrites and related lavas; also zeolite zones in basaltic lavas where it is associated with chabazite, thomsonite, and phillipsite. Also spelled gismondite.