A monoclinic mineral, (Na, Ca)_2-3Al₃(Al, Si)₂Si₁₃O₃₆.12H₂O) ; zeolite group, with extensive substitution of NaSi for CaAl and K predominant over Na; forms in cavities in basalt and andesite, as skarn druses, as diagenetic product in silicic vitreous tuffs, in bentonitic clays, and as authigenic mineral in limestone or sandstone.