The crystal structure of a natural erionite from Nizhnyaya Tunguska, Siberia (schematic formula Va3.SAGW72 * 32H,O) has been refined to a residual wR of 0.044. The erionite framework can be described by a sequence AABAAC . . . of 6-membered rings of tetrahedra. The (Si, Al) distribution (24% of Al in Tl and 31% of Al in T2) is consistent with the strong disorder found in tetrahedral sites for all zeolites describable by a sequence of 6-rings. Erionite, as in the closely related zeolite offretite, shows a slight enrichment of Al in the single 6-ring. K is at the center of the cancrinite cage and coordinates 6 framework oxygen atoms. Ca atoms alternately occupy three sites, quite near each other, on the triad axis parallel to c, at the center of the erionite cage. Their coordination number varies between 6 and 9. The marked differences in T-O distances found between hydrated and dehydrated erionite are explained in terms of interactions between extraframework ions and framework oxygens.