hydrated sodium-potassium-calcium aluminosilicate mineral in the zeolite family, one of the most abundant zeolites present in sedimentary rocks. Its chemical composition is approximately represented by the formula (Na₂,K₂,Ca)₂Al₄Si₁₄O₃₆·15H₂O. It forms woolly, fibrous crystals that have an internal molecular structure similar to that of chabazite. Erionite is the principal mineral present in several tuff deposits in the U.S. states of Oregon, California, Nevada, and Arizona; a few of these have been quarried for building stone since the 19th century.