Erionite is a naturally occurring fibrous mineral belonging to the zeolite group that is typically found in hydrothermally altered volcanic ash deposits. While erionite and other zeolites have similar optical, structural and chemical properties, erionite has been classified as a Group 1 Carcinogen by IARC due to its link to malignant mesothelioma in Turkey. The document discusses using polarized light microscopy, scanning electron microscopy, and transmission electron microscopy to compare erionite to other naturally occurring fibrous zeolites like mesolite, natrolite, thomsonite and mordenite.