As with the six regulated asbestos minerals (chrysotile, amosite, crocidolite, anthophyllite, tremolite, and actinolite), the zeolite mineral, erionite, can exhibit a fibrous morphology. When fibrous erionite is aerosolized and inhaled, it has been linked to cases of lung cancers, such as malignant mesothelioma. Importantly, fibrous erionite appears to be more carcinogenic than the six regulated asbestos minerals. The first health issues regarding erionite exposure were reported in Cappadocia (Turkey), and more recently, occupational exposure issues have emerged in the United States. Erionite is now classified as a Group 1 carcinogen. Thus, identifying the geological occurrence of erionite is a prudent step in determining possible exposure pathways, but a global review of the geological occurrence of erionite is currently lacking. Here, we provide a review of the >100 global locations where erionite has been reported, including: 1) geological setting of host rocks; 2) paragenetic sequence of erionite formation, including associated zeolite minerals; 3) fiber morphological properties and erionite mineral series (i.e., Ca, K, Na); and 4) a brief overview of the techniques that have been used to identify and characterize erionite. Accordingly, erionite has been found to commonly occur within two major rock types: felsic and mafic. Within felsic rocks (in particular, tuffaceous layers within lacustrine paleoenvironments), erionite is disseminated through the layer as a cementing matrix. In contrast, within mafic (i.e., basaltic) rocks, erionite is typically found within vesicles. Nevertheless, aside from detailed studies in Italy and the United States, there is a paucity of specific information on erionite geological provenance or fiber morphology. The latter issue is a significant drawback given its impact on erionite toxicity. Future erionite studies should aim to provide more detailed information, including variables such as rock type and lithological properties, qua Indeed, erionite has been identified in various geological formations globally ( Figure 1 ), and due to health concerns, significant research has been undertaken in the United States to identify the geological occurrences of erionite (e.g., Sheppard, 1996 ; Van Gosen et al., 2013 ) and in Italy (e.g., Giordani et al., 2017 , 2016 ), although the latter study only focused on one region of Italy (the Lessini Mountains). Despite Giordani et al. (2017) listing some global erionite occurrences, a comprehensive review and synthesis of literature about the worldwide geologic occurrences of erionite is lacking. Dakota and Old Sarihidir (following Carbone et al., 2011 ); (B) ternary diagram of Na-C-K; (C) Mg and K composition of different erionite species; (D) comparison of (Na + Ca) values against K; (E) comparison of (Na + Ca) against Mg; (F) frequency of Si/(Si + Al) ratios. 3 Geology 3.1 Global occurrence Erionite has been found worldwide in many different countries, as shown in Figure 1 , yet often in publications, much of the key geological data useful in characterizing erionite (e.g., rock units, paleoenvironment, apparent mode of formation) is missing or incomplete. Erionite has been found within rocks used for road aggregates in the United States ( Van Gosen et al., 2013 ), and in the rock used to construct houses in Turkey ( Carbone et al., 2007 ). Hence understanding the geological occurrence, formation processes, and geographic distribution of erionite-bearing rock is important. 3.2 Geological settings of erionite As with other zeolites, erionite is usually identified within volcanic and volcanically-derived rocks ( Figures 1 , 4A ), where the minerals typically form via diagenesis or hydrothermal alteration ( Mumpton, 1979 ; Coombs et al., 1997 ). Nevertheless, erionite has also been identified in sedimentary and metamorphic rocks ( Lehtinen, 1976 ; Sheppard, 1996 ; Schmieder and Jourdan, 2013 ). The characteristic host rocks that erionite typically occurs in are basalt and tuffs, but for this review, the volcanic rocks have been classified based on their wt% SiO 2 composition. The classification via SiO 2 is because when describing certain rock types, such as tuff, the definitions can vary according to the author. Note that for many published studies, the erionite formation processes were not reported by the authors, which, again, limits the possibility of accurate geospatial mapping of potentially hazardous erionite-bearing units. Notwithstanding this, from the literature summarized in Supplementary Table S1 , six characteristic geological settings can be recognized for erionite formation, outlined below. (1) Hydrothermal alteration of silica-rich volcanic deposits—heated hydrothermal fluids alter the surrounding host rock and cause the precipitation of erionite and other minerals ( Bargar and Beeson, 1981 ; Van Gosen et al., 2013 ). (2016) showed that Fe found in some erionite analyses was actually coming from iron-bearing nano-particles on the surface of the erionite fibers. The fact that iron is not found in the erionite crystal structure of natural samples also has a sound geological basis. This is because, during the zeolitization process, iron typically present as Fe 2+ in the host tuffs is leached, oxidized, and precipitated later as secondary iron-bearing phases like iron hydroxides ( Gualtieri et al., 2018 ). Second, analysis of associated zeolites would assist in determining the paragenetic sequence of formation, which is useful for geological studies. Third, the effects of preparation methods on the morphology of erionite fibers should be explored, and optimum milling protocols could be identified to enhance the replicability of results. Finally, major differences between erionites from magmatic vesicles and those that have crystallized in volcaniclastic sedimentary rocks need to be fully explored in terms of toxicity and hazard.