Erionite-Ca |K2(Ca0.5,Na)8(H2O)30| [Al10Si26O72]
Erionite-Na |K2(Na,Ca0.5)7(H2O)30| [Al9Si27O72]
Erionite-K |K2(K,Na,Ca0.5)7(H2O)30| [Al9Si27O72]
Morphology:

Erionite sprays, Agate Beach, Lincoln Co., Oregon, USA. Width of image 9 mm.
Hexagonal, 6/m2/m2/m
Single crystals as hexagonal prisms terminated by a pinacoid with sizes under 3 mm
Fibrous and wool-like
Common forms: {0001} and {1010}
Physical properties:
Cleavage: poor, prismatic
Hardness: 3.5 – 4
Density: 2.02 - 2.13 g/cm3
Luster: vitreous
Streak: white
Optical properties:
Color: colorless to pale tan or pink, colorless in thin section
Uniaxial (+ or -). ω = 1.455-1.477,
ε = 1.459 – 1.480, δ= 0.003 – 0.005
Crystals with lower Si/Al tend to be negative (Passaglia et al. 1998)


Prismatic erionite replacement of rhyolite tuff in the Big Sandy Formation, Mohave Co., Arizona, USA. Width of image 0.48 mm (from Sheppard and Gude 1973)
Crystallography:
Space Group: P63/mmc
Unit cells:
Erionite-Ca a 13.333 Å c 15.091 Å
(Harada et al. 1967)
Erionite-Na a 13.214 Å c 15.048 Å
(Sheppard and Gude 1969)
Erionite-K a 13.227 Å c 15.075 Å
(Passaglia et al. 1998)
Names:
Erionite was described and named by Eakle (1898) for woolly masses occurring in welded rhyolite tuff at the old Durkee opal quarry, Swayze Creek, Baker County, Oregon, USA. The name is from the Greek word for wool, alluding to its appearance. The Subcommittee on Zeolites of the International Mineralogical Association, Commission on New Minerals, Nomenclature and Classification (Coombs et al. 1997) elevated the name to series status, and named three new species based on the dominant extra-framework cation: erionite-Ca, type example from Mazé, Niigata Prefecture, Japan (Harada et al. 1967), erionite-K, type example from Rome, Malheur County, Oregon, USA (Eberly 1964), and erionite-Na, type example from Cady Mountains, San Bernardino County, California, USA (Sheppard et al. 1965). Dogan and Dogan (2008) proposed a new general chemical formula for the three erionite minerals, erionite-Ca, erionite-Na, erionite-K, based on the mean elemental values of the most abundant extra-framework cations within the erionite found in various areas.
Crystal structure:
Erionite is one of the 6-ring zeolites, a group that also includes offretite and levyne among others (Gottardi and Galli, 1985). The framework (type ERI) is constructed of 6-rings in the sequence AABAAC…, first proposed by Staples and Gard (1959), and confirmed by later refinements, e.g. Alberti et al. (1997). This stacking arrangement produces columns of cancrinite cages alternating with double 6-ring (D6R) cages (formed by the A 6-rings) and of erionite cavities between the B or C 6-rings.

For clarity in this diagram only the tetrahedral sites and T-T linkages have been plotted.

Like in most 6-ring zeolite structures the Si and Al are randomly distributed in the T-sites.
K-cations (shown as purple circles) occur in the cancrinite cages coordinated with 6 of the framework oxygen anions at distances of 2.92 Å. There are two such positions per unit cell, and these are filled (or nearly filled) in all erionite. All the remaining cations and all water molecules (blue circles) are in the erionite cages. In the erionite-Ca, refined by Alberti et al. (1997), there are three partially occupied positions Ca1 (here as yellow circles), Ca2 (green), and Ca3 (red). Each is coordinated with water molecules.
Even though there are six cation sites within a single erionite cavity, only five can be occupied simultaneously. The two Ca3 sites (red) are too close for simultaneous occupancy. With two cavities per unit cell, the limit of the cation content is about 10 (not counting the K-cations, which are in cancrinite cages). Most crystals of high Si/Al erionite-Ca have about 5 cations per unit cell (about 2.5 cations per erionite-cavity). Low Si/Al, erionite-Na, like those that occur as overgrowths on levyne, has up to 10 cations per unit cell (5 per cavity), requiring the maximum cavity occupancy. The structure of an erionite-Na composition was recently reported (Cametti et al. 2013) with the cation site locations close to those shown here.

Because of similarities in the framework structure of levyne, offretite, and erionite, these three minerals commonly exhibit epitaxial intergrowths. Most common is epitaxially oriented erionite on levyne, but offretite and erionite may alternate along a single prism as well. A recent synchrotron nano-diffraction data combined with three-dimensional electron diffraction (3DED) shows that the erionite from Tuzkoy (Cappadocia, Turkey) is not associated with offretite (Giacobbe et al. 2023).

The dehydration dynamics of erionite were studied by Schlenker et al. 1977; Ballirano and Cametti 2012; Ballirano and Pacella 2016. The thermal behavior of erionite-Na has been found to deviate significantly from that of erionite-K and erionite-Ca. Internal ion-exchange mechanism during dehydration results in the depletion of some extraframework sites counterbalanced by an increase of others (Ballirano and Cametti 2012).
High-pressure experiments performed using potentially penetrating P-transmitting fluids showed significantly low compressibilities, suggesting the P-mediated intrusion of new molecules into the structural voids (Battistion et al. 2022).
Chemical composition:
Erionite composition varies both in the Si,Al content of the framework and the cation content of the erionite cavities.
R2+ - R+ - Si compositional plot and Na - Ca - K plot of the erionite series analyses from Passaglia et al. (1998) and others (click on either drawing to get a larger image). Squares (solid and open) represent samples from cavities in basaltic rocks, and circles represent samples from diagenetically altered pyroclastic rocks. Solid squares represent erionite from epitaxial overgrowths on levyne, and open squares from other associations in basalt cavities. The offset of points toward K corner in both plots is the result of essential K occurring in the cancrinite cages (Deer et al. 2004).
Occurrences:

The global occurrence and geological setting of erionite has been recently reviewed by Patel et al. (2022). Erionite has been reported as a minor to rare mineral, generally associated with clinoptilolite, chabazite, phillipsite, analcime and mordenite, from over 100 locations worldwide, mainly in the USA. Erionite commonly occurs in alkaline environments (Bisroy 2002), such as in saline lakes
Diagenetic alteration of rhyolitic volcanic rocks within saline lakes
Erionite, associated with clinoptilolite, chabazite, phillipsite, and K-feldspar, replaces Miocene to Pleistocene rhyoliteic tuff deposited in alkaline, saline lakes in the western USA (Sheppard et al. 1965; Sheppard and Gude 1973) and in rhyolitic ignimbrite of Upper Miocene-Pliocene age together with lacustrine sediments in Cappadocia, Turkey, (Temel and G?ndo?du 1996) and in Miocene dacitic tuffs in lacustrine environments in central Turkey (Karakaya et al. 2015). Another occurrence of this type is from Agua Prieta, Mexico (Cochemé et al. 1996).

Hydrothermal alteration of silica-rich volcanic rocks

Erionite has been found along with mordenite and clinoptilolite in the upper parts of drill cores from some active hydrothermal areas in Yellowstone National Park, Wyoming, USA (Bargar and Keith 1995).
Diagenesis in altered basalt
There are many occurrences of erionite that formed as lining in vesicles and fractures in basalt (e.g. Sheppard et al. 1974). Common associated minerals are clinoptilolite, phillipsite, chabazite, and mordenite. The zeolites formed from diagenetic reaction with groundwater (Tschernich 1992, Patel et al. 2022).
Hydrothermal alteration in basalt and dolerite
In this mode of occurrence, zeolites including erionite occur in veins and in basalts and dolerites (Patel et al. 2022). Examples include in dolerite from Mt Adamson, Antarctica (Vezzalini et al. 1994), and in basalt from Lessini Mounts, Italy (Mattioli et al. 2016).

Another mode of occurrence is in volcaniclastic beds in Miocene sedimentary rocks in the Auckland area, New Zealand. The zeolite minerals analcime, chabazite, clinoptilolite, erionite, mordenite and phillipsite are variously present and were formed by the alteration of volcanic glass in during diagenesis (Sameshima 1978, Davidson and Black 1994).

Uses:
Erionite is the only natural zeolite having applications in fuel processing. Mobil Oil Co. developed the Selectoforming process for the removal of low octane normal alkanes by selective hydrocracking on erionite containing about one tenth percent platinum. Among the cracking products C7-C9 hydrocarbons are practically missing due to the particular pore structure, where these hydrocarbons are captured in the erionite cage and thus fractionated (Chen et al., 1968; 1969).
Potential health hazard:
Great attention has been initially paid by the scientific community and authorities in Turkey, where the relationship between erionite and lung cancer was first demonstrated (Carbone et al. 2011, Giacobbe et al. 2023). Epidemiological and experimental studies have shown that erionite fibers have the highest carcinogenic potential of any mineral fibers, including crocidolite and chrysotile asbestos (e.g. Carbone et al. 2011, Patel et al. 2022). Therefore, erionite-bearing sedimentary rock should not be used for any purpose unless it is totally and effectively controlled to prevent exposures during mining, processing, handling, and utilizing the materials, to humans as well as animals (Anonymous 1986; Dogan and Dogan 2008).The mechanisms that induce cyto- and geno-toxic damage seem to be related to the presence of iron in erionite (Gualtieri et al. 2016).
References:

Alberti, A., Martucci, A., Galli, E., and Vezzalini, G. (1997) A reexamination of the crystal structure of erionite. Zeolites 19, 349-352.

Anonymous (1986) Asbestos and Other Natural Mineral Fibres, Environmental Health Criteria 53, IPCS International Programme on Chemical Safety, World Health Organization, Geneva, 1986. (see sections 2.2.1 Fibrous zeolites, and specifically 7.2.3 Fibrous zeolites - erionite (p. 106), and Conclusions, section 9.3.2).

Ballirano, P. and Cametti, G. (2012) Dehydration dynamics and thermal stability of erionite-K: Experimental evidence of the “internal ionic exchange” mechanism. Microporous Mesoporous Mater., 163, 160-168.

Ballirano, P. and Pacella, A. (2016) Erionite-Na upon heating: dehydration dynamics and exchangeable cations mobility. Sci. Rep., 6(1), p.22786.

Bargar, K.E. and Keith, T.E.C. (1995) Calcium zeolites in rhyolitic drill cores from Yellowstone National Park. In: Ming, D.W. and Mumpton, F.S. (eds.) Natural Zeolites ’93, International Committee on Natural Zeolites, Brockport, New York, 69-86.

Battiston, T., Comboni, D., Pagliaro, F., Lotti, P., Hanfland, M. and Gatta, G.D. (2022) High-pressure behavior and crystal-fluid interaction in natural erionite-K. Mater. Chem. Phys., 292, 126760.

Bisroy, R. (2002) Activity diagrams of zeolites and implications for the occurrences of zeolites in Turkey and of erionite worldwide. Clays Clay Miner. 50, 136-144.

Cametti, G., Pacella, A., Mura, F., Rossi, M. and Ballirano, P. (2013) New morphological, chemical, and structural data of woolly erionite-Na from Durkee, Oregon, USA. Am. Mineral., 98, 2155-2163.

Carbone, M., Baris, Y. I., Bertino, P., Brass, B., Comertpay, S., Dogan, A. U., et al. (2011) Erionite exposure in North Dakota and Turkish villages with mesothelioma. Proc. Natl. Acad. Sci. U. S. A. 108, 13618–13623.

Chen, N.Y., Maziuk, J., Schwartz, A.B., and Weisz, P.B. (1968) Selectoforming, a new process to improve octane and quality (of gasoline). Oil and Gas Journal 66, 154-157.

Chen, N.Y., Lucki, S.J., and Mower, E.B. (1969) Cage effect on product distribution from cracking over crystalline aluminosilicate zeolites. J. of Catal. 13, 329-332.

Cochemé, J. J., Lassauvagerie, A. C., Gonzalez-Sandoval, J., Perez-Segura, E., and Münch, P. (1996) Characterisation and potential economic interest of authigenic zeolites in continental sediments from NW Mexico. Mineral. Deposita 31, 482–491.

Coombs, D. S., Alberti, A., Armbruster, T., Artioli, G., Colella, C., Galli, E., et al. (1997) Recommended nomenclature for zeolite minerals: report of the subcommittee on zeolites of the International Mineralogical Association, Commission on New Minerals and Mineral Names. Mineral. Mag. 35, 533–571.

Davidson, K. J., and Black, P. M. (1994) Diagenesis in Early Miocene Waitemata Group sediments. Geoscience Rep. Shizuoka Univ. 20, 135–142.

Deer, A., Howie, R., Wise, W.S., and Zussman, J. (2004). Rock Forming Minerals. vol. 4B. Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites. The Geological Society, London.

Dogan, A. and Dogan, M. (2008) Re-evaluation and re-classification of erionite series minerals. Environ. Geochem. Health, 30, 355-366.

Eakle, A.S. (1898) Erionite, a new zeolite. Am. J. Sci. 156, 66-68.

Eberly, P.E., Jr. (1964) Adsorption properties of naturally occurring erionite and its cationic-exchanged forms. Am. Mineral. 49, 30-40.

Giacobbe, C., Moliterni, A., Di Giuseppe, D., Malferrari, D., Wright, J. P., Mattioli, M., Ranieri, S., Giannini, C., Fornasini, L., Mugnaioli, E., Ballirano and P., Gualtieri, A. F. (2023) The crystal structure of the killer fibre erionite from Tuzköy (Cappadocia, Turkey) IUCrJ, 10, 397-410.

Gottardi, G. and Galli, E. (1985) Natural Zeolites. Springer-Verlag, Berlin.

Gualtieri, A.F., Gandolfi, N.B., Pollastri, S., Pollok, K. and Langenhorst, F. (2016) Where is iron in erionite? A multidisciplinary study on fibrous erionite-Na from Jersey (Nevada, USA). Sci. Rep., 6, 37981.

Harada, K., Iwamoto, S., and Kihara, K. (1967). Erionite, phillipsite and gonnardite in the amygdales of altered basalt from Mazé, Niigata Prefecture, Japan. Am. Mineral. 52, 1785-1794.

Karakaya, M.Ç., Karakaya, N., and Yavuz, F. (2015). Geology and conditions of formation of the zeolite-bearing deposits of southeast Ankara (Central Turkey). Clays Clay Min. 63, 85-109.

Mattioli, M., Cenni, M., and Passaglia, E. (2016) Secondary mineral assemblages as indicators of multistage alteration processes in basaltic lava flows: Evidence from the Lessini Mountains, Veneto Volcanic Province, Northern Italy. Period. Mineral. 85, 1–24.

Passaglia, E., Artioli, G., and Gualtieri, A. (1998) Crystal chemistry of the zeolites erionite and offretite. Am. Mineral. 83, 577-589.

Patel, J.P., Brook, M.S., Kah, M., and Hamilton, A. (2022). Global geological occurrence and character of the carcinogenic mineral, erionite: A review. Front. Chem.10:1066565.

Sameshima, T. (1978). Zeolites in tuff beds of the Miocene Waitemata Group, Auckland Province, New Zealand. In: Sand, L.B. and Mumpton, F.S. (eds.) Natural zeolites, occurrence, properties and uses. Oxford and New York: Pergamon Press, 309–317.

Schlenker, J.L., Pluth, J.J. and Smith, J.V. (1977) Dehydrated natural erionite with stacking faults of the offretite type. Acta Cryst. B, 33, 3265-3268.

Sheppard, R.A., Gude, A.J. III., and Munson, E.L. (1965) Chemical composition of diagenetic zeolites from tuffaceous rocks of the Mojave Desert and vicinity, California. Am. Mineral. 50, 244-249.

Sheppard, R.A. and Gude, A.J. III (1969) Diagenesis of tuffs in the Barstow Formation, Mud Hills, San Bernardino County, California. U. S. Geological Survey, Professional Paper 634, 35 pp.

Sheppard, R.A. and Gude, A.J. III. (1973) Zeolites and associated authigenic silicate minerals in tuffaceous rocks of the Big Sandy Formation, Mohave County, Arizona. U. S. Geological Survey, Professional Paper 830, 36 pp.

Sheppard, R.A., Gude, A.J. III., Desborough, G.A., and White, J.S. (1973) Levyne-Offretite Intergrowths from Basalt near Beech Greek, Grant County, Oregon. Am. Mineral. 59, 837-842.

Staples, L.W. and Gard, J.A. (1959) The fibrous zeolite erionite: its occurrence, unit cell, and structure. Mineral. Mag. 32, 261-281.

Temel, A., and Gündo?du, M.N. (1996) Zeolite occurrence and the erionite-mesothelioma relationship in Cappadocia, Central Anatolia, Turkey. Min. Deposita, 31, 539-547.

Tschernich, R.W. (1992) Zeolites of the World. Geoscience Press, Phoenix, Arizona, USA.

Vezzalini, G., Quartieri, S., Rossi, A., and Alberto, A. (1994) Occurrence of zeolites from Terra Victoria Land (Antarctica). Terra Antarct. 1, 96–99.

Updated: May 2025.