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 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).
       
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. Although the structure of such an erionite-Na composition has never been refined, the cation sites are probably 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.
   
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:  
  Diagenetic alteration of sediment  
  Erionite, associated with clinoptilolite, chabazite, phillipsite, and K-feldspar, replaces rhyolite tuff deposited in alkaline, saline lakes and rarely in deep marine sediment (Sheppard et al., 1965, Sheppard and Gude, 1973).
  Cavities in altered basalt  
  There are many occurrences of erionite in cavities of basaltic lavas. Common associated minerals are clinoptilolite, phillipsite, chabazite, and mordenite. Some occurrences show pervasive rock alteration, suggesting hydrothermal alteration, while others have limited alteration, consistent with diagenetic reaction with groundwater (Tschernich, 1992).
  Hydrothermal alteration  
  Erionite has been found in the upper parts of drill core from some active hydrothermal areas in Yellowstone National Park, Wyoming, USA (Bargar and Keith, 1995).
       
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:  
  Compared to other mineral particles, erionite has been shown to have greater pathogenicity than asbestos. 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).
       
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).

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.

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. Journal of Catalysis 13, 329-332.

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. eochem. Health, 30, 355-366.

Eakle, A.S. (1898) Erionite, a new zeolite. American Journal of Science 156, 66-68.

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

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

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

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

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. American Mineralogist 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. .

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

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