Mordenite |Na2,Ca,K2)4(H2O)28| [Al8Si40O96]
       
Morphology:  
Mordenite needles up to 7 mm on pink heulandite in altered basalt
Stevenson, Skamania County, Washington, USA.
  Orthorhombic mmm or mm2
Single crystals are thin fibers,
0.1 to 10 mm long
Common forms: {100}, {010}, {110} and {101}
 
Physical properties:
  Cleavage:  {100} perfect, {010} distinct
Hardness:  3 - 4
Density:  2.12 - 2.15 g/cm3
Luster:  vitreous to silky
Streak:  white
   
Optical properties:                               
  Color:  white, yellowish or pinkish
colorless in thin section
Biaxial (+ or -)
α = 1.472 - 1.483, β = 1.475 - 1.485,
γ = 1.477 - 1.487, δ = 0.004 - 0.005
2Vz = 76-104°
a = Y, b = Z, c =X, O.A.P. = (100)
     
Crystallography:
  Unit cell
   a = 18.052 - 18.168 Å   
   b
= 20.404 - 20.527 Å
   c
= 7.501 - 7.537 Å
Z = 1
Space group:
   The average space group is Cmcm.
   The true space group is probably Cmc21.
       
Name:  
  Mordenite was described and named by How (1864) for material found along the shore of the Bay of Fundy, 3-5 km east of Morden, King’s County, Nova Scotia, Canada. It is named after the discovery and type locality.
       
Crystal structure:  
  The crystal structure of mordenite was determined by Meier (1961) and refined by Gramlich (1971) on Na-exchanged natural crystals from Challis, Idaho, USA. The topology of the framework is characterized by 5-member tetrahedral rings, which are part of the mor composite building unit (see MOR). These building units are linked by edge-sharing into chains along c, which are in turn linked together by 4-rings to form a puckered sheet perforated with 8-ring holes. These permeable sheets are oriented parallel to (010). Linking these sheets together with 4-rings, 12-ring channels are formed parallel to [001]. The 8-ring holes of successive sheets do not align to make channels parallel to [010] (see fig. 2 in Simoncic and Armbruster, 2004).
  Ever since mordenite was synthesized (Sand, 1968), it has been known that some synthetic mordenite can accept cations or molecules larger than 4.5Å, while natural mordenite cannot. Explanations for small-port mordenite have remained controversial. In an effort to solve the problem, Simoncic and Armbruster (2004) recently refined a natural and a synthetic, large-port  mordenite and found that both structures exhibit the same defect features. Domains of the entire Cmcm framework structure are reproduced by a non-crystallographic (001) mirror plane at z = 0 and z = 1/2. These domain shifts do not influence or obstruct the 12-membered ring-channels characteristic of this zeolite (and therefore, do not effectively solve the small-port problem).
 

 
    Projection of the structure of mordenite onto (001), based on the refinements by Alberti et al. (1986) and Simoncic and Armbruster (2004). Si,Al distribution in tetrahedral sites is largely disordered, but Al is concentrated in the T3 tetrahedra, which are here shaded light green.  Most non-framework cations are in the 8-member ring channels. Red circles represent Ca2+ sites with about 50% occupancy; purple circles, K+ cations with 25% occupancy; and yellow, Na+ with about 50% occupancy. The remaining K+ and Na+ probably occur in the other sites with H2O molecules (blue) in the 12-ring channels, parallel to [001].  
  A consequence of refinement with the space group Cmcm is that an oxygen (O8) is located on an inversion center (black dots in the figure), causing the T-O8-T bond angle to be 180°. Liebau (1961) showed that such straight angles are energetically unfavorable and are unlikely to occur. Refinement in the space group Cmc21 removes the inversion center and reduces T-O8-T bond angle from 180 to 155-167° (Simoncic and Armbruster, 2004).
  The refinement of the Elba mordenite by Alberti et al. (1986) and another from Jarbridge, Nevada, USA by Simoncic and Armbruster (2004) determined the non-framework cation sites and occupancy. Site Ca (in the figure above), bonded to oxygen of T3 tetrahedra is 50% occupied by Ca2+, while K+ occurs in the K sites only when the Ca-site is not occupied.
Chemical composition:
  Mordenite is a high-silica zeolite, in which the Si,Al content of the framework and the cation content of the erionite cavities are moderately variable. The compositional range is illustrated in the figure below. TSi ranges from 0.80 to 0.86 (38.7 to 40.9 Si per unit cell). The non-framework cation compositions are mostly Na-dominant. Mordenite samples from cavities in basalt have less than 0.8 K ions per unit cell, whereas those from rhyolitic, tuffaceous rock have decidedly more K, in many samples more than one ion per cell. In at least five localities, Yucca Mountain, Nevada, U.S.A.; Yeongil, Korea; Polyegos Island, Greece, island of Samos, Greece (Pe-Piper and Tsolis-Katagas, 1991) and eastern Taiwan (Lo and Hsieh, 1992), analytical results show at least some samples with K as the dominant cation.
  Judging from several complete analyses, the water content of mordenite is probably 28 H2O molecules per unit cell, which corresponds to about 14.2 weight percent H2O in the analysis.
 

R2+ - R+ - Si plot of the mordenite compositions from Passaglia (1975) and Deer et al. (2004). Black squares represent samples from cavities in basaltic rock, the yellow circles represent authigenic mordenite in tuffaceous, sedimentary rock, and red diamonds, two samples from active hydrothermal areas at Yellowstone National Park, Wyoming, USA. (Adapted from Deer et al., 2004).
 

 

   
Occurrences:
  Mordenite is a common alteration product of pyroclastic sediment and sedimentary rocks, as well as lava flows. These occurrences range from low temperature diagenesis of vitric tuff in lacustrine sediment, to thermal aureoles surrounding volcanic vents, and to active hydrothermal systems. The following was largely adapted from Deer et al. (2004).
  Diagenesis and burial metamorphism of sediment and sedimentary rocks
  Mordenite crystallizes over a range of physical conditions, as a replacement of volcanic glass or pre-existing zeolite phases, from the surface temperatures of lakes in arid climates to those of the laumontite subfacies of very low grade metamorphic rocks (> 110°C).
  Hydrologically closed systems
    Mordenite is one of several diagenetic zeolite and clay minerals replacing rhyolitic tuff in lacustrine sediment in the western U.S. About 1000 m of fluvial and lacustrine rocks with interspersed rhyolitic tuff beds 1 cm to 2 m thick comprise the Miocene Barstow Formation in the Mud Hills of western San Bernardino County, California, USA. The tuff beds have been variably replaced by zeolites, including mordenite, commonly associated with clinoptilolite, and less commonly with analcime, phillipsite-Na, and chabazite-Na. The zeolites formed during diagenesis by reaction of silicic glass with interstitial water. Sheppard and Gude (1969) suggest that the interstitial water was likely saline and alkaline, based partly on the occurrence of molds of common saline minerals. The entire section was probably never buried by more than 1000 m of overlying sediment.
    Similar environmental conditions probably existed in west-central Nevada, USA, where tuffs beds were placed by mordenite, ferrierite, and clinoptilolite. In two localities (Rice et al., 1992, and Sand and Regis, 1968) pyroclastic beds dominate the section, that also includes lacustrine sediment.
    Sparse mordenite occurs with smectite, analcime, clinoptilolite, and potassium feldspar replacing thin, vitric tuffs interbedded with lake sediment of the Eocene Lake Gosiute forming parts of the Green River Formation, Wyoming, USA. The distribution of the zeolite species is related to water salinity within the lake (Surdam and Parker, 1972; Goodwin, 1973; and Ratterman and Surdam, 1981).
   

Mordenite, associated with clinotpilolite-K, is a replacement product of vitric tuff and perlite, and a common phase in cavities and fractures in the tuff beds in the Guryongo area, Korea (Noh and Boles, 1989). The pyroclastic rocks are interbedded with lacustrine shale, siltstone, and coal beds. Textural evidence supports the widely held notion that zeolite replacement of shards occurs following solution of the glass. Much of the mordenite in the Guryongo area may have followed the crystallization of smectite, but some replaced clinoptilolite. Judging from the low grade of interbedded lignite, Noh and Boles (1989) suggest temperatures never exceeded 100° C.

  Hydrologically open systems
    There are several occurrences of mordenite in thick accumulations of pyroclastic rocks in which the distribution of mordenite and other zeolites indicates control by temperature. One of the most extensively studied is Yucca Mountain, Nevada, USA. Here a 3000 m sequence of Miocene silicic ash-flow tuff, fall-out tuff, lava flows, and flow breccia has been explored by extensive drilling. The volcaniclastic material has been variably altered to clay, zeolite, feldspar, and silica minerals. Near the surface rocks remain glassy, but zones with mordenite and clinoptilolite, analcime, and albite follow in succession with depth and with distance from the Timber Mountain caldera, the source for the volcanic products. Most of the mordenite crystallized following dissolution of glass shards, but Sheppard et al. (1988) show scanning electron micrographs of some mordenite forming after dissolution of clinoptilolite. Experimental work by Bish et al. (1982) suggests that conversion of clinoptilolite to mordenite occurs between 120° to 180°C.
    Significant expanses of ignimbrite on the island of Polyegos, Greece, have been replaced by mordenite and clinoptilolite (Kitsopoulos 1997). Mordenite comprises 40% to 90% of the altered ignimbrite, while clinoptilolite is much less abundant. Textural evidence shows that mordenite has replaced clinoptilolite and that illite replaces smectite. These relations suggest that mordenite and illite crystallized from interstitial fluids heated by subsequently emplaced rhyolite domes.
    Much of the rhyolite tuff associated with domes and flows of Ponza Island, Italy, have been altered to bentonite and to a mordenite-smectite assemblage (Passaglia et al., 1995). Alteration of the glass is thought to have occurred by percolating waters, heated by post-volcanic events.
  Marine sediment from arc-source terrains
    Mordenite is widespread in pyroclastic rocks of Japan, especially in the Green Tuff region of northern Honshu (Utada, 1971; Iijima and Utada, 1972). In this region of Japan zeolite distribution is related either to depth of burial or proximity to intrusions, giving a sequence of zones formed by diagenesis into very low-grade metamorphism. Zones of burial diagenesis and metamorphism defined by Iijima and Utada (1972) are:
   I: altered glass zone,
   II: alkali clinoptilolite zone,
   II:I clinotpilolite-mordenite zone,
   IV: analcime zone, and
   V: albite zone.
Using geothermal gradients in oil fields in this region, Iijima and Utada (1972, Fig. 8) suggest that the lower temperature limit of zone III is about 50°C and the upper limit about 90° C. Mordenite is most abundant in silicic tuff and less common in pyroclastic rocks with a mafic composition.
    In the metamorphic aureole surrounding an intrusive mass in the Tanzawa Mountains of central Japan (Seki et al., 1969b; Seki et al., 1972) mordenite occurs in the outer most zone. In the southern Tanzawa Mountains mordenite occurrence is sparse, but in the eastern part of the range the abundance is much greater, especially in the pumice-tuff beds. All pyroclastic rocks are andesitic, but the higher abundance in pumice beds may be related to the higher reactivity of glass (Seki et al., 1972).
    Mordenite has been reported from two settings in California, USA, that may have formed by very low grade metamorphism. Surdam and Hall (1968) describe the replacement of the Miocene Obispo Tuff in San Luis Obispo County by mordenite, comprising up to 75% of the rock. Also in the California Coast Ranges Murata and Whiteley (1973) found pervasive laumontite as cement and fracture filling in the Miocene Briones Sandstone, and in one locality mordenite and clinoptilolite occur in opal veins cutting a tuff.
    Aguirre et al. (1978), Offler et al. (1980), Aguirre and Offler (1985), and Levi et al. (1989) show that the rocks in basins along the Andes of Peru and Chile have not been deformed and that grade increases with depth from zeolite to greenschist facies. In the Santiago area of Chile Levi et al. (1989) divide the zeolite facies into a shallow mordenite subfacies, which generally overlies the laumontite subfacies. The affected rocks are Mesozoic marine and continental lavas and volcaniclastic rocks of mafic to intermediate composition. These are overlain by Cenozoic subareal andesitic flows and clastic debris. Interestingly the mineralogic breaks tend to be at unconformities.
  Diagenesis of mafic lava flows
    Mordenite is a relatively common zeolite in basalt cavities in localities (Tschernich, 1992). It is most common in host rocks that are oversaturated with respect to silica, such as tholeiitic basalt. Rarely is there any evidence of hydrothermal alteration, such as extensive alteration of the host rock. Common associated minerals are other high-silica zeolites, commonly clinoptilolite, and silica minerals. A sense of temperature control on mordenite occurrence in basalt cavities comes from Walker’s (1960) description of Iceland localities. Mordenite is restricted to cavities in tholeiitic basalt of eastern Iceland, exposed in the sea cliffs at Teigarhorn in Berufjord. From sea level to the 300 m elevation mordenite is abundant and associated with heulandite, stilbite, scolecite, and epistilbite. Above 300 m mordenite occurs only with chalcedony, quartz, and clay, while above about 750 m the cavities are empty.
  Hydrothermal alteration
    Mordenite has been found in drill core from most geothermal fields, leading to the interpretation that mordenite is characteristic of hydrothermal alteration. At Wairakei, New Zealand, where host rocks are rhyolitic, it has been found between the 73-300 m depths at temperatures 150-230°C (Steiner, 1953; Coombs et al., 1959). In the andesitic to dacitic Katayama geothermal area, Onikobe, northeast Japan, mordenite occurs at depths less than 100 m and at temperatures less than 100° C (Seki et al., 1969a). At Hanawa (Seki, 1966) mordenite is in the lowest temperature zones, i.e. outside or above the laumontite zone. In drill core in the rhyolite host rocks of several thermal areas of Yellowstone National Park, Wyoming, USA, mordenite occurs in 10 of the 15 studied. Temperatures range from 90° to 160°C, and depths, between 15 and 300 m. The pH measured in the hole at Y-3 was 8.12 at 88 m depth, at Y-6, 6.4, and 8.8 in the major thermal spring near the Y‑1 hole (Bargar and Beeson, 1984, 1985; Honda and Muffler, 1970). Mordenite is rare in core from all geothermal areas of Iceland, which may be related to the fact that the host rock is basalt.
    Mordenite is a characteristic mineral in the hydrothermal aureoles surrounding Kuroko-type deposits in northern Honshu, Japan (Utada, 1988). It replaces vitroclastic particles, as well as the groundmass of volcanic, lithic grains in the outermost of several zones. The hydrothermal zones are thought to have formed after the ores were deposited and covered with sediment. The capping sediments allowed cooling hydrothermal waters to spread outward, becoming alkaline and allowing zeolite deposition. The temperatures of zeolite formation are probably consistent with that of the mordenite-clinoptilolite zone of diagenesis, less than 120-150°C.
    Mordenite with dachiardite-Na and ferrierite occurs on joint surfaces and in cavities in chlorite schist and amphibolite in the tunnels through Tanzenberg near Kapfenberg, Styria, Austria (Postl et al., 1985). This type of occurrence in non-volcanic host rocks suggests the zeolite grew in warm fluids flowing through the fracture system.
 
Uses:
  Sedimentary deposits of mordenite are present in several countries, especially in Bulgaria, Hungary, Japan and the United States. Quarried material is generally substantial, e.g., a recent estimate of the yearly production in Japan is 150,000 tons (anonymous, 2006).
  Apart from generic applications in the fields of agriculture and building industry (as dimension stone), uses are known as sorbent and molecular sieve (Colella, 2005). Gas separation processes are reported for the production of high-grade O2 from air by pressure-swing operating generators. Full-scale plants based on mordenite-rich tuffs have been operating in Japan since the end of the 1960.
  With its interconnecting channels of puckered twelve and eight membered-ring openings, mordenite has also been found to be an effective lube dewaxing catalyst with the incorporation of a noble metal hydrogenative function. The catalytic dewaxing process developed by the British Petroleum Company in the early seventies (Bennett et al., 1975) is a catalytic cracking process, which employs a bifunctional platinum/H-mordenite catalyst.
       
Potential health hazard:
  Because of the fibrous nature of some crystalline morphological occurrences of mordenite the handling of such fibrous mordenite minerals may pose health risks similar to those of erionite and asbestos (Lilis, 1981; Stephenson et al., 1999). Therefore, mordenite-bearing sedimentary rock should be carefully examined by SEM to determine crystalline morphology prior to use. Fibrous forms should be used only with suitable precautions to prevent exposures to humans and animals, particularly respiratory, during mining, processing, handling, and utilizing the materials.
       
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