The zeolitic rocks of Akrotiri, on Santorini Island (Aegean Sea, Greece), can be grouped according to the zeolite minerals present. The first group includes zeolitic rocks that contain only clinoptilolite, the second group contains clinoptilolite and mordenite and the third group contains only mordenite. Clinoptilolite accounts for up to 56 wt.% and mordenite for up to 69 wt.% of the rocks. All samples contain feldspars (8–36 wt.%), clay minerals (6–8 wt.%), quartz (3–6 wt.%), opal-CT (2 wt.%), amphibole (2–4 wt.%) and amorphous materials (4–7 wt.%). The studied samples were classified chemically as andesites or dacites. The ammonium-exchange capacity of the studied samples was 104–158 meq 100 g–1. According to Commission Implementing Regulation (EU) No. 651/2013, zeolitic rocks that contain ≥80 wt.% clinoptilolite, ≤20 wt.% clay minerals and are free of fibrous minerals and quartz can be used as feed additives in animal husbandry. Zeolites with fibrous habit (mordenite, erionite, secondarily roggianite and mazzite) and SiO2 minerals such as quartz, cristobalite and tridymite can be dangerous to both humans and animals. The mineralogical study showed that, due to their low clinoptilolite content and the presence of both quartz and fibrous mordenite, the studied zeolitic rocks do not conform with European Regulation No. 651/2013. As a result, their use as feed additives and nutrition supplements is prohibited.

The Tertiary silicic volcanic rocks in the Silent Canyon Caldera beneath Pahute Mesa, of the Department of Energy's Nevada Test Site have been divided into three vertical mineralogical zones that vary in thickness and transgress stratigraphic boundaries. Zonal contacts are generally sharp. Zone 1, the uppermost zone, includes unaltered or incipiently altered rhyolitic glass. Zone 2 is characterized by a predominance of clinoptilolite and subordinate amounts of smectite, cristobalite, and mordenite. Zone 3 is a complex mineral assemblage that includes analcime, quartz, calcite, authigenic K-feldspar and albite, kaolinite, chlorite, and mixed-layer illite/smectite. The mixed-layer clay shows an increase in ordering and a decrease in expandability with depth.

Shortly after deposition and after shallow burial, hydration of relatively impermeable, highly porous vitric rocks resulted in the rapid formation of the Zone 2 assemblage (except mordenite). This stage of alteration resulted in a net porosity loss and negligible mass transfer. Continued burial and rise in temperature led to a dehydration stage in which the Zone 2 assemblage was replaced by the Zone 3 minerals. The dehydration stage resulted in a porosity increase and an increase in permeability of several orders of magnitude. This process, like the earlier reactions, also conserved mass. Precipitation of mordenite followed the formation of this zonal configuration. The diagenetic zones below Pahute Mesa were caused by: (1) changing pore-water chemistry in an essentially closed hydrologic system; (2) disequilibrium or kinetic precipitation of metastable phases; and (3) a higher thermal gradient than that now present.

After a temperature-dependent period when little dissolution occurs, the dissolution of rhyolitic glass can be described by dC/dt = k(Cs − C), where Cs is the concentration of dissolved silica at saturation, C is the instantaneous silica concentration, and k is a rate constant equal to 1.6 × 10−5, 3.0 × 10−5, and 4.5 × 10−5 sec−1 at 115°, 130°, and 140°C respectively, in 2 M Na-K carbonate solution at 1 kbar pressure. At 130°C, a Cs value of 0.177 M SiO2 is reached in 30 hr, and phillipsite, clinoptilolite, and mordenite begin forming at 34, 64, and 76 hr, respectively, in 2 M CO3, 1:1 Na/K. During glass dissolution and zeolite formation, the concentration of Al as Al(OH)4− is buffered at 3.7 × 10−4 M by an unidentified phase. The ratio of SiO2 to Al(OH)4− at the onset of zeolite formation is 475. In 2 M CO3 solution, phillipsite crystallization begins at 144 hr at 115°C, at 34 hr at 130°C, and at 20 hr at 140°C. Phillipsite crystallization begins at 48 hr in 1.5 M CO3, at 168 hr in 1.0 M CO3, and in excess of 550 hr in 0.2 M CO3 at 140°C. In addition to OH− catalysis, CO32- appears also to catalyze the glass-dissolution and zeolite-formation processes.

Thermodynamically, phillipsite is unstable relative to clinoptilolite and mordenite in silica-rich alkaline hydrothermal solutions. Phillipsite forms first, followed by clinoptilolite, and then mordenite. Phillipsite formation is favored by runs of one-week duration, temperatures less than 150°C, and K-rich fluids. Clinoptilolite formation is favored in runs of more than one week, temperatures less than 150°C and K-rich fluids. Mordenite formation is favored by runs of more than one week, temperatures greater than 140°C, and Na-rich fluids. In 8-day runs at 140°C, clinoptilolite formation was favored by liquid: solid reactant (volume: mass) ratios less than 1.0, mordenite by ratios from 0.85 to 1.5, and phillipsite by ratios greater than 1.5. The mechanism of formation of the different zeolites, particularly phillipsite, may involve silica-cyclic tetramers which are abundant in concentrated solutions under alkaline hydrothermal conditions but which are almost absent in dilute low-temperature solutions. Thus, the results of hydrothermal experiments may not be directly applicable to zeolite formation at low temperatures.

Mordenite occurs in hydrothermally altered rhyolitic volcanic rocks at the margin of a Late Miocene lake basin on the island of Samos, Greece. The mordenite was identified by its X-ray powder diffraction pattern and appearance in scanning electron micrographs. Electron microprobe analyses show high Si, Ca, and K, low Na, and high balance errors. The b cell dimension is consistent with a high Si/(Al + Si) ratio, and the balance errors are apparently due to Na deficiency. The missing Na was estimated from the amount required to give (Si + Al) = 48 and yielded analyses comparable with X-ray fluorescence analyses of samples predominantly of mordenite. Even after this correction, the mordenite had a low Na:K ratio compared with most analyses reported in the literature. The peculiar chemistry of the mordenite may have resulted from a high-temperature metasomatic origin as a result of basaltic volcanism at the basin margin and characterized by hydrothermal circulation of alkaline lake water rich in K.

Clay minerals and zeolites, candidate backfill minerals for nuclear waste repositories, were treated with saturated NaCl brine and Mg-rich (Mg-Ca-Na-K) brine at 200°C and 300°C for 4 weeks under a confining pressure of 30 MPa. The Al concentrations released in NaCl brine were lower than those in Mg-rich brine at both temperatures indicating that the Mg-rich brine is more acidic than the NaCl brine under these hydrothermal conditions. The Si concentrations in both brines were low because of the relatively acidic conditions developed during the hydrothermal treatment. As determined by X-ray powder diffraction or by specific Cs and Sr sorption measurements, no alteration could be detected in clay minerals treated with NaCl brine at 200°C. Among the zeolites tested, only phillipsite and erionite altered to analcime in NaCl brine at 200°C. Zeolites and most of the clay minerals tested did not alter in the Mg-rich brine treated at 200°C. Vermiculite altered to randomly interstratified vermiculite/K-vermiculite (mica-like) by selective K uptake from the Mg-rich brine.

At 300°C, the clay minerals did not greatly alter, whereas the zeolites altered to analcime and/or albite in the presence of the NaCl brine. In the Mg-rich brine, Al-rich montmorillonite from Wyoming did not alter, whereas Al-poor montmorillonite from Texas altered to randomly interstratified montmorillonite/ illite at 300°C. Vermiculite collapsed to form K-vermiculite (~ 10.2 Å) by the selective uptake of K from the Mg-rich brine at 300°C. Most of the zeolites altered to smectites in the Mg-rich brine at 300°C because of the acidic conditions generated by the hydrolysis of Mg. The selective Cs-sorption Kd decreased from 11,700 for untreated phillipsite to 240 and 15 for the hydrothermally produced analcime/albite mixtures from the phillipsite at 200° and 300°C, respectively, in NaCl brine. These results suggest that montmorillonites and mordenites are relatively more resistant than vermiculite or other zeolites at elevated temperatures and pressures in concentrated hydrothermal brines expected in a salt repository.

The vitric matrix of pre-caldera acid tuff and tuff breccia of the Santorini volcano, Aegean Sea, Greece has been generally replaced by one or more of the following authigenic minerals: K-rich and (K,Ca)-rich clinoptilolite, mordenite, opal-CT, and clay minerals. Halite is also present in some samples. Initial compositional inhomogeneities between the dacitic blocks in tuff breccia and tuff seem to have controlled the type of K-rich heulandite-group zeolite that formed. Mordenite postdates the heulandite-group zeolites and opal-CT. Some mordenite has replaced the rims of glass shards. The alteration minerals are not related to vertical or lateral zonation, and the irregular distribution of their assemblages is attributed to variations in heat flow, ionic activity in interstitial waters, and permeability. The pyroclastic rocks were in a region of active heat flow during and after their emplacement. The formation of authigenic silicates may have led to the sealing of open spaces and fractures, imposing barriers to permeability and subdividing the original open system into smaller closed systems. As alteration progressed, some of the trapped water in each individual domain was consumed in hydration reactions. Salts could have been concentrated by such a process, and halite probably precipitated from solutions of appropriate composition in the individual closed systems.

Yucca Mountain is being studied as a potential site in southern Nevada for an underground, high-level nuclear waste repository. A major consideration for selecting this site is the presence of abundant zeolites in Miocene ash-flow tuffs underlying the region. Beneath Yucca Mountain four diagenetic mineral zones have been recognized that become progressively less hydrous with depth.

Zone I, the shallowest zone, is 375–584 m thick in the central part of Yucca Mountain, but 170 m thick to the north. Zone I contains vitric tuffs that consist of unaltered volcanic glass and minor smectite, opal, heulandite, and Ca-rich clinoptilolite. Zone II thins south to north from 700 to 480 m and is characterized by complete replacement of volcanic glass by clinoptilolite with and without mordenite, and by lesser amounts of opal, K-feldspar, quartz, and smectite. Zone III thins south to north from 400 to 98 m thick and consists of analcime, K-feldspar, quartz, and minor calcite and smectite. Heulandite occurs locally at the top of zone III in the eastern part of Yucca Mountain. Zone IV occurs in the deepest structural levels of the volcanic pile and is characterized by albite, K-feldspar, quartz, and minor calcite and smectite.

Clinoptilolite and heulandites in zone I have uniform Ca-rich compositions (60–90 mole % Ca) and Si:Al ratios that are mainly between 4.0 and 4.6. In contrast, clinoptilolites deeper in the volcanic sequence have highly variable compositions that vary vertically and laterally. Deeper clinoptilolites in the eastern part of Yucca Mountain are calcic-potassic and tend to become more calcium-rich with depth. Clinoptilolites at equivalent stratigraphic levels on the western side of Yucca Mountain have sodic-potassic compositions and tend to become more sodium-rich with depth. Despite their differences in exchangeable cation compositions these two deeper compositional suites have similar Si:Al ratios, generally between 4.4 and 5.0. Analcimes have nearly pure end-member compositions, typical of these minerals formed by diagenetic alteration of siliceous volcanic glass; however, K-feldspars are Si-rich compared to the ideal feldspar formula.

Bulk-rock contents of Si, Na, K, Ca, and Mg of zeolitic tuffs generally differ significantly from stratigraphically equivalent vitric tuffs, suggesting that zeolite diagenesis took place in an open chemical system. Both the whole rock and the clinoptilolite are relatively rich in Ca and Mg in the eastern part of Yucca Mountain and rich in Na in the western part. The Ca- and Mg-rich compositions of the zeolitized tuffs in the eastern part of Yucca Mountain may be due to cation exchange by the sorptive minerals with ground water partially derived from underlying Paleozoic carbonate aquifers.

Diagenetic zones become thinner and occur at stratigraphically higher levels from south to north across Yucca Mountain, probably due to a higher geothermal gradient in the northern part of the area. The diagenetic zones were established when the geothermal gradient was greater than it is today, probably during the thermal event associated with the development of the Timber Mountain-Oasis Valley caldera complex north of Yucca Mountain.

Hypothetical model structures for magadiite and sodium octosilicate, based on the structure of the zeolite dachiardite, are proposed that consist of layers of 6-member rings of tetrahedra and blocks containing 5-member rings attached to both sides of the layers. The infrared (IR) and nuclear magnetic resonance spectra of magadiite and sodium octosilicate have features in common with spectra of zeolites in the ZSM-5 and mordenite groups. A peak at 1225 cm-1 in the IR spectra of magadiite and sodium octosilicate is characteristic of zeolites containing 5-member rings, such as ZSM-5- and mordenite-type zeolites. The defect structures of pentasil zeolites may therefore be akin to layered alkali metal silicates containing zeolite-like domains, in which part of the silanol groups from adjacent silicate layers are condensed (cross-linked) forming siloxane linkages.

Perlite, which occurs at the contacts between dacite and lacustrine tuffs of Miocene age in the Guryongpo area, Korea, has undergone more sluggish and incomplete diagenetic alteration than the surrounding zeolitic tuffs. Alkali-clinoptilolite, mordenite, smectite, K-rich gel-like glass, low-cristobalite, and K-feldspar are characteristic diagenetic phases in the altered perlite.

Hydration of the glass to form perlite resulted in the expulsion of cations, mainly Na, from the glass into the pore fluid and in the relative enrichment of K in the perlite. Hydration of the glass also resulted in increased alkalinity of the pore fluid, which, in turn, affected the nature and behavior of subsequent glass dissolution. Textural observations and chemical data on the early diagenetic phases indicate a sequence of incongruent dissolution reactions, which depended on silica activity and alkalinity of the ambient pore fluid: Reaction (1) 12.5 perlitic glass + 3.88 K+ + 0.65 H+ + 15.4 H20 = smectite + 9.5 gellike glass + 4.03 Na+ + 0.25 Ca2+ + 10.55 H4SiO4 released Ca and silica which were consumed by reaction (2) perlitic glass + 0.1 Ca2+ + 0.1 H4SiO4 + 0.1 H+ + H20 = clinoptilolite + 0.1 K+ + 0.2 Na+. The paragenesis from glass via smectite to alkali zeolites in most glass-bearing rocks may be explained by a sequence of such dissolution reactions. Still later reactions involved the transition from less-silicic clinoptilolite to an assemblage of silicic Na-clinoptilolite + mordenite and the crystallization of the gel-like glass to K-feldspar.

605 m of sediments were cored in Hole 841 of the Ocean Drilling Program (ODP) at the Tonga Trench margin. The sedimentary sequence consists mainly of Miocene vitric siltstones, vitric sandstones, and volcanic conglomerates. A major consideration for selecting this site was the presence of abundant authigenic minerals (40% to 70% of the whole rock), which consist of K-feldspars, clays, thaumasite (Ca3Si(OH)6CO3SO4, 12H2O), and zeolites. The zeolite minerals include phillipsite, clinoptilolite, analcime, mordenite, chabazite, heulandite, wairakite, and erionite. The increasing amount of analcime from 257 mbsf to 470 mbsf, and the joint occurrence of mordenite and wairakite in this zone of Miocene tufts, seems to be induced by the heat flow from a major intrusive sequence of basaltic andesite sills and dikes. This abundance of analcime in response to the thermal pulse could explain the unusual Na-depleted porewater compositions observed in ODP Hole 841.

The low-temperature alteration of a rhyolitic rock from Kimolos Island, Aegean, Greece, yielded an alteration profile characterized by gradual transition from fresh glass to bentonite containing homogeneous Chambers-type montmorillonite and then to a mordenite-bearing bentonite. Both mordenite and smectite were formed from poorly crystalline precursors, which probably had compositions comparable to that of the crystalline end-product. However, their composition may have been modified to some degree after reaction with the fluid phase. Particle length and width measurements of smectite crystals yielded lognormal profiles, which suggest supply-controlled crystal growth in an open system or random ripening in a closed system. The former mechanism is in accordance with the observed sustained supply of Mg and Fe by the fluid phase throughout the alteration profile and is believed to be the dominant formation mechanism of smectites in bentonites in general. In the mordenite-bearing zone, random ripening is expected in domains with low permeability, in which the system was essentially closed, favoring the formation of mordenite. The level of supersaturation with respect to smectite was probably lower in the mordenite-bearing zone. Smectite probably affected pore-fluid chemistry either through ion exchange or via dissolution of initially formed K-bearing smectite. The latter process raised the K+/(Na+ + Ca2+) activity ratio of the pore-fluid favoring K-bearing mordenite.

Clinoptilolite and mordenite occur as diagenetic products of medium-grained, moderately welded and poorly sorted pyroclastic flows belonging to the Tertiary calc-alkaline volcanism of Sardinia. Both clinoptilolite and mordenite occur within pyroclastic flows of the same stratigraphic unit. Mordenite frequently occurs in the late volcanic sequences from Anglona area (northern Sardinia). Textural features indicate that zeolites are products of glass alteration. Thin sections show either complete alteration of glassy shards by clinoptilolite and mordenite or unaltered shards with clinoptilolite or mordenite confined to the cineritic matrix. During the zeolitization process, interacting fluids were important in the mobilization and distribution of alkali elements. The compositional variations of clinoptilolite and mordenite within a single sample showed trends that suggest steps in a continuous process probably evolving towards equilibrium conditions.

Progressive alteration by seawater of an andesite in the Aegean Island of Milos and an ignimbrite in the Aegean Island of Kimolos, Greece, formed bentonites with or without zeolites. Both profiles are dominated by migration of alkalis and uptake of Mg, Fe and H2O, while Al and Ti are immobile. The relative removal of alkalis controls the formation of either smectite or zeolites. The behavior of Ca and Si depends on the chemistry of the parent rock. In the rhyolitic profile, alteration is controlled by gain of Mg, Fe2+ and Ca and loss of Na, K and Si, while in the andesitic profile by gain of Mg and Fe2+ and loss of Na, K and Ca. In both profiles, significant uptake of SO4- was not observed. Moreover Zr, Nb, V and Ni are immobile and have been enriched residually, while Sr, Rb and Y are lost in both profiles. Thorium is immobile in the rhyolitic profile but is leached in the andesitic profile. Also, the rare earth elements (REE) display fractionation in both profiles; the degree of fractionation increases with the degree of alteration to bentonite. Fractionation of the REE in both profiles and mobility of Th in the andesitic profile are related to the existence of monazite (rhyolitic profile) and apatite (andesite profile). The REE and Th appear to partition into phosphates rather than smectite.

The mobility of Y coupled with the immobility of Nb increases the Nb: Y ratio with advancing alteration, rendering discrimination diagrams that use this ratio to determine the nature of the protoliths misleading. Mass balance calculations showed that in the smectite-rich zones the water: rock (WR) ratio might be as high as 13:1 in both profiles, while in the zeolite-bearing zones it is about 5.5:1. Such WR ratios explain the observed extensive mass transfer and suggest that the pore fluid chemistry might overprint the chemical characteristics of the parent rocks controlling smectite and bentonite chemistry.

The ammonium acetate saturation (AMAS) method was used to study the cation-exchange capacity (CEC) of zeolitic volcaniclastic materials from Santorini and Polyegos Islands, Greece. The AMAS method was tested with respect to the time required to saturate the samples of zeolites with ammonium (NH4+) ions, the efficient liberation of NH4+ ions, and the suitability of two widely used techniques to measure ammonia (NH3) (Kjeldahl ammonia distillation technique and the ammonia electrode technique). By using the inductively coupled plasma-mass spectrometry (ICP-MS) technique and measuring the number of cations exchanged during ammonium acetate (NH4OAc) saturation, it was found that the saturation period for zeolitic materials must be increased to 12-day cycles to ensure effective saturation. Following NH4OAc saturation, the AMAS method produces NH4+ solutions, after the NH4+-saturated samples of zeolites are washed with 10% NaCl. The amount of the NH4+ ions in solution (i.e., exchangeable cations) is a measure of the CEC. The NH4+ ions can not be directly measured and must be converted to NH3. The Kjeldahl ammonia distillation technique and the ammonia electrode technique for measuring NH3 give identical CEC results. However, the ammonia electrode technique, when used directly with the NH4+ samples of zeolite without 10% NaCl treatment, generally gives higher CEC values. The amount of NH4+ treated (converted to NH3), when the NH4+-saturated zeolitized samples were used directly, was higher than the amount of the NH4+ treated when the NH4+ solutions were obtained after washing the NH4+-saturated zeolitized samples with 10% NaCl. Therefore, washing with 10% NaCl does not facilitate the release of all NH4+ initially within the zeolite structure. A modified AMAS technique is proposed for measuring the CEC of zeolitic material.

The vitric component of the silicic pyroclastic flows and surge deposits (Prassa ignimbrite unit), from the northwestern exposures of the pyroclastics formation of Polyegos Island, South Aegean Sea Volcanic Arc in Greece, is replaced by authigenic zeolite and clay minerals. Mordenite dominates, and clinoptilolite (heulandite type 3), illite and illite-smectite (I-S) are subordinate. Opal-CT, quartz, feldspar, biotite and halite complete the mineralogical suite. In the southwestern part of the pyroclastics formation (Myrsini pyroclastics unit), kaolinite, halloysite, alunite and amorphous silica are the major mineralogical constituents as a result of a strong hydrothermal alteration by solutions enriched in SO42-. Scanning electron microscope (SEM) examination proved that the 1st type of zeolites formed within the area were heulandite minerals, following the formation of smectite, as a result of the activity of pore fluids within the volcaniclastic pile. There was no substantial evidence to support a hypothesis that mordenite was formed with the heulandite minerals after the initial stages of glass dissolution, other than some very minor mordenite that was formed from a gel-like type of material. The majority of mordenite present is most often draped over and formed from the crystals of heulandite minerals. Heulandite minerals often show advanced dissolution effects. The mineral dissolution was due to the emplacement of rhyolite lava domes and flows and the associated temperature rise and circulation of hydrothermal fluids, which had also mixed with seawater. The volcanic activity raised the temperature and changed the pore fluid chemistry, and the more unstable members of the heulandite minerals group were transformed to mordenite. Clinoptilolite (heulandite type 3), which was found within a few samples, was thermally more stable than any heulandite type 1 or 2 phases initially present. Therefore, clinoptilolite was either transformed more slowly or within a very few cases, it has not been affected at all. A heulandite-minerals-derived material acted as the major precursor for the formation of mordenite. The temperature increase within the area and the later hydrothermal alteration effect are also indicated by the illitization of the smectite. Mordenite is also found to have formed from I-S clays. Overall, mordenite formed as a result of elevated temperature and high Na+ concentration.

Silicic vitric tuffs of the Alaçatı (Çeşme) area, west of Izmir (Turkey), are partly altered to authigenic zeolites and clay minerals. Mordenite and clinoptilolite-heulandite accompanied by smectite are the main alteration products of the tuffs. Scanning electron microscope examination indicates that mordenite forms mainly from a gel-like precursor and to a lesser degree from volcanic glass. Mordenite and clinoptilolite-heulandite have formed by the hydrolysis and dissolution of silicic vitric tuffs by thermal waters which circulated through the fracture zone into porous vitric tuffs. These thermal waters, which resulted from mixing of thermal (Na-Cl type) and groundwaters, provided the extra Na and Ca that are necessary for the formation of mordenite. Various percentages of calcite are present in the altered tuffs. The enrichment of calcite in the altered tuffs indicates that thermal waters supply Ca, sourced from dissolution of Triassic carbonate rocks.

The alteration minerals in the Alaçatı (Çeşme) area do not seem to be laterally zoned. The occurrence of the zeolites seems to be related to inferred fracture zones trending NE—SW. These fracture zones are important because they provide porosity and permeability in the tuffs for zeolite formation. The large glass content and permeability of tuffs accompanied by the favorable hydrochemical condition resulted in zeolitization of the Alaçatı (Çeşme) area.

Mordenite and clinoptilolite have replaced glass shards and pumice in vitric tuffs that are the products of ash fall-out into lake basins of late Quaternary age in the Taupo Volcanic Zone. The vitric tuffs are intercalated with siltstone and diatomite and overlie pumice-rich, rhyolitic ignimbrite. A Zr/TiO2-Nb/Y immobile element ratio plot indicates that the vitric tuffs, like the ignimbrite, are of rhyolitic composition. X-ray diffraction and scanning electron microscopy studies indicate that the mordenite and clinoptilolite are accompanied by authigenic K-feldspar and Opal-CT. The zeolites and other authigenic minerals are very fine grained (<10 µm), with open meshes of acicular mordenite crystals that result in low densities (0.7–1.0 g cm−3) in mordenite-rich tuffs. From Pearce element ratio analysis of whole-rock chemical analyses, only Na and K appear to have been mobilized during alteration. The zeolite deposits are associated with sinter, hydrothermal eruption breccias and silicified fault breccias that represent surface or near-surface manifestations of geothermal activity. Plant material extracted from a sinter overlying one of the deposits has a 14C age of 8498±60 BP, which is interpreted to be the age of zeolite deposition for this deposit. Mordenite and clinoptilolite occur in the lower-T (60–110°C) parts of some active or recently active geothermal systems elsewhere in the Taupo Volcanic Zone. The main fluid in these systems is weakly saline (alkali-chloride) water heated by geothermal activity. The Ngakuru zeolite deposits are interpreted as the products of the reaction of vitric tuffs with this type of water in the near-surface part of recently active geothermal systems.

The zeolites and coexisting minerals of the silicic vitric tuffs in the Alaçatı (Çeşme) area, west of İzmir (Turkey), were studied. Mordenite is the most abundant zeolite in tuffs of the Alaçatı area and usually coexists with clinoptilolite-heulandite, smectite and calcite. Opal-CT was identified by means of its crystal morphology and EDX spectrum. Scanning electron microscopy (SEM) revealed the relative age relationships between the zeolites and coexisting minerals, namely mordenite, clinoptilolite-heulandite, smectite, calcite, and, in addition, opal-CT. Smectite consistently crystallized earlier than any of the zeolites, and it occasionally coats the outer walls of some of the vitric material. The zeolites are commonly located on the smectite, although some mordenites were observed to be in direct contact with glass shards that lacked a smectite coating. Clinoptilolite-heulandite formed after smectite and before mordenite. Opal-CT is seen to postdate both smectite and needle-shaped mordenite. Calcite was probably the latest mineral to crystallize in the Alaçatı tuffs. The zeolites in the tuffs of the Alaçatı area formed by dissolution of silicic vitric tuffs by Na- and Ca-rich thermal waters which passed through the fracture zone. The appearance of zeolites together with smectite along this zone may be attributed to a semi-open system which subdivided into smaller closed systems. Small changes in the pH and chemical composition of the thermal waters during alteration produced the corrosion effects observed by SEM. Small amounts of clinoptilolite-heulandite were corroded prior to crystallization of coexisting mordenite. The different compositions of the thermal waters were probably inherited from water that resulted from mixing of thermal and groundwaters.

Tuffs of the Tertiary Colbún Formation near Quinamávidain central southern Chile have been mapped and their mineralogy analyzed. The pyroclastic rocks present a maximum outcropping thickness of 120 m and are dominated by vitreous lapilli and minor lithic tuffs, the products of active volcanism nearby. About 10% of the tuffs consist of lenses of fine banded tuffs with a high leaf content that were deposited in shallow lakes during quiescent periods between periods of volcanic activity. This tuff sequence is pervasively transformed to clinoptilolite/heulandite and mordenite with variable amounts of plagioclase, minor quartz and smectite. Factors thought to have influenced this conversion to zeolites are a humid climate following deposition combined with a slightly elevated heat flow. Local hydrogeological conditions have modified the cation-hydrogen ion ratios across the study area favoring the formation of clinoptilolite/heulandite and mordenite with medium-minor smectite in the center and south, and a more abundant presence of smectite in the north of the study area.

Strong gas-mineral interactions or slow adsorption kinetics require a molecular-level understanding of both adsorption and diffusion for these interactions to be properly described in transport models. In this combined molecular simulation and experimental study, noble gas adsorption and mobility is investigated in two naturally abundant zeolites whose pores are similar in size (clinoptilolite) and greater than (mordenite) the gas diameters. Simulated adsorption isotherms obtained from grand canonical Monte Carlo simulations indicate that both zeolites can accommodate even the largest gas (Rn). However, gas mobility in clinoptilolite is significantly hindered at pore-limiting window sites, as seen from molecular dynamics simulations in both bulk and slab zeolite models. Experimental gas adsorption isotherms for clinoptilolite confirm the presence of a kinetic barrier to Xe uptake, resulting in the unusual property of reverse Kr/Xe selectivity. Finally, a kinetic model is used to fit the simulated gas loading profiles, allowing a comparison of trends in gas diffusivity in the zeolite pores.