The most prominent authigenic reaction in Holocene tuffaceous sediments at Teels Marsh, Nevada, is the hydration of rhyolitic glass by interstitial brines and the subsequent formation of phillipsite. This reaction has the form: rhyolitic glass + H2O → hydrous alkali alumninosilicate gel → phillipsite. Phillipsite is the most abundant authigenic phase in the tuffaceous sediments (>95%), analcime is the next most abundant phase, and clinoptilolite occurs as a trace mineral in the <2-mm fraction. Analcime forms by the reaction of phillipsite and Na+. Gaylussite and searlesite also are common authigenic phases at Teels Marsh. The concentration of silica in the interstitial brines is controlled by one or more of the authigenic reactions at less than 100 ppm. A stoichiometric equation for the reaction of phillipsite to analcime at Teels Marsh is:

$$0.43N{a + }\, + {K_{0.43}}N{a_{0.57}}AlS{i_{3.1}}{O_{8.2}}\, \cdot \,3.2{H_2}O\, \to \,NaAlS{i_2}{O_6}\, \cdot \,{H_2}O\, + \,1.1{H_4}Si{O_4}\, + \,0.43{K + }.$$

Sodium and potassium activities of brines associated with both phillipsite and analcime were used to estimate the equilibrium constant for this reaction as 3.04 × 10−5. The ΔG0 value for the reaction is +6.2 kcal/mole at 25°C and 1 atm pressure. The estimated ΔG0 value of phillipsite, using this reaction, is −1072.8 kcal/mole at 25°C and 1 atm.

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.

Exchange isotherms for the pairs Na-K and Na-Ca were measured by use of 0.1 N solutions at 5°, 35°, and 70°C in phillipsite from Tecopa, California (3.63 Al/32 oxygen unit cell), and Oki Islands, Shi-mane Prefecture, Japan (6.31 Al/32 oxygen unit cell). All isotherms except those for Na-Ca at 5°C were reversible. Free energy was evaluated for all reversible exchanges. The thermodynamic affinity sequences were K > Na > Ca in both phillipsites. The selectivity for K in competition with Na and that for Na competing with Ca became larger at the lower temperatures. The siliceous phillipsite preferred the larger cation more strongly for the Na-K system, and Na more strongly for the Na-Ca system than the aluminous phillipsite.

An assemblage of clay minerals, Na-zeolites, Fe- and Sb-sulfides, and borates occurs in cavities in the basalt flow that underlies the main borax ore body exposed by the U.S. Borax open pit at Boron, Kern County, California. Analcime and saponite are widespread in cavities and fractures near the top of the basalt. Part of the basalt recently exposed, although restricted in distribution, contains a much more diverse assemblage of diagenetic minerals. Although only a few different minerals occur in each cavity, the composite order of deposition is: ferroan saponite, pyrrhotite, saponite, phillipsite, gmelinite, clinop-tilolite, herschelite, analcime, greigite, rhodochrosite, searlesite, borax, calcite, and colemanite. In some cavities early-formed zeolites appear to have dissolved as later ones crystallized.

Microprobe analyses of the zeolites yielded the following compositions: phillipsite, Na4.28K0.08(Al4.36-Si11.64O32)·xH2O (Si/Al = 2.67); gmelinite, Na6.64K0.10(Al6.80Si17.22O48· xH2O (Si/Al = 2.53); herschelite, Na2.96K0.24(Al3.21Si8.79O24)· xH2O (Si/Al = 2.75); clinoptilolite, Na6.48K0.56(Al7.15Si28.77O72) · xH2O (Si/Al = 4.02); analcime, Na14.02K0.09(Al13.33Si34.44O96)· xH2O (Si/Al = 2.58).

These zeolites formed by reactions between the basalt and evolving fluids, which were controlled by the development of an overlying, Na-borate lake. The wide distribution of saponite, analcime, and searlesite suggests that these minerals formed by the diagenetic reaction between basaltic glass and Na- borate water. The localized occurrence of the complex mineral assemblage, including some of the saponitic clay and the zeolites, phillipsite, gmelinite, and clinoptilolite, formed through a different process, perhaps deuteric or hydrothermal alteration of the basalt, before the lake developed. Because these zeolites are extraordinarily rich in Na, cation exchange may have taken place as the pore fluids became increasingly Na rich. In the latest stages of mineral growth, each of these zeolites was apparently partially dissolved and epitaxially overgrown by a second generation crystal, phillipsite on phillipsite, herschelite on gmelinite, and sodian heulandite on clinoptilolite.

Ash layers from Searles Lake, California, were sampled in core of drill hole KM-3, which penetrated 693 m of lacustrine sediment deposited in a playa-lake complex over the past 3.2 my. Lake water changed from moderately saline and slightly alkaline (pH ~7.5) to highly saline 2.04 my ago and to highly saline and alkaline (pH ~9.5) 1.28 my ago. As a result of brines flushing downward, the upper 291 m of sediment, spanning the past 1.28 my, contain highly saline, alkaline pore fluid. Silicic ash layers in contact with highly saline, alkaline pore fluid were first altered to phillipsite and meriinoite and then to K-feldspar and searlesite. The transformation of phillipsite and/or meriinoite to K-feldspar required more than 45,000 years and was largely completed in 140,000 years. Tephra layers in contact with moderately saline, slightly alkaline pore fluid vary from uncemented vitric ash containing minor smectite to bentonites in which glass is wholly altered to smectite, clinoptilolite, analcime, and opal. Layers with much fine tephra are more altered than the coarser, better-sorted layers. Alteration is attributed to hydrolysis in essentially a closed hydrologic system, in which the alteration of glass to smectite raised the pH, aSiO2, and (Na+ + K+)/H+ activity ratio to the level where clinoptilolite formed. Some diffusion and/ or fluid flow is, however, indicated by the loss of SiO2 during the alteration of ash layers to smectite and by anhydrite deposited during and after clinoptilolite in some tuffaceous sandstones.

Zeolites having the structures of phillipsite, merlinoite, and gobbinsite were synthesized from clear solutions at 80°C in the system Na2O-K2O-Al2O3-SiO2-H2O and their morphologies, cell parameters, and compositions determined. At 3.5 M silica concentration, the formation of merlinoite (synthetic zeolite W) is favored over the formation of phillipsite (synthetic zeolite ZK-19) by solution conditions of high pH (> 13.6) and low Na/(Na + K) ratios (<0.5).

Using the information obtained from the synthesis experiments, the presence of merlinoite was predicted in sediments from Searles Lake, a saline, alkaline lake in California with ideal physiochemical conditions for its formation. Merlinoite was subsequently discovered to occur in tuffaceous sediments as part of an authigenic silicate zonation pattern from phillipsite → phillipsite + merlinoite → merlinoite → K-feldspar with increasing depth. Because of the close similarities in the physical properties of phillipsite and merlinoite, merlinoite may be much more common as an authigenic mineral than is currently realized.

Phillipsite is a common constituent in diagenetically altered rhyolitic vitric tuffs of Cenozoic saline, alkaline-lake deposits where it occurs as spherulites or aggregates of prismatic crystals. This phillipsite has chemical and physical properties that differ from phillipsites that occur in rocks of other compositions and depositional environments, but previously it had not been completely characterized. Published analyses of five samples from four deposits in the southwestern United States show that the phillipsite is very siliceous and alkalic; the Si/(Al + Fe3+) ratio is 3.08–3.37, alkalis greatly exceed the divalent exchangeable cations, and the Na/K ratio is 1.05–3.05. The mean index of refraction (1.451–1.470) is much lower than that generally reported for the phillipsite group and can be correlated with the relatively high Si and alkali contents. Monoclinic unit-cell parameters show the following ranges: a = 9.931–9.991 Å, b = 14.142–14.155 Å, c = 8.650–8.706 Å, ß = 124.57–125.07°, and V = 1000.3–1007.4 Å3. The especially small b dimension is consistent with the high Si content.

The Cs selectivity of several natural zeolitic tuffs and synthetic zeolites was measured. Phillipsite-rich tuffs from California and Nevada exchanged 13.5 and 23.7%, respectively, of the Cs present in simulated alkaline defense waste containing 0.00025 M CsCl in 5.5 M NaCl-NaOH solution from the Savannah River Plant, Aiken, South Carolina; whereas mordenite-rich tuffs from Arizona and Nevada exchanged less than 12.7%. The immobilization or fixation of Cs in phillipsite unlike other zeolites can be achieved by heating the zeolite at 600°C for 4 hr in air and collapsing the silicate (aluminate) tetrahedral rings around the Cs ions to produce a Cs-feldspar-type phase. Treatment of the Cs-exchanged phillipsite-rich tuff at 800° to 1000°C resulted in pollucite, CsAlSi2O6, which also “locks in” the Cs ions in its structure. The fixation of Cs exchanged in phillipsite can also be achieved by the formation of pollucite upon hydrothermal treatment at 300°C and 30 MPa pressure within 12 hr. These results suggest that phillipsite-rich tuffs are good candidates for Cs immobilization by heat treatment at low temperatures after they have been used as sorbents in waste decontamination.

Core KM-3 at Searles Lake, California, comprises 693.4 m of lacustrine sediments deposited over the past 3.2 m.y. The lake water evolved from moderately saline, slightly alkaline, and dominated by Na, Ca, Cl, SO4, and HCO3 + CO3 to a highly alkaline brine dominated by Na and CO, ions. Sediments are chiefly muds and evaporites. Montmorillonite and illite are the principal detrital clay minerals supplied to Searles Lake at present and probably during the late Pliocene and Pleistocene.

The drill core is divided into three diagenetic zones on the basis of clay-mineral reactions. The upper zone (0-291.1 m) contains authigenic Fe-illite, Mg-smectite, K-feldspar, and analcime, which average 60–70% of the <2-μm silicate fraction of mud samples. The principal silicate reactants are detrital montmorillonite and kaolinite, which have been largely consumed. The middle zone (291.1–541.6 m) also contains authigenic Fe-illite, Mg-smectite, K-feldspar, and analcime, but they form only 20 to 30% of the <2-μm silicate fraction, and considerable detrital montmorillonite and kaolinite remain unaltered. In muds of the lower zone (541.6–693.4 m), a small amount of clinoptilolite is the only authigenic silicate mineral identified, although authigenic montmorillonite probably coexists here as well. Vitric ash is the silicate reactant, and detrital clay minerals apparently remain unaltered in the lower zone.

Diagenetic zoning reflects the pore-water chemistry, in which pH may have been the most important parameter. Sediments of the upper zone were deposited in highly alkaline lakes of variable salinity, and sinking brines with a pH of 9.0–10.0 have saturated all sediments. Sediments of the middle zone were deposited in lakes of moderate to high salinity. The pH was overall lower than in the upper zone, although it probably exceeded 9.0, at least locally, during silicate diagenesis. Moderate salinity and a slightly alkaline pH (~ 7.5–8.0) are inferred for both the lake and pore water of the lower zone.

Oxygen-isotope values of authigenic Fe-illite, Mg-smectite, and K-feldspar, and phillipsite in the upper diagenetic zone reflect a high degree of evaporative concentration and presumably of salinity. Equilibrium water values calculated for 22°C from the oxygen-isotopic composition of authigenic phyllosilicates range from −3.5 to +1.9%, averaging −1.2% (SMOW). Higher salinities are suggested for K-feldspar, for which water values range from +1.8 to +4.8%, averaging +3.4‰ The water value for phillipsite is +0.3‰ By comparison, rainfall at Searles Lake has an average δ18O value of about −9.8%, and a brine sample has a value of +4.0‰

Mud samples of the middle and upper diagenetic zones tend to be rich in Fe-illite or Mg-smectite, but not in both, indicating that the two minerals are favored to some extent by different environments. Fe-illite seems to be generally favored by oxidizing conditions and probably by a playa environment, and Mg-smectite seems to be favored by reducing conditions and an open-water environment. One strati-graphic unit of the upper zone does not fit this pattern and contains abundant Fe-illite in sediments of a deep perennial lake.

Batch reactor experiments were performed at 150°C, 175°C, and 200°C to determine the effect of high pH KOH solutions on the mineralogy of the Opalinus shale. In these experiments, the change in solution quench pH at 25°C, solution composition, and mineralogy were monitored as a function of time for up to ≈ 50 days. Runs were performed in 50 ml titanium hydrothermal reactor vessels. Each reactor was charged with 0.5–5.0 grams of the 80–200 mesh size fraction of Opalinus shale, and 25 ml of solution (0.08 and 0.008 m KOH). Under these high pH conditions, the general sequence of reaction products observed is the formation of phillipsite, followed by K-feldspar ± K-rectorite. Phillipsite is a metastable intermediate that eventually transforms to K-feldspar. This sequence of mineral reaction products is very different from that found in the NaOH system.

Experiments on zeolitization were conducted on four synthetic monocationic glasses (Na, K, Ca, or Mg-rich glass) with Si/Al molar ratios of 2.67, similar in acidity to many volcanic glasses of partially zeolitized Italian tuffs. The products of the hydrothermal treatment at 100, 150, and 200°C of single glasses or glass mixtures with deionized H2O or monosaline solutions (NaCl, KCl, CaCl2) were characterized by X-ray diffraction, thermal, microscopic and chemical analyses. Chemical analyses of mother liquors were also performed. Mineral assemblages, containing chabazite, phillipsite, analcime, and K-feldspar, very similar to those found in altered, volcaniclastic alkali-trachytic or trachytic glass deposits were produced. Potassium was essential to chabazite and phillipsite crystallization, although phillipsite was obtained also in Ca-Na mixed systems. Sodium was necessary for analcime formation. Calcium plays only a secondary role in zeolitization, and magnesium does not favor zeolite crystallization but promotes the formation of smectite. Glass composition determines the mineral assemblages obtained and hence in those commonly found in nature.

Experimental alteration of obsidian was performed in 0.001 to 0.5 N NaOH and KOH solutions at 150 and 200 °C for 1 to 30 d. The products were examined by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy dispersive X-ray analysis (EDX). Changes in chemical composition and pH value of solutions during the reactions were also measured. As the pH of reacting solutions was increased, smectite, phillipsite and rhodesite crystallized progressively in NaOH solutions, while smectite, merlinoite and sanidine grew successively in KOH solutions. In addition, a small amount of less-soluble, poorly ordered boehmite was present as products of all the experiments. Smectite mainly appeared at slightly high pH, Si/Al and Na/K conditions, whereas rhodesite should be produced in extremely high pH, Na/K and Si/Al conditions. Sanidine was also formed in conditions of very high pH and Si/Al and very low Na/K. In intermediate conditions of pH and Si/Al, crystallization of phillipsite was stimulated in solutions of Na/K > 10, while formation of merlinoite was favored in conditions of Na/K < 1.

The chemical and the mineralogical composition of a group of pumiceous tuffs associated with recent salic volcanic episodes from Tenerife (Canary Islands) have been studied. The investigation focused on the two main types of pyroclastic deposits of the zone: ash-flows and ash-falls. The samples can be classified chemically as trachytic and phonolitic rocks with an intermediate silica content and a high percentage of alkali cations (Na+ and K+). The mineralogical composition, determined by X-ray diffraction, scanning electron microscopy, and optical microscopy, shows the occurrence of zeolites (mainly phillipsite, with lesser chabazite and analcime), associated with the parent glass. K-feldspar (sanidine) and calcite are accessory minerals. Zeolites are significantly more abundant in the ash-flow deposits. Zeolite formation by hydrothermal weathering in closed-system conditions varies according to the nature and the origin of the pyroclastic deposits. Tenerife phillipsites differ from typical diagenetic, lacustrine, and deep-sea phillipsites, both in chemical and mineralogical features. Alkali cations exceed divalent cations in the unit-cell that, assuming a monoclinic symmetry, has the following parameters: a = 8.46–10.55 Å, b = 14.21–14.40 Å, c = 7.80–8.70 Å, and β = 105°–110°.

Geologic mapping and crystal-chemical analysis of Middle-Upper Miocene volcanics in the Sandıklı-Afyonkarahisar region of Turkey, coupled with published zeolite analyses has revealed that western Turkey hosts unique zeolitic mineral assemblages with distinct paragenetic sequences. The present investigation focused on pyroclastic deposits, including low to intermediate potassic trachytic/trachyandesitic tuffs (LPT) and high potassic tephriphonolitic, tephritic, and trachybasaltic tuffs (HPT), each of which contains various styles of zeolites. Optical petrography, X-ray powder diffraction, and chemical analyses have revealed varying degrees of lithification, probably related to differences in initial emplacement temperature, depositional mechanism and thickness, chilling rate, and extent of mafic composition. Zeolitization was further influenced by meteoric flushing in a hydrologically open system. Chabazite in the LPT from the Selçik area occurs extensively as coatings and infillings of pores. Phillipsite in the HPT found in the Ballık, Küfeke, and Ömerkuyu areas dominates the assemblage and is accompanied by chabazite and minor amounts of analcime. Analcime was probably generated by alteration of leucite which is found as a pyrogene mineral. Alkali zeolites or Ca-bearing zeolites formed as a consequence of the addition of Ca and/or the removal of Na (i.e. dissolution of analcime). The paragenetic sequence may be described as: analcime/phillipsite → chabazite → calcite. The characterization of these assemblages may lead to better exploitation strategies for natural zeolitic resources in the region.

The Plio-Pleistocene deposits of the Olduvai Basin in northern Tanzania consist of a sequence of lacustrine and fluvial sediments. They contain various amounts of zeolite minerals, the formation of which is related to an interaction of volcanic material or detrital clays with saline alkaline lake water and groundwater. Petrographic characteristics of zeolite occurrences provide information about their conditions of formation. They were studied for all four main stratigraphical units that are recognized at Olduvai (Beds I to IV), sampled in the southeastern part of the basin. In the lake-margin deposits of Bed I and the lower part of Bed II, chabazite is the dominant zeolite mineral accompanied by phillipsite and minor amounts of erionite and clinoptilolite. Chabazite commonly occurs as part of altered volcanic rock fragments, characterized by partial or complete dissolution of volcanic glass and the formation of chabazite inside vesicles, following the development of thin smectite coatings. It also formed within the sediment matrix, requiring extended periods of impregnation of the deposits by saline alkaline solutions. Chabazite also occurs extensively as coatings and infillings of pores, developed during periods of subaerial exposure which were characterized by high groundwater levels. Phillipsite formed at a later stage, from more evolved solutions, with higher K/Na ratios than during chabazite formation. The fluvial deposits of Bed IV, Bed III and the upper part of Bed II have a high analcime content. They also contain various amounts of chabazite, phillipsite and natrolite. All zeolite minerals mainly occur in pores. The predominance of analcime indicates a higher salinity and alkalinity than during the preceding period with sedimentation and diagenesis in a lake margin environment. Early development of zeolite occurrences, shortly after the deposits became exposed during breaks in sedimentation, is recorded for some intervals, where zeolites are covered by illuvial clay coatings or by sparitic carbonate cement. In most intervals, however, zeolites mainly formed at a later stage.

The current study demonstrates how co-existing zeolite and clay minerals formed by the alteration of tephra in a closed-basin lacustrine and lake-margin environment can retain the overall composition of the original bulk tephra for many elements, even when diagenetic conditions and resulting authigenic mineral assemblages change. Zeolite and clay minerals co-exist in the closed-basin, saline-alkaline lacustrine altered tephra of Pleistocene Olduvai Gorge, Tanzania, and their diagenetic histories can be reconstructed using variations in their textures and compositions. The authigenic minerals in the altered tephra of the Olduvai paleolake form a classic ‘bull’s-eye’ pattern, with clay-dominated tephra in the distal lake margin, chabazite and phillipsite in the proximal margin, and phillipsite ± K-feldspar in the intermittently dry lake and lake center. Fifteen representative samples of altered volcanic ash lapilli (designated Tuff IF) were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and scanning electron microscopy (SEM) to determine their authigenic mineral assemblages and bulk compositions, and to texturally and compositionally compare their clay mineral and zeolite components.

Textural observations indicate that clay minerals formed first, followed by zeolites and finally feldspars. Clay minerals, however, persist even in the most altered samples. The overall composition of Tuff IF shows only limited change in Fe, Si, Al, and Na between fresh, clay-altered, and zeolite-dominated diagenetic environments, despite significant differences in authigenic assemblage. Where zeolites dominate the assemblage, the remaining clay minerals are rich in Mg, Fe, and Ti, elements that are not readily incorporated in zeolite structures. Where clay minerals dominate, they are more Al-rich. A ‘mixing model’ combining clay-mineral and zeolite compositions yields a close approximation of the original volcanic glass for most elements (exceptions including Mg, Ca, and K). This initial composition was preserved in part by the redistribution of elements between co-existing clay minerals and zeolites.

A natrolite-cemented palagonite ash tuff unit is reported in a palaeo-basin in northeast Jordan. Phillipsite and chabazite are also identified. The zeolites were formed due to transformation of volcanic glass granules into palagonite by the reaction with percolating water in a closed hydrological system. Consequently, Si, Al, Ca, Na and K are leached out and precipitated in a series of authigenic layers developed at the extreme edge of the granules. The recorded paragenetic sequence is smectite → phillipsite → chabazite → natrolite → analcime → calcite. The natrolite studied is chemically similar to those reported in the literature with minor variations. The phillipsite and chabazite studied are chemically different from the other Jordanian phillipsite and chabazite reported. The latter are chemically equivalent to those formed under open hydrological systems, whereas the phillipsite and chabazite in this study are chemically equivalent to those formed in saline lakes and arid soil environments. This conclusion is based on the (Na+K)/(Na+K+Ca+Mg) ratio and the Si/Al ratio.

This report embodies recommendations on zeolite nomenclature approved by the International Mineralogical Association Commission on New Minerals and Mineral Names. In a working definition of a zeolite mineral used for this review, interrupted tetrahedral framework structures are accepted where other zeolitic properties prevail, and complete substitution by elements other than Si and Al is allowed. Separate species are recognized in topologically distinctive compositional series in which different extra-framework cations are the most abundant in atomic proportions. To name these, the appropriate chemical symbol is attached by a hyphen to the series name as a suffix except for the names harmotome, pollucite and wairakite in the phillipsite and analcime series. Differences in spacegroup symmetry and in order—disorder relationships in zeolites having the same topologically distinctive framework do not in general provide adequate grounds for recognition of separate species. Zeolite species are not to be distinguished solely on Si : Al ratio except for heulandite (Si : Al < 4.0) and clinoptilolite (Si : Al ⩾ 4.0). Dehydration, partial hydration, and over-hydration are not sufficient grounds for the recognition of separate species of zeolites. Use of the term ‘ideal formula’ should be avoided in referring to a simplified or averaged formula of a zeolite.

Newly recognized species in compositional series are as follows: brewsterite-Sr, -Ba; chabazite-Ca, - Na, -K; clinoptilolite-K, -Na, -Ca; dachiardite-Ca, -Na; erionite-Na, -K, -Ca; faujasite-Na, -Ca, -Mg; ferrierite-Mg, -K, -Na; gmelinite-Na, -Ca, -K; heulandite-Ca, -Na, -K, -Sr; levyne-Ca, -Na; paulingite-K, -Ca; phillipsite-Na, -Ca, -K; stilbite-Ca, -Na.

Key references, type locality, origin of name, chemical data, IZA structure-type symbols, space-group symmetry, unit-cell dimensions, and comments on structure are listed for 13 compositional series, 82 accepted zeolite mineral species, and three of doubtful status. Herschelite, leonhardite, svetlozarite, and wellsite are discredited as mineral species names. Obsolete and discredited names are listed.

Phillipsite was crystallized from a high siliceous aluminosilicate mixture containing Zn cations and methylene blue (MB). The presence of MB did not affect the crystallization, but it resulted in a substantial amount of dye being anchored to the zeolite, despite its narrow channels. Dye-free synthetic phillipsite modified with MB solution showed markedly lower dye content, which, however was considerably higher than those in the MB-treated natural phillipsite and mordenite. The ultra violet/visible (UV-vis) spectra of the dye-modified synthetic phillipsites indicated the presence of MB monomers and oligomers, whereas the spectra of the modified natural zeolites showed protonated MB also. The electron spin resonance spectra of samples crystallized with MB indicated the presence of paramagnetic species.

Supposed 'merlinoite' crystals from Monte Somma, Vesuvius (Italy) and Fosso Attici, north of Rome (Italy) represent highly twinned coherent intergrowths between merlinoite and phillipsite on a submicroscopic level. The MER (Immm, a ≈ 14.1, b ≈ 14.2, c ≈ 9.9 Å) and PHI (P 21/m, a ≈ 9.9, b ≈ 14.3, c ≈ 8.7 Å, β = 124.8°) frameworks of similar composition are assembled from identical tetrahedral units, though with a different connectivity. Coherent intergrowth and twinning of the two frameworks lead to P42/mnm pseudosymmetry, which is diagnostic of the intergrowth. Under ambient conditions merlinoite has Immm symmetry or I4/mmm if twinned. a low-symmetry model of space group P121/m1 (a ≈ 14.2, b ≈ 14.2, c ≈ 10 Å, β = 90°) allows structure refinement and quantification of the two frameworks.

Upon in situ dehydration to 250°C the evolution of the unit-cell volume of the Monte Somma merlinoite/phillipsite intergrowth displays an intermediate trend between previously studied pure merlinoite from the Khibiny massif (Russia) and Ba-rich phillipsite.

The Monte Somma crystal studied by temperature-dependent single-crystal X-ray diffraction methods also contained a subordinate chabazite inclusion with no coherent structural relationship to the merlinoite/phillipsite framework. Thus, the modification of the chabazite framework on dehydration could also be studied.