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Home , Clinoptilolite, Ferrierite, Heulandite, Mordenite, Phillipsite, Stilbite

Eur. J. . 1989,1,479-487

The genesis of

GLAUCoGOTTARDIt*

Istituto di Mineralogia e Petrologia, Università di Modena, via S. Eufemia 19,1-41100 Modena,

Abstract: The equilibrium diagrams of zeolites and the different possibilities of synthesizing zeolites starting from chemicals, , and natural glasses are reviewed so to have a general picture of the conditions of crystallization of these minerals. Subsequently, a description and interpretation is given of the geological environments where zeolites crystallize in nature.

Key-words: , diagenesis, very-low-grade , hydrothermalism, volcanic glass.

1. Introduction and generally contain some M+ ca­ tions, which are almost absent in , yu­ This topic has been the subject of so many publi­ gawaralite and wairakite, so the con­ cations (e.g. Hay, 1978, 1986; Iijima, 1978, 1980; centration in the system may influence the given Kastner & Stonecipher, 1978; Surdam & Shep- boundaries. Additional diagrams on these zeolites pard, 1978) over the last ten years, that one may can be found in the literature, but none is known wonder "Why another one?". As a matter of fact, to the author for zeolites other than those men­ all these previous studies give detailed informa­ tioned here. Field and laboratory evidence suggests tion on rock-forming zeolites, generally ­ that some other alkali zeolites may have a stabil­ lized from natural glasses during diagenesis, but ity field; this is certainly true for , they omit any consideration of zeolites in veins the siliceous alkali-rich variant of heulandite, and and vugs of massive rocks. The author also aims is also probably true for and . to give information on this latter occurrence, al­ though it is impossible to fill the gap entirely, because of the paucity of detailed research. 3. Synthesis of zeolites

The literature on the synthesis of zeolites is very 2. Equilibrium diagrams involving zeolites extensive; a comprehensive review is given in Barrer (1982). The synthesis of a zeolite is com­ Only a few zeolite equilibrium diagrams have monly accomplished from a recipe: changing the been published so far: data are known only for ingredients and the way of cooking may change , laumontite, wairakite, , heulan- the final result. This fact is certainly in some way dite, and yugawaralite. For analcimes with Al/Si connected with the nucleation difficulties encoun­ ranging from 1:3 to 1:1.7, diagrams at P(H2O) of tered, especially with some zeolite species that are 2 and 5.15 kbar may be found in Kim & Burley (1980). Cho et al (1987) show a P(H2O)/T dia­ gram where stability fields for stilbite, heulandite, * The author who was a member of the editorial yugawaralite, laumontite, wairakite and lawsonite board of this journal passed away shortly after having are indicated. It is important to note that stilbite submitted this manuscript.

0935-1221/89/0001-0479 $ 2.25 © 1989 E. Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1 Downloaded from http://pubs.geoscienceworld.org/eurjmin/article-pdf/1/4/479/4001658/479_gseurjmin_1_4_479_488_gottardi.pdf by guest on 01 October 2021 480 G. Gottardi

very common in nature. For instance, laumontite the amount of active silica the more nuclei are has never been synthesized, although it is one of generated, so larger can be obtained by the few zeolites whose stability diagram is known; diluting the active silica source; higher tem­ these studies have been performed only using peratures favour the formation of larger crystals natural crystals. In other words, laumontite crys­ (Barrer, 1982, p. 177 and 178, and cited references). tals may be easily grown but not nucleated. Near­ Raising the pH has the same effect as raising the ly the same is true for natrolite, also a common temperature, but a too high pH (> 13) favours the natural zeolite; its nucleation is possible only in reaction of the liquid with the first formed crystals the presence of nuclei of well crystallized to give a new crystal phase. () but not from gels alone (Senderov & The Si/Al-ratio is normally different in the Khitarov, 1971). There is a "magnesium mys­ liquid phase and in the crystallized solid; it is usu­ tery": if Mg is added to any mixture intended for ally higher in the liquid, but may be higher in the zeolite synthesis, only sheet silicates ( miner­ solid if the ratio is as low as 2 in the liquid. als) are obtained, but Mg is present in many natu­ The alkali metal cations may be in solution as ral zeolites (, mazzite, , offretite, bases, but also in part as salts (chlorides, sul­ and ). Phases equivalent to these Mg- phates, etc.), which may or may not be present in bearing zeolites may be synthesized in the pres­ the synthesized crystals. The crystals enclosing the ence of an organic such as tetramethyl am­ anions may correspond to well-known natural monium (TMA), which is unlikely to occur in phases (, , etc.), which bear an­ natural enviroments. in their framework cavities, but may also be Three kinds of ingredients will be considered a synthetic phase (e.g. ZK-5) without a natural here: chemicals, minerals, and natural counterpart. glasses.

3.2. Synthesis with silicate minerals 3.1. Synthesis with chemicals There are many publications (see Barrer, 1982, The literature is vast and commonly in patents. Typ­ p. 216 ff.) on how zeolites may be obtained from ical ingredients are SiO2 (as silica-gel or soluble clays plus alkali; this is important not only from silicate) + Al2O3 (as soluble aluminate or alumi­ the point of view of industry, because of the lower nium hydroxide gel) + a pure or mixed strong price of these raw materials, but also from a geo­ base + H2O with hydrothermal treatment from logical point of view, because some zeolite occur­ 80 to 350°C; the strong base may be an inorganic rences in soils may have formed by this kind of one (alkali metal or alkaline earth) or an organic process. one (e.g. tetramethylammonium hydrate). Zeo­ The already mentioned synthesis of natrolite lites are always obtained from alkaline media, the from nepheline + NaOH (Senderov & Khitarov, pH ranging from 14 down to 8.5. At a lower pH, 1971) is also an example of this kind. synthesis is still possible, but longer times are A particular example of zeolite synthesis is giv­ needed to form a detectable amount of zeolite. en by Balagna et al. (1977), studying the alteration The mechanism of nucleation and growth of of a biotite granodiorite in contact initially with zeolites is certainly complex and not easy to un­ distilled water at 300°C and 0.33 kbar for several derstand. Some reactants (SiO2, AI2O3) have low months. The overgrown phases included vermicu- solubilities in the pH range of interest, but they lite (a clay mineral), (a zeolite), and form gels and colloidal particles which dissolve ashcroftine (a zeolite-like mineral). rapidly as soon as possible; so the silica available for synthesis in the liquid phase depends on the rate at which the "solid" phase dissolves and re­ 3.3. Synthesis with natural glasses places the ions subtracted from the solution by the process of nucleation and growth of crystals. There is much literature also on this type of syn­ The first nuclei of the newly formed alumino-sili- thesis, which is particularly important for Earth cate are very poorly crystalline and unstable (Aiel- scientists. Obviously, "natural glass" means almost lo et al., 1971) and are subsequently transformed only volcanic glass in , although the rare into better crystallized material; also, when the impact glasses are also natural and may alter to reaction is started from clear solutions. The larger zeolites. For example the work of Colella et al.

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(1977) employed rhyolitic pumices from Lipari generally thought of as a low-temperature pro­ with added alkali: , merlinoite, phillip- cess, where "low" means "less than 200°C". site, and analcime were crystallized, depending on Comprehensive reviews on the diagenesis of zeo­ temperature and NaOH/KOH-ratio. lites are given in the references quoted at the be­ More akin to geological conditions are syn­ ginning of this paper. theses with glass plus water; unfortunately, publi­ Two kinds of diagenesis must be distinguished: cations of this kind are few. Holler & Wirsching above the water table, and below the water table. (1978) obtained mordenite from a rhyolitic glass, Diagenesis of zeolites above the water table may chabazite, , and analcime from a phono- occur in soils, in hydrologically open systems, in litic glass (final pH up to 9.5) and also from a hydrologically closed systems, and in geoauto- basaltic glass. Note that the phonolitic glass is claves. Diagenesis below the water table may oc­ altered to the same zeolites in nature. Similar re­ cur in marine sediments and as burial diagenesis sults were obtained by De Gennaro et ah (1988) (the latter has a continuous transition to very- with a trachytic glass at 200° C. Phillipsite crystal­ low-grade metamorphism). lized, and the final pH in the solution was 10.

3.4. General remarks on synthesis 4.1.1. Diagenesis in soils

The synthesis of zeolites requires high pH, but It is well-known that soils of arid and semi-arid very high values are not necessary. If in the lab­ regions may be very rich in carbonate (or oratory a pH of9.5 is sufficient, in a natural envi­ bicarbonate); even crystallization of the corres­ ronment a lower crystallization rate is possible ponding mineral (trona) is possible. Hence, it is with pH = 8.5. High activities of the zeolitic com­ predictable that the concentration of sodium bi­ ponents are also obviously necessary, and magne­ carbonate may rise by evaporation to such a value sium concentration must be low. Anyway, a natu­ as to start a reaction with clay minerals to form ral glass and water are sufficient ingredients to zeolites (mainly analcime). A typical example is reach this pH and to "cook" a zeolite. Also, sil­ the analcime in the alkaline soils of San Joaquin icates and water are possible ingredients: Stevens Valley, California (Baldar & Whittig, 1968) and (1932) showed that distilled water in contact with Ruzizi, Burundi (Frankart & Herbillon, 1970). powdered silicates develops a pH of 7.2 to 10.8 in a short time; he tested lepidolite, phillipsite, stil- bite, muscovite, biotite, , K-, al- 4.1.2. Diagenesis in hydrologically open systems bite, leucite, etc. This is in accordance with the high salinity (mainly NaCl) and high pH values of Meteoric water, when percolationg slowly in a the sediment pore-waters (see Fyfe et ah, 1978); thick layer, becomes more alkaline and saline thus, even where a natural glass is absent, condi­ (and higher in pH) as the water reaches greater tions for the crystallization of zeolites may be depths; near the surface, the glass is altered to clay reached easily in nature. minerals, a little deeper to zeolites (clinoptilolite, In this way, it is quite understandable that na­ chabazite, phillipsite), deeper still to analcime, tural zeolites are generally related to the presence and at the lowest level to alkali-. A char­ of volcanic glass (sometimes impactite glasses); acteristic vertical zonation is the result of this zeolites may also be the product of deposition genesis. Typical examples are the Vieja Group, from a water solution which dissolved pre-exist­ Texas (Walton, 1975; montmorillonite, clinoptilo­ ing mineral crystalline assemblages. lite and analcime zones) and the John Day For­ mation, Oregon (Hay, 1963; fresh glass and cli­ noptilolite zones; K-feldspar is locally present). 4. The genesis of zeolites Generally, only one or two zeolite species are formed in open systems. The palagonitic tuff of 4.1. Diagenesis (and very-low-grade metamorphism) Oahu, Hawaii, is an exception because, according to Hay & Iijima (1968a, b) it was zeolitized in Diagenesis is the crystallization of a mineral by an open system with the formation of as many as alteration (secondary transformation) of the pre­ seven zeolites: faujasite, phillipsite, gismondine, existing components of a sediment; diagenesis is chabazite, , natrolite, analcime.

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4.1.3. Diagenesis in a hydrologically closed system high enough to char any wooden remains. There is also a difference in the manner of sedimenta­ A hydrologically closed system is a closed basin tion, because tuffs build a layer of constant thick­ without effluent streams; at its centre, a saline ness which covers the pre-existing geomorphol- alkaline lake is or was present. From this lake or ogy, whereas the ash-flow behaves like a liquid dried zone, the water leaves the system by evapo­ and its deposit fills the valleys. ration, so closed systems are typical of desert Lenzi & Passaglia (1974) showed that an ig- areas. As in open systems, in this case also there is nimbrite in central Italy is homogeneously zeolit­ a slow water flow towards the central zone; this, ized to chabazite, although two tuffs, one above however, occurs over an impermeable sedimen­ and one below the ignimbrite, both totally lack tary layer, and hence it is nearly horizontal and zeolites and both have the same chemical compo­ not vertical as in the open system. During its slow sition as the ignimbrite. This fact cannot be ex­ flow, meteoric water may become more saline by plained by zeolitization in an open system, so the dissolution of surface minerals (like silicates) and authors related the complete zeolitization to the by evaporation, but it may also loose high temperature and the high vapour pressure carbonate as precipitates: thus, the water compo­ which existed inside the rock unit immediately sition often evolves towards basic alkali-rich solu­ after its deposition. Aleksiev & Diourova (1975) tions, i.e. a brine. If the surface sediments are proposed a similar model for the genesis of a cli- made up of tuffs, this tendency of surface water noptilolite-rich submarine ignimbrite from Bulga­ towards greater alkalinity is enhanced by the ria: these two authors also coined the name ready dissolution of the volcanic glass, and zeolite "geoautoclave". crystallization is highly probable. Concentric Matsubara et al (1978), who did not know of zones of authigenic minerals are so formed, with the two above mentioned publications, proposed an outer and upper ring of fresh or altered glass again a similar genesis for the erionite of some plus clay minerals, an intermediate ring with zeo­ "welded-tuffs" of . lites, a successive inner ring with analcime and a Autoclave genesis needs further study. It has final central with alkali-feldspars. Evaporitic min­ been suggested (D.E.W. Vaughn, pers. comm.; erals are common in the central zone, which may R.L. Hay, pers. comm.) that some difference in coincide with a (dried) lake. the features of the glass (like the grain size of the A good example is Lake Tecopa, California glass fragments, and hence their different surface/ (Sheppard & Gude, 1968), where the zeolitic ring volume ratios) could be the key to explain why one contains phillipsite, clinoptilolite, and erionite tuff is zeolitized and the other is not. and is followed by the central feldspar zone with­ out an analcime zone. Many other examples have been described in the Miocene-Holocene sedi­ 4.1.5. Diagenesis in marine sediments ments of the Western . Zeolite-bearing marine sediments can be distin­ guished in shallow and deep sea environments. 4.1.4. Genesis in geoautoclaves It is not easy to give a short review of the broad variety of shallow sea zeolite-bearing sediments; This type of zeolite crystallization was proposed however, they are generally volcaniclastic, may independently by three groups of scientists for now crop out on land or not and the zeolitization three occurrences quite far from one another. It may have occurred when they were above or be­ cannot be considered as diagenesis, because it is low sea level. The andesitic tuffite occurring in the thought to occur at temperatures perhaps as high Northern Apennines, Italy (Sersale et al, 1963; as 200° C, but it is described here because there is Gianello & Gottardi, 1969), at the boundary no sharp difference between the tephra zeolitized between Oligocene and Miocene, was certainly in this way and tephra which has undergone di- deposited in a shallow sea, but it is zeolitized to agenetic zeolitization. phillipsite at only one place and unzeolitized else­ Ignimbrites or ash-flow deposits are rocks sim­ where, so that zeolitization probably occurred ilar to tuffs in appearance and mineralogical when the tuffite was already on land. Zeolitiza­ composition, and they have often been considered tion below sea level probably would be homo­ as tuffs in the past, although, immediately after geneous and widespread. An example of a shal­ their deposition, the temperature inside them was low sea sediment, zeolitized beneath sea level, is

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given by the analcimic tuffite in the Gulf of Na­ I. Clay-carbonate. ples (MÜller, 1961). Carbonate deposition is possible only above the The formation of zeolites in hyaloclastites is carbonate compensation depth which means only currently thought to occur below sea level, by at moderate depth, hence near islands or conti­ reaction of sea water with fine-grained basaltic nents, where the volcanism is essentially acid and glassy fragments. The fine grain size is a pre-re- favours the formation of clinoptilolite. By con­ quisite necessary to increase the surface/volume trast, deep-sea clays are deposited at great depth ratio and hence the reaction rate. In larger masses, in regions remote from continents where a silica- the reaction rate would be too low. The reaction undersaturated, alkaline volcanic activity is pos­ of acid (silica-rich) glasses with water is slower. sible, favouring the subsequent formation of phil­ Small glass fragments are typically produced by lipsite. explosive volcanic activity in the shallow sea (hya­ loclastites s.s.) but may also be the result of frag­ II. High-low accumulation rate. mentation during the rapid chilling of a small Near the continents or islands, the rate of sedi­ flow that entered the sea (a large lava flow would mentation is high and, for the reasons given under not be chilled rapidly). The first product of the I., clinoptilolite formation is favoured. glass + water reaction, also called "palagonitiza- tion", is a "palagonite", which is a hydrated mod­ III. Pacific-Atlantic. ification of glass. The final products are clay min­ Because ocean-ridge cannot be considered erals (smectites), zeolites (mainly phillipsite, but here, there is no basic volcanism in the Atlantic also chabazite and gonnardite), , and iron capable of spreading glass shards in the ocean. hydroxides (Cristofolini et al., 1973; Honnorez, However, acid volcanism exists at the ocean mar­ 1978). gins, and its debris is also brought to the ocean by In deep sea sediments, a zeolite content of 1 to rivers. In the Pacific Ocean, both types of volca­ 2% is quite common. Clinoptilolite and phillipsite nism are present, as are phillipsite and clinoptilo­ are more frequent, analcime is rare. The distribu­ lite. tion of the first two zeolites is interesting and may be summarized as follows: IV. Latitude higher or lower than 50° No explanation is apparent for this distinction. Phillipsite Clinoptilolite more frequent more frequent V. Holocene-Cretaceous. Phillipsite prevails over clinoptilolite in Holocene I. in clayey sediments in calcareous and other sediments because basic glasses break down to sediments phillipsite faster; the presence of basic glasses in II. where accumulation where accumulation rate ancient sediments is minor for causes related to rate is less than 10m/106 is more than 10m/106 the geological history of the Earth. Acid glasses years years need a longer geological time to be altered to III. in Pacific Ocean both in Atlantic and clinoptilolite. Pacific Oceans IV. between latitudes at all latitudes 50° N and 50° S V in Upper Miocene to in Lower Miocene to It is useful to note that acid glasses mostly orig­ Holocene sediments Cretaceous sediments inate from the explosive activity of continental VI. in sediments in sediments containing volcanoes, whose glass fragments are thrown high containing basic volcan- rhyolitic glass or from and, after a trajectory in the atmosphere, are ic glass glass in the pres­ spread in the oceans. Basic glasses are spread di­ ence of excess silica rectly in the oceans without any atmospheric tra­ jectory; submarine central volcanoes erupt , According to Petzing & Chester (1979), this which are fragmented during their rapid chilling distribution may be explained simply by the fact in contact with water, and the splinters are spread that phillipsite crystallizes from basic (silica-poor) by currents (volcanoes of mid-ocean ridges do not glasses, and clinoptilolite from acid (silica-rich) contribute to the spread of basic glass shards). glasses (cf. point VI.) with the following argu­ The pH in pore-water solutions from deep sea ments: drillings ranges from 7 to 8 (Hay, 1978).

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4.1.6. Burial diagenesis and very-low-grade meta- A characteristic of burial diagenesis in the morphism small number of zeolite species which crystallize: clinoptilolite, mordenite, analcime, and laumon­ This mode of genesis has been studied mainly in tite; some chabazite and phillipsite may also occur the circumpacific countries (Japan, New Zealand, in the altered glass zone. This is true only without Western United States). Different zones with in­ hydrothermal influence, which produces more creasing depth are recognized; these zones are the zeolite species (f.i. stilbite, , epistilbite, same (fresh or altered glass, zeolite, analcime, thomsonite, etc.) (Utada, 1980). feldspar) as already mentioned for open system Finally, it may be useful to note that the mor­ diagenesis, but burial diagenetic zones are much phology of all sedimentary (sensu lato) zeolites is thicker (order of magnitude for thickness is 100— the same as that for the hydrothermal zeolites 1000 m instead of 10 m). If any hydrothermal described below (but diagenetic zeolites from some influence is present, the zones do not have hori­ tuffs may be anhedral), although their average zontal boundaries and are thinner. size is only 10 to 20 µm, whereas the crystals of hydrothermal origin are commonly in the mm or cm range. Table 1. Zones of burial diagenesis according to Iijima (1985): a sodic and a calcic series are shown, and temperature boundaries are also given. 4.2. Hydrothermal genesis Sodic Calcic 4.2.1. Geothermalfields I Altered glass I Altered glass Sediments in geothermal fields commonly contain 41-55°C zeolites, especially if they include tuffs with glassy II Clinoptilolite- II Clinoptilolite components. The hot (50 to 150° C) water dis­ Mordenite (more calcic) solves glass and silicates in the deeper and hotter 81-91°C parts and deposits zeolites in the whole sequence. III Analcime III Heulandite A large variety of zeolite species (6 to 8) is charac­ 120-124°C teristic of this kind of genesis, which features a IV Albite IV Laumontite continuous transition to burial diagenesis. Cli­ noptilolite and mordenite, the only two zeolites common in burial diagenesis, are also commonly abundant in geothermal fields, but in this envi­ Coombs et al (1959) introduced the "zeolite- ronment are associated with stilbite, chabazite, facies" as a of very low grade. thomsonite, epistilbite, dachiardite, yugawaralite, Utada (1970, 1971) gave a detailed sequence of erionite, wairakite, etc. zeolite zonation under burial diagenesis and very- A typical example are the zeolitic sediments low-grade metamorphism. Iijima (1985) distin­ of the Yellowstone National Park (Honda & guished two sequences, a Na-rich and a Ca-rich Muffler, 1970; Bargar et al, 1981) which include one, and was the first to give the temperature at opaline sinter, glacial sediments, and rhyolitic zone boundaries (see Table 1). These tempera­ lavas and tephra. tures were measured in drill holes in Tertiary and Cretaceous marine strata, interbedded with silicic vitric volcaniclastic rocks and in Japanese oil- and 4.2.2. Hydrothermal ore deposits with zeolites gas-fields (the general applicability of these temperature boundaries is still open to debate). If Many ore deposits, which were formed by crystal­ Iijima's scheme is compared with the equilibrium lization from hot (150 to 400° C) solutions under diagram for calcic zeolites (Cho et al, 1987), a pressure in veins or in porous rocks, locally con­ general agreement can be observed. In detail, tain zeolites (also as fine museum specimens). A however, there are differences; so Iijima's heulan- good example is the sulphide deposit at Andreas- dite-laumontite T-boundary (124° C) is too low berg, W. Germany, with large, well-formed crys­ according to the diagram, which however was tals of and stilbite. The of drawn for a system devoid of alkali, a situation the deposit of Kongsberg, Norway (Neu­ not possible in nature. mann, 1944), should also be noted here.

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4.2.3. Late hydrothermal deposition in pegmatites then zeolites are formed in regions at lower temperature. In a typical diagenetic model, sur­ The temperature of crystallization of pegmatites face waters percolate through fractures of the ba­ (450 to 550° C) is certainly too high for zeolites, salt flow, dissolving the glass and deposit­ but some pegmatites do contain zeolites which ing zeolites in regions close to the dissolution probably were formed at a lower temperature as a area. However, the temperatures involved in both late hydrothermal deposition on pre-existing processes could be similar (water can be consid­ pegmatitic minerals. A well-known example is the ered hydrothermal even if T < 100° C, and dia­ Elba granodiorite pegmatite, which contains the genesis may also occur above 100° C, if at the zeolites mordenite, dachiardite, stilbite, epistil- appropriate depth) and the distance between the bite, heulandite, and chabazite; these were depos­ sites of dissolution and deposition is not necessar­ ited on the typical pegmatitic minerals: feldspar, ily zero in diagenesis, and not necessarily very , tourmaline, beryl, pollucite, and lepidolite great in hydrothermal genesis. Therefore we can (Pederzolli Gottardi, 1968). suppose a continuum between the two models, with no clear-out distinction between them. It ist worthwhile noting that autometamor- 4.2.4. Hydrothermal veins andgeodes infeldspathic phism must be excluded, because the vacuoles of rocks all freshly solidified basaltic lavas are devoid of zeolites, notwithstanding the fact that thermal The laboratory experiment by Balagna et al. waters are common in volcanic areas. (1977), described in section 3.2., shows that zeo­ Benson & Teague (1982) have described a case lites may crystallize by reaction of water with the (basalt of the Pasco Basin, Washington) where the minerals of a granodiorite; the crystallization of diagenetic model is more suitable. The model is zeolites in veins and geodes of granite, granodior­ supported by the association of zeolites with iron- ite, gneiss, and similar coarse-grained and magnesium-rich smectites, the distribution of feldspar-bearing non-volcanic rocks is due to a these smectites and the species of zeolite which are similar process of dissolution-precipitation caused present. The same authors emphasize that the by rising hydrothermal fluids. An example is the zeolite paragenesis in the basalt of and chabazite and stilbite in veins of the Adamello Northern Ireland are different with respect to the granite (Alberti, 1971). Pasco Basin paragenesis, and for both occurren­ ces they consider a hydrothermal genesis as more probable. 4.3. The genesis of zeolites in geodes and vugs of In my opinion, the hydrothermal hypothesis is basalts more probable for many occurrences because: 1. Fine large zeolite crystals, apart from basalt Most of the fine zeolite specimens which embel­ geodes, always have a hydrothermal genesis; di­ lish mineral collections are set on the walls of agenetic zeolites are commonly micorcrystalline geodes and vugs of basic lavas (basalts sensu lato.) (nearly 15 µm in size). The genesis of these crystals could be considered, 2. The number of zeolite species which are as those occuring in non-volcanic rocks, as due to formed by diagenesis is generally limited to cli- hydrothermal deposition. Nashar & Davies (1961) noptilolite, mordenite, analcime, chabazite, phil- and Nashar and Basden (1966), however, have lipsite; this number is slightly larger in "closed found some evidence in favour of the hypothesis systems" (erionite and ferrierite may also occur) that zeolites crystallized by reaction of meteoric and increases further only in the presence of a waters slowly percolating in rock fractures, with hydrothermal influence. Hydrothermal genesis is the glass components at surface temperature, or a possible source of all zeolite species; in the vugs nearly so, as in "open system" diagenesis. of basalts, the variety of zeolites is great. In par­ Schematically, the difference between the two ticular, fibrous zeolites (e.g. natrolite) are frequent hypotheses can be described as a function of two in vacuoles of basalts, but very rare as diagenetic parameters: temperature, and distance between minerals. the sites of glass dissolution and the sites of crystal 3. The average chemical composition of zeolites deposition. In a typical process of hydrothermal from vugs is different from that of microcrystal- deposition, thermal water dissolves the glassy line zeolites from sediments (Alberti & Brigatti, components in the hottest part of a basalt flow, 1985).

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4.4. Magmatic genesis Cho, M., Maruyama, S., Liou, J.G. (1987): An experi­ mental investigation of heulandite-laumontite equilibrium at 1000 to 2000 bar Pfluid Contr. Miner­ Analcime may have a magmatic origin, but this al. Petrol, 97,43-50. seems to be a rare event. Roux & Hamilton (1976) Colella, C, Aiello, R., Di Ludovico, V. (1977): Sulla have shown that analcime can crystallize from a merlinoite sintetica. Rend. Soc. Ital. Miner. Petr., 33, silicate melt only in a very narrow temperature 511-518. and pressure range. In the majority of cases, the Coombs, D.S., Ellis, A.J., Fyfe, W.S., Taylor, A.M. analcime of lavas is sodium exchanged leucite. (1959): The , with comments on the interpretation of hydrothermal syntheses. Geochim. The only definitely magmatic analcime has been Cosmochim. Acta, 17,53—107. found by Luhr & Carmichael (1981) in lavas of Cristofolini, R., Di Girolamo, P., Stanzione, D. (1973): minette composition in the Colima volcanic com­ Caratteri genetici e mineralogici di jaloclastiti- plex, Mexico: this genesis has been proven by del Γaltopiano Ibleo (Sicilia) e dell'isola di Procida isotope analysis. (Campania). Rend. Soc. Ital. Miner. Petr., 29, 497— 552. Magmatic genesis has been attributed also to De Gennaro, M., Colella, C, Franco, E., Stanzione, D. natrolite, but evidence thereof does not seem to (1988): Hydrothermal conversion of trachytic glass be sufficient. into zeolite. 1. Reactions with deionized water. N. Jb. Miner. Mh., 1988,149-158. Frankart, R. & Herbillon, A.J. (1970): Presence et Acknowledgements: Thanks are due to Carmine genèse d'analcime dans les sols sodiques de la Basse Colella, Richard L. Hay, and Richard A. Shep- Ruzizi (Burundi). Bull. Groupe Franc. Argues, 22, pard for reading the manuscript and suggesting 79-89. some improvements. The financial support of Fyfe, W.S., Price, N.J., Thompson, A.B. (1978): Fluids in the earth's crust. 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