Pure & Appl.Chern., Vol.52, pp.2II5—2l3O.Pergamon Press Ltd. 1980. Printed in Great Britain.

Plenary Paper — Geology and Mineralogy

GEOLOGY OF NATURAL ZEOLITES AND ZEOLITIC ROCKS

Azuma lijima

Geological Institute, Faculty of Science, University of Tokyo,7-3-1 Hongo, Tokyo 113, Japan

ABSTRACT

Present status, particularly development in the latest decade, ofthe geology of natural zeolites and zeolitic rocks has been overviewed.Emphases are focused on the classification of genetical occurrences andthe synthesis deduced from the nature..

INTRODUCTION

Nearly two centuries have passed since zeolite was discovered andnamed by Cronstedt in 1756. Collection and mineralogical descriptionof pretty, coarse crystals were the primary studies of zeolites for along time till early this century. It is a great monument that theconcept of zeolite facies was established by Eskola in 1936 as the low—est grade of the metamorphic facies. The concept was greatly developedby Coombs in New Zealand, having further been visualized by lijima,Seki and Utada in Japan. Researches on zeolites in alkaline, salinelake deposits by Hay, Gude and Sheppard in USA made our eyes open tothe zeolite formation at or near the Earth's surface. From deep—seasediments zeolites were already reported by Murray and Renard in 1891.

The Deep Sea Drilling Project (1968—1975) by the Glomar Challengercruises shed light on the deep—sea zeolites. Our knowledge on naturalzeolites and zeolitic rocks has rapidly expanded during the latest twodecades. The growth of study on natural zeolites has been and will bestimulated by increasing needs of them as a potential natural resource,instead of synthetic zeolites, particularly in Africa, East and WestEurope, Japan, USA and USSR. A recent result is summarized in "Naturalzeolites: occurrence, properties and use" Eli.

This paper has been prepared for an overview of geology of naturalzeolites and zeolitic rocks in the world. A brief note on zeolitemineralogy is followed by classification of genetic types of occurrenceand a geological synthesis on zeolite formation. Readers can refer to

Coombs[2], Hay [3, 4], lijima and Utada [5, 6], lijima [7], Kossovskaya[8], Sheppard [9], Surdam and Sheppard [10], Gottardi and Obradovic[11], Sersale [12], Kastner and Stonecipher [131 and their references.

2115

2116 AZIJMA IIJIMA

Table 1. Composition of natural zeolites and composition of hostigneous rocks.

Species R Dominant cation Host rock

R = Si: (Si+Al-I-Fe).U: ultrabasic rock, B:A: acidic rock.

NATURAL ZEOLITES

basic rock, I: intermediate rock, and

Zeolites are a group of basic, hydrous aluminosilicate minerals.The name was created by Cronstedt in 1756 from the Greek cw and XtOowhich mean 'boiling stone' [14]. They possess an open alumino—silicateframework structure containing channels filled with water molecules andcations. The water molecules are easily dehydrated by heating and re—hydrated in the air without significant changes of the framework. Thecations are usually exchangeable at low temperature below 100°C.

Gismondine 0.50—0.57 Ca U BPhillipsite 0.52—0.77 Ca, Na, or K U B I AThomsonite 0.50—0.52 Ca U BGonnardite 0.52—0.58 Na U BAshcrof tine 0.55 K UMesolite 0.60 Ca, or Na U B I

Scolecite 0.60 Ca U BNatrolite 0.60 Na U B I

Cowlesite 0.60 Ca BEdingtonite 0.60 BaGarronite 0.63 Ca B I

Analcime 0.63—0.74 Na U B I AChabazite 0.63—0.80 Ca, or Na U B I AGmelinite 0.67 Ca, or Na BLevyne 0.67 Ca BLaumontite 0.66—0.71 Ca U B I AWairakite 0.67 Ca B I APaujasite 0.71 Na U BOffretite 0.72 Ca, or Mg BHarmotome 0.73 Ba BMazzite 0.73 Na BStilbite 0.72—0.78 Ca, or Na B I AHeulandite 0.73—0.80 Ca B I AErionite 0.74—0.79 Ca, Na, or K B I AYugawaralite 0.75 Ca B I ABrewsterite 0.75 SrEpistilbite 0.75 Ca B I ABarrerite 0.77 Na AStellerite 0.78 CaPaulingite 0.78 K BDachiardite 0.78—0.86 Ca, or Na B AClinoptilolite 0.80—0.85 Ca, Na, or K B I ANordenite 0.81—0.85 Ca, or Na B I AFerrierite 0.85 Mg, or K B I A

Geology of natural zeolites 2117

Twenty—three types of frameworkhavebeen known In natural zeolites[14] and thirty—four species are listed in Table 1. Cottardi [14] em—phasizes that the ratio R=Si:(Si+Al+Fe) of a zeolite, which representsthe percentage of the tetrahedra occupied by Si, is an important chemi—cal parameter with respect to the Order—Disorder relation in zeolites;zeolites are classified into"basic"with O.5O<R<O.625, "intermediate"with O.625<R<O.75, and "acidic" with O.75<R. These three classes seemto be closely related to the chemical composition of host rocks includ—ing zeolites as shown in Table 1. Zeolites in undersaturated, alkalirocks largely belong to "basic", whereas those in oversaturated, acidicrocks are "intermediate" to "acidic". Zeolites often show a wide com—positional variation even in the same species [14, 15]; in particular,

phillipsite [16, 17], chabazite [18, 19], clinoptilolite—heulandite[20, 21, 22, 23], erionite [24], ferrierite [25], and analcime [26, 27,28]. The variation reflects various factors, which act on the zeoliteformation, such as P—T conditions, chemistry of original material andpore water, and others as discussed later.

GENETIC TYPES OF OCCURRENCE OF ZEOLITES

Zeolites form at present or formed in the past in various sedimentor rocks under, varying physical and chemical environments. Geneticclassification of occurrence has been attempted by some workers; e.g.,

Hay [j, 4], Iijima and Utada [6], Iijima [7], Sheppard [9], Gottardiand Obradovic [11] and Mumpton [29]. Much work in the field and labor-atory has explicitly proved that temperature is an important factor tocontrol zeolite formation [30—35]. Another significant factor is thechemistry of pore water in which zeolites precipitate [3, 10, 13, 36,37]. As a result of the geothermal or chemical gradient, zeolites com-monly occur in a vertically or laterally zonal arrangement which isusually mappable as zeolite zones. Therefore, temperature and natureof pore water should be emphasized as the criteria to classify thege—netic types of occurrence. In addition, zeolite species formed areinfluenced by the original material such as acidic or basic volcanicglass, clay and etc. Nine genetic types are classified as shown inTable 2. More than two types frequently overlap one another to make acomplex type. It is sometimes difficult to distinguish a specific typeof occurrence from the shallow—burial diagenesis, percolating ground-

water, low—temperature hydrothermal and contact metamorphism withoutregional mapping of zeolite zones.

Magmatic Primary Zeolites. Analcimes that are considered as aprimary mineral of late formation occur in some alkali rocks. The dis-tinction between primary and secondary analcimes, however, is not easywhen the analcime occurs as interstitial grains [38]. In plutonic rockssuch as teschenite, essexite, and syenite, analcimes sometimes occur asa main constituent with or without nepheline. Analcime occurs as smallglobules in inclusions of analcime trachybasalt in the phonolite ofTraprain Law. The globules originate probably in a magmatic emulsionformed by the separation of a water—rich magma into two immiscible liq-uids [39]. In volcanic rock analcime is known as a primary constituentin some basalts, where typically it is restricted to the groundmass

[38].

2118 AZUNA IIJIMA

Table 2. GenetIc types of occurrence of zeolites.

A. Zeolites for',n at elevated tenrperature3 the .aones being primarily

caused by geothermal gradient.1. Nagmatic primary zeolites2. Contact metamorphism3. Hydrothermal4. Burial diagenesis (or metamorphism)

B. Zeolites form at or near the surface condition, the zones being

principally caused by chemical gradient.5. Percolating groundwater6. Weathering7. Alkaline, saline lake deposits

C. Zeolites form at low tenrperature, any zones being not recognized.8. Marine environment

D. Zeolites form in inrpact craters.9. Impact crater

Contact Metamorphism. Zeolite is widespread in contact aureolesof the vulcanoplutonic complexes which intruded into a thick sequenceof porous tephra, volcaniclastic sediments or feldspathic sandstonefilled with pore water. Good examples are reported from the TanzawaMountains [31], the Misaka Mountains [40], the Motojuku area [41] andsome other areas [32] in the Green Tuff region of Japan, where Neogenebasic to acidic tephra and volcaniclastic sediments with a thickness ofsome kilometers accumulated in marine basins.

Metamorphic minerals appear to make a concentric pattern aroundintrusive bodies. In andesitic tephra and andesitic sandstones of theTanzawa Mountains, five metamorphic zones are distinguished toward a

quartz—diorite mass [31]; 1) stilbite—(clinoptilolite)— vermiculite,2) laumontite—mixed layer—chlorite, 3) pumpellyite—prehnite—chlorite,4) actinolite greenschist, and 5) amphibolite. Heulandite, mordeniteand analcime are rarely found in the zone 1. Wairakite and yugawara—lite occur commonly in the high—temperature part of the zone 2. Sekiet al. [31] insisted that the lowest—grade metamorphism of the zeolitefacies can be divided into the high solid pressure type characterizedby stilbite and/or heulandite and the low solid pressure typecharacterized by mordenite. However, heulandite and stilbite form atthe low solid pressure in andesitic tephra of the Motojuku area [411.

The effect of chemical composition of host rocks to zeolite assem-blages is explicitly documented in the western Misaka Mountains [40],where Late Miocene granitic complex intrudes basaltic and rhyodacitictephra which were alternately laid in the Miocene marine basin. Thealteration zones and mineral assemblages are shown in Figure 1. On theone hand, clinQptilolite, mordenite, analcime, heulandite, laumontiteand albite appear in each of the same zones of both acidic and basic

tephra; on the other hand, phillipsite, stilbite, mesolite, thomsonite,epistilbite, yugawaralite and wairakite are found only in vesicles andvugs in basalts.

Hydrothermal. Zeolites precipitate extensively from alkaline toweakly acid hot water in active geothermal areas; e.g., Wairakei, New

Geology of natural zeolites 2119

IIIa b

IVa b

— in both basic and acidic tephrain basaltic tephra— .— unstable relic

Figure 1. Zeolite zones in Neogene acidic and basic tephra in contactaureole in the western Nisaka Mountains, central Japan. The zoning isreferred to Burial Diagenesis. Slightly modified [40].

Zealand [42, 43], Yellowstone Park, Wyoming [33], Iceland [34], andOnikobe in Japan [44]. The zeolite assemblage and zone are principallycontrolled by temperature and chemical composition of host rocks and

subordinately by permeability, composition of geothermal fluids, andage of geothermal area and host rocks.

In Quaternary basaltic flows and hyaloclastic rocks of Iceland,zeolite zones are principally resulted from geothermal gradient in thelow—geothermal areas where the top 1 km—depth is less than 150°C. Fourzeolite zones are commonly discriminated; 1) chabazite—thomsonite, 2)mesolite—scolecite, 3) stilbite, and 4) laumontite. By contrast, zeo—lites do not make any clear zone in the high—geothermal areas where thetop 1 km—depth is more than 200°C [34].

Zeolite zones in the Tertiary basalts of eastern Iceland [45] andNorthern Ireland [46] are similar to those in the Quaternary basalts inthe low—geothermal areas of Iceland besides the occurrence of a zone ofNa zeolites, analcime and natrolite, beneath the top chabazite—thotnson—ite zone. They are considered to have formed by geothermal gradient.

In acidic tephra of geothermal areas, zeolite assemblages are notdifferent from those in basalts in the deep, high—temperature zones butquite distinct in the shallow, low—temperature zones. Four zones are

vertically arranged in Onikobe [44]; 1) mordenite, 2) lautnontite, 3)laumontite—wairakite, and 4) wairakite. Yugawaralite and analcime arecommon in the laumontite zone. In Yellowstone Park [33] analcime orheulandite zone intervenes between the top clinoptilolite—mordenite andthe laumontite zones. The content of water molecules in the zeolitesdecreases with increasing temperature.

Submarine hydrothermal activities produce a peculiar alterationpattern. Analcime and mordenite occur in a zone surrounding the hydro-thermal clay zones which enclose the Miocene Kuroko polysulf ides oredeposits in Odate [47] and Nishiaizu [48] in the Green Tuff region ofTohoku, Japan. The analcime and mordenite formed by reaction of daci-tic glass with the Na—rich hydrothermal water which migrated from theclay zones into porous vitric tuff. The clinoptilolite—mordenite zonesuperimposes on the hydrothermal alteration zones. Ferrierite islocally associated with mordenite and clinoptilolite [49].

ClinoptiloliteMordenitePhillipsiteAnalcimeHeulanditeStilbiteMesol iteLaumontiteThomsoniteEpistilbiteYugawaral iteWairakiteAlbitePrehnitePumpellyiteEpidoteLow cristobaliteQuartzSmectiteChloriteSericite

2120 AZUMA IIJIMA

lake deposits of the San Simon valley in Arizona by Loew in 1875 [74]and from ferruginous clay of Duingen in Germany by von Seeback in 1862

[11].Modern saline lakes are restricted in the semiarid tracts of the

Earth; e.g., in the rift valley of East Africa and Israel and in theBasin and Range province of the western United States. The Quaternaryand Neogene saline lake deposits which contain zeolites are also wide-'

spread in the western United States [3, 4, 10, 36, 74 — 76] and in East

Africa [3, 10, 68—70]. The Paleogene deposit is the well—knownGreen River Formation of Wyoming, Utah and Colorado [3, 28, 77—80].

In acidic tuffs of alkaline, saline lake deposits, diagenetic mm—erals occur in lateral zones contrasted with the vertical zones due toburial diagenesis. A typical occurrence is seen in the Late Pleistocene

Tecopa Lake Beds in southern California [36]. Rhyolitic glass shardsremain unaltered in the flood plain and lake margin facies in whichfresh—water prevails. They alter to phillipsite, clinoptilolite anderionite associated with some chabazite and mordenite In the salinewater facies. Small amounts of analcime may replace the precursor zeo—lites. K—feldspar occurs in the supersaline fades of the lake center,is commonly associated with saline minerals like trona, and replacesthe precursor zeolites. The content of analcimes in alkaline, salinelake deposits tends to increase with age [3, 4]. Analcimes do not makea clear zone in the Late Pleistocene Tecopa Lake Beds, but occur in a zone.between the zeolite and the K—feldspar zones in the Pliocene Big SandyFormation, and are the principal zeolitesin the Eocene Green River For-mation. Authigenic albite that replaces analcime is only found in theGreen River Formation which was buried at the maximum depth of approxi-mately 2 km [28].

The lateral zones of the diagenetic facies, viz, fresh silicicglass/zeolites/analcime/K—feldspar are undoubtedly caused by a chemicalgradient of pore water within the alkaline, saline lake deposits. Con-sequently, this type is often said as the closed—system groundwatertype [9, 10]. The hydrology and geochemistry of alkaline, saline lakedeposits are reviewed by Surdam and Sheppard [10].

Ca—clinoptilolite and (Si—rich heulandite) with little erionitereplace silicic vitric tuffs in the Eocene Wagon Bed Formation in Wyo—ming [81]. These zeolites alter to K—feldspar in the lake center. Nophillipsite and çhabazite and little analcime are recognized in thesaline lake deposit. Boles and Surdam [81] consider that the lake wasnot highly alkaline with a pH less than 9.

Marine sediments. Zeolite from deep—sea sediments was recorded byMurray and Renard in 1891 [82]. Much work has been done since then,but the Deep Sea Drilling Project (1968—1975) and the InternationalPhase of Ocean Drilling (1976—) shed light upon the whole deep—sea zeo—lites, of which overview was made by Hay [4], Iij ima [7] and Kastner

and Stonecipher [13].K-'zeolites such as phillipsite and clinoptilolite are dominant.

Harmotome is commonly associated with phillipsite. Analcime is muchless common that the K—zeolites and occurs usually in basaltic sediments.Other zeolites including chabazite, erionite, laumontite, ginelinite,natrolite, and thoinsonite are very rare in basic volcanics. A littleamount of laumontite was found in Upper Miocene to Cretaceous, terrige-'nous and volcanic sediments of the D.S.D.P. cores at Site 323 in theBellingshausen Abyssal Plain off Antartica 83 . However, its authi—

genesis seems not to be evident.

Geology of natural zeolites 2121

Phillipsite is common in post—Miocene brown clay, vitric siliceousand calcareous oozes, and basic volcanic sediments in the slow sedimen—tation areas of the Pacific and Indian Oceans [7, 13, 84]. The fieldof stability of phillipsite seems to be restricted, because the crystalfaces are corroded not only by circulating sea water in the surficialbottom sediments within a depth of about 1—2 m but also by pore waterin the sediments below about 4 m [84]. The pore water in phillipsite—rich sediments poses a lower silica and higher potassium environment.The origin of deep—sea phillipsite is probably related with basalticglass, considering the common association with palagonite and basaltic

glass fragments [4, 7, 13] and from a high Ti/Al ratio, of phillipsite—containing sediments [85]. The moderately higher content of silica inthe deep—sea phillipsite suggests the contribution of biogenic silica[19]. Deep—sea phillipsite occurs exceptionally in acidic vitric tuffat Site 205 [86].

Clinoptilolite is conunon in Paleogene and Cretaceous calcareoussediments and terrigenous clay in the Atlantic Ocean and the PacificMargin, though it is not uncommon in post—Miocene brown clay and othersediments in the Pacific and Indian Oceans. Clinoptilolite crystalsare not corroded. The pore water in clinoptilolite—rich sediments su—gests a higher silica environment [7]. Clinoptilolites are mostlyoriginated from acidic glass in vitric tuffs and vitric sediments dueto burial diagenesis particularly in the rapid sedimentation areas ad-jacent to cotitinents and island arcs [7]. In the slow sedimentationareas where phillipsite may occur, clinoptilolites probably form byreaction of basaltic glass with pore water rich in dissolved silica of

a biogenic or weathering origin [4, 7, 13, 87]. Also, X—ray amorphousclay and smectite may be the original material of some clinoptilolite

[4, 13, 87].The decrease in the amount of phillipsite in the Paleogene and

Cretaceous sediments is interpreted that composition of host sediments,particularly volcanic material, changes from basaltic to andesitic [88];that phillipsite may alter to either K—feldspar or analcime [7]; and

that phillipsite may be replaced by clinoptilolite or sepiolite [13,87]. However, these interpretations are still not entirely proved.

Impact Crater. Zeolites are discovered in suevite and lake sedi-ment of the Nrdlinger Ries impact crater in Bavaria, West Germany [89,90]. Glass in suevite alters to analcime and smectite, while microcav—ities of suevite are filled by analcime, clinoptilolite, phillipsite,erionite and harmotome. Zeolites are common in the upper suevitesequence, in which analcime and clinoptilolite occur in bituminous,dolomitic shale and sandy sediments with some altered suevite.

GEOLOGICAL SYNTHESIS ON NATURAL ZEOLITE FORMATION

Source and Distribution of Zeolite. Natural zeolites occur as analteration product of glass of a volcanic or impact origin, amorphousclay, and aluminosilica gel. They replace crystalline materials suchas nepheline, plagioclase, precursor zeolite, and smectite. Also, theyprecipitate in cavities and veins from hot or cold solution. However,fine—grained glass fragments are the most important raw material ofnatural zeolites for almost all types of occurrence, because oftheir high reactivity and similar chemical composition to certain zeo—lites. In fact, zeolite—rich rocks composed of one or some species of

2122 AZUNA IIJIMA

zeolite are largely derived from altered vitric tuffs, in which thecontent of zeolite is sometimes over 90 percent of the rock. Clinopti—lolites in deep—sea sediments and marine strata are often interpretedas originating in clay material owing to the negative evidence that'volcanic glass can't be seen'. However, this evidence does not neces—

sarily deny the origin of volcanic glass, because altered, very fine—grained, vitric fragments in zeolitic clays and claystones are usuallyimpossible to be identified under the microscope, as Kossovskaya [8]pointed out.

The distribution of zeolitic rocks Is usually very closely relatedto the site of volcanism. Zeolites of the hydrothermal, contact meta—

morphism, burial diagenesis, and percolating groundwater origins arecommonly widespread in the younger orogenic belts and hot spots wherethick deposition and contemporaneous volcanism occurred: viz., in the

Circum—Pacific region including New Zealand, Japan, Korea, Sakhalin,Kamchatka, Alaska, western United States, Chile, and other potentialareas; in the Tethys region including Spain, France, Italy, Switzer—land, Yugoslavia, Bulgaria, Rouznania, Hungary, Turkey, north Iran,South Russian Basin, Ural Basin and Central Asia; and in Hawaii andIceland. Clinoptilolites in acidic vitric tuffs and vitric sedimentsinterbedded with the deep—sea deposits in the rapid sedimentation areasadjacent to continentsand island arcs seem to be caused by burial dia—genesis [7, 91]. Volcanism as well as semi—arid climate are stressedby Surdam and Sheppard [10] in terms of the geological situations ofthe alkaline, saline lake deposits which contain abundant zeolite. Therift valley in East Africa and the Basin and Range Province in westernUnited States fulfill the condition.

Most if not all known zeolite ore deposits are derived from vitrictuffs: e.g., clinoptilolites of Itaya in Japan [92], Hector in Cali—fornia [93], Northeast Rhodopes in Bulgaria [94], the Tokay in Hungary[95], and Dzegvi and Khekordzula in Georgia and Ay—dag in Azerbaydzhan[96]; mordenites of Itado and Shiroishi in Japan [92]; and chabazitesof Bowie in Arizona [74], and Campania in Italy [11, 12].

Zeolitization of Volcanic Glass. Much work on zeolitization ofvolcanic glass has been done both in the field and laboratory.Hydration of glass is often considered to be the preliminary stage ofzeolitization [97]. The rate of hydration is represented by a simpleequation, S2=CT where S is thickness of hydrated rind on glass, T isyears, and C is a constant relating to temperature and composition of

glass and pore water [98, 99]. However, a complicated chemical reac-tion concurs with the hydration and active migration of such componentsas Na, K, Ca, Mg, silica and even alumina occurs between glass and porewater[47, 64, 100]. On the one hand, the leached alkalis increase pHand alkalinity of.pore water, accelerating dissolution of glass: ontheother hand, silica and alumina released from the glass are utilized forconstructing zeolite frameworks. In the Teels Marsh and Lake Magadi,volcanic glass is dissolved to form an aluminosilica gel which changesto phillipsite in water of pH over 9, while such gel is not found inwater of pH 8—9 in which clinoptilolite forms [10].

The rate of zeolitization is represented by a function with multi—variables such as time, temperature, composition of glass, and composi-tion and flow rate of pore water. Zeolites may grow very fast in hightemperature geothermal fluids, considering the results of hydro-thermal experiments. Even in pure water, chabazite, phillipsite andanalcime are synthesized, from basaltic glass at 200°C for 5 days whilemordenite and analcime are produced from rhyolitic glass at 250°C for

Geology of natural zeolites 2123

40—60 days [101]. The experiments are accelerated by adding a littleamount of NaOH or KOH (35, 101]. Basaltic ash laid on the Surtsey fromNovember 1963 to April 1964, is altered to zeolite, perhaps chabaziteby heated sea water of below 100°C after 7 years [102]. Phillipsite isassociated with fresh glass and gel in rhyolitic vitric ash of 600 y.b.p. in the Teels Marsh, an alkaline, saline playa—lake of Nevada. Abun—dant chabazite and phillipsite were formed by percolating groundwaterin alkali—trachytic glass fragments of the Neapolitan Yellow Tuff ofl—l.2XlO y.b.p.; the zeolitization is estimated to have continuedfor 4—5 X lO years though it is now inactive [103]. Not only phillip—site and chabazite but also analcime occur in basanite tuff of Koko

Crater of 3.5Xl04 y.b.p, by reaction of sideromelane with percolatinggroundwater 14]. Phillipsite crystals grow up to 45 pm for 1.5X105years in deep—sea sediments of the Indian Ocean [104]: the rate ofgrowth is correlative to the slow rate of sedimentation which is 0.5—0.8 mm/103 years [105]. Approximately 106 to years are necssaryfor rhyolitic or dacitic glass in marine sediments to change to clino—ptilolite and mordenite due to the present—day burial diagenesis whichare influenced by geothermal gradient and the rate of burial and sedi-mentation [7, 30, 51]. The rate of zeolitization is proportional tothe content of silica in glass under the sane physicochemical conditions

it is signficantly accelerated by increasing temperature and increasingpH, alkalinity and salinity of pore water.

Formation of Zeolite Species. Formation of zeolite species is in-fluenced by various factors. Chemical composition of original materialof the zeolite and composition of host rocks primarily control zeolitespecies particularly at low temperature. Zeolites in alkali rocks areof "basic" species excepting analcime and faujasite, while those inacidic rocks are "acidic" and "intermediate" ones (Table 1). Even

oc

• 0 acidic glass250 • 0 basic glass

200 )2

150

03100

•450 09

05 00 1607p8

10

•2 •1 0 1 2 3 4 5 6 7

TIME(Iog year)Figure 2. Rate of zeolitization of volcanic glass in various occur-rences. Solid symbols represent analcime. 1) mordenite—analcimeand 2) chabazite—phillipsite-analcinie by hydrothermal synthesis inpure water [101]; 3) mordenite by hydrothermal synthesis in weaklyalkaline geothermal fluid in the Shikabe geothermal well [109]; 4)chabazite on the Surtsey [102]; 5) phillipsite in the Teels Marsh[10], 6) phillipsite—chabazite in the Neapolitan Yellow Tuff [103];7) phillipsite—chabazite—analcime in the Koko Crater Tuff [4]; 8)phillipsite in the Indian Ocean [104]; 9) clinoptilolite in the. Nii—gata oilfield [30]; and 10) clinoptilolite at Site 439 [51].

2124 AZUMA IIJIMA

Burial Diaenesis. The concept of burial metamorphism was esta—bushed by Coombs [50] based on the study of the Triassic geosynclinaldeposits in New Zealand. Zeolitic burial diageriesis is defined thatzeolites form progressively within a thick sequence of strata with in—creasing burial depth [3, 4, 5, 7]. It has been best documented in theTertiary and Cretaceous geosynclinal basins of Japan, where marine andfresh—water sequences are commonly interbedded with acidic vitric tuffsand volcaniclastic beds [7, 30, 51—53]. Two zeolitic reaction seriesare recognized in acidic tuffs and volcaniclastic sediments: one isalkali zeolite reaction series, acidic volcanic glass + H20 -'alkaliclinoptilolite + mordenite + low cristobalite -' analcime + quartz + H20+ albite + H20; and the other is calcic zeolite reaction series, alkaliclinoptilolite -' heulandite + quartz -' lauxnontite + quartz. In acidictuffs the alkali zeolite reaction series is practical to set up

because it occurs more commonly than the calcic zeolite reactionseries. As a result, four diagenetic zones are constructed; namely,Zone I is a zeolite—free zone characterized by glass shards which alterpartially or sometimes wholly to smectite and opal; Zone II is a zonecharacterized by clinoptilolite, mordenite or both which originate fromacidic glass; Zone III is a zone characterized by analcime replacing theprecursor zeolites, which often coexist as relic; and Zone IV is a zonecharacterized by albite replacing analcime. Plagioclase in tuffs andsandstones usually changes to albite, too.

The zeolite zones are widespread and mappable. Boundaries of eachzone is rather gradational. The zoning is primarily controlled by geo-thermal gradient. In the Niigata, Akita and Tenpoku oilfieids of Japan,the present—day zeolitic burial diagenesis is certainly proceeding inthe Quaternary and Neogene marine strata. Five drillholes, penetratingthe strata, show that the top of Zone flI (analcime) occurs at 84°—9l°Cin a depth range between 1700 and 3500 m; and that the top of Zone IV(albite) is at 120°—l24°C in a depth range between 2500 and 4500m[30,51]. The wide depth ranges, compared with the restricted temperatureranges, suggest that PH2O and the difference between P total and PH2Odo not practically influence the reaction temperature in those depths.The good correlation of the zeolite zones with the degree of coalifica—tion [54—58] supports the progress of temperature—dependent reactionsduring the burial diagenesis. The transition of Zone I/Il occurs at25°—60°C in the depth range between 635 and 1900 m. The change ofacidic glass to zeolite is probably influenced by composition of porewater, time and temperature.

The calcic zeolite reaction series roughly concurs in the analcimezone, though laumontite can coexist with albite in Zone IV. Therefore,Zone lEE is sometimes subdivided into flla (analcime—heulandite) and lEEb(analcime—laumontite) [7, 51]. Heulandite and laumontite are common inandesitic and basaltic volcaniclastic rocks; e.g. the Triassic depositsof New Zealand [50, 59] and the Paleogene flysch of the North Helvetic[60, 61] and the northern Apennine [11]. Shallower zeolite zones aremissing due to denudation in the Triassic of New Zealand. The reactionheulandite -' laumontite is influenced by not only temperature but alsoPH2O, PCO2 and the activities of silica and cations in pore water [59,62]. The reaction occurs in Miocene dacitic tuff at 120°C at the depthof 4310 m of the MITI—Masugata Hole in the Niigata oilfield, where thepresent—day burial diagenesis is in progress.

Phillipsite, habazite and erionite seem not to occur in acidicvitric tuffs in marine Or fresh—water deposits by burial diagenesis.

Geology of natural zeolites 2125

Percolating groundwater . Zeolites precipitate from groundwaterpercolating through a sequence of porous pyroclastic material rich inreactive glass. Consequently, they are often said as the open—systemgroundwater type [4, 9]. This type of zeolitization was established byHay in the John Day Formation in Oregon [63]. Most spec1acular examplewould be Late Pleistocene palagonite tuffs on Oahu Island, Hawaii [64,65]. Many small tuff cones of nephelinite and basanite ash—fall depo—sits demonstrate a characteristic alteration pattern. The surface oftuff cones are covered with subtropical soil, which is followed by acarapace of fresh tuffs or slighly—altered, opal—cemented tuffs with athickness of 12—18 m. Below the carapace with a sharp contact, orangeto brown zeolitic palagonite tuffs constitute the main part of thetuff cones. The upper zone of the palagonite tuffs is dense, cementedwith phillipsite, chabazite and Mg calcite, while the lower zone of itis rather porous and characterized by analcime partially replacing theprecursor zeolites and partially cementing pores. On the foot of thesoutheast wall of Kahauloa Crater, below the analcimic tuffs are thesubpalgonite tuffs which overlie slightly—altered, opal—cemented tuffs.This alteration pattern is interpreted as suggesting the chemical gra—dient produced by reaction of percolating groundwater with sideromelaneparticles. The pH and the concentrations of dissolved ions increase asthe groundwater percolates through porous sideromelane tuffs, until zeo—lites precipitate in interstitial pores. A 12—18 meter—thick columnof sideromelane ash is needed to form zeolitic palgonite tuffs. It isremarkable that palagonitization is hardly proceeding in sideromelanetuffs below the groundwater table so that zeolites do not form.

In rhyolitic vitric tuffs of the John Day Formation, a 200 meter—thick column is necessary for percolating groundwater to gain salinitysufficient to precipitate clinoptilolite [63]. How thick it is as com-pared with 12—18 m in ultrabasic tuffs on Oahu! Below the clinoptilo—lite zone with a thickness of 700 m, analcime formed by reaction of theclinoptilolite with alkaline pore water, making the analcime zone. Thevertical arrangement, viz., fresh glass/clinoptilolite/analcime, is thesame as that formed by burial diagenesis in acidic vitric tuffs. Thesharp boundary between fresh glass and clinoptilolite zones is charac-teristic in the open—system groundwater type and the radiometric age ofauthigenic minerals is a useful tool for the specification [4].

Zeolitized vitric tuffs of the Quaternary alkali rocks in southernItaly, e.g., chabazite and phillipsite in alkali—trachytic tuffs inCampania and the Neapolitan Yellow Tuff of Canpi Flegrei, are consider-ed to form percolating groundwater [11, 12].

Weathering. Zeolites are formed by chemical weathering in alkalisoils and land surfaces in semi—arid tracts. Recent to Pleistoceneexamples are reported from the San Joaquin valley of California [66],Ruzizi in Burundi [67] and the Olduvai gorge in Tanzania [3, 68—70].Regional Mesozoic analcimolites interbedded with redbed facies inUtah [71], Congo Basin [72] and Central Sahara [73] were probably form-ed by weathering of non—tuffaceous fluvial claystones on flood plainssoon after deposition [4]. It is remarkable that analcime and illiteare formed by reaction of smectite with the Na—HCO3—C02 soil water inalkali soils. Hay [4] wrote a nice review and nothing adds to it.

Alkaline, saline lake deposits. Many zeolites form in alkaline,saline lake deposits of various ages from Recent to Upper Permian. Theearliest records of sedimentary zeolites were indeed from the saline

P.A.A.C. 52/9—F

2126 AZUMA IIJIMA

Table 3. Geological synthesis andoccurrence of natural zeolites. Theindicated temperature is merely approximate.

Type of occurrence Temperature Zeolite species

Deep—sea sediments 4°—50°C Ph, Cp, (An)

Earth's surface

weatheringAlkaline, saline lake

Percolating groundwater(in basic tephra)

200 — 50°CPh,Gi,(He)

Ph,F,He,La,

Cp,Fa,

Ch,Go,

Er, Mo,Na, An,

Cp,An,St,

Wa,

Ch,Th,etc.and

Er, Mo,Me, Sc,except

Yu

Percolating groundwater(in acidic tephra)

Shallow burial diagenesis

Low-temperature hydrothermal

25°—l00°c

Deep burial diagenesisModerate—temperature hydro—

thermal>100°C La, An

Low-grade metamorphism.

High—temperature hydrothermal>200 C Wa, Yu, An

Magmatic primary An

Ph: phillipsite, Cp: clincvptilolite, Ch: chabazite, Er: erionite, Mo:mordenite, Gi: gismondine, Fa: faujasite, Go: gonnardite, An: analcime,Na: natrolite, He: heulandite, Th: thomsonite, Me: mesolite, Sc: scole—cite, St: stilbite, La: laumontite, Wa: wairakite, Yu: yugawaralite.

though they occur in various sediments and host rocks, phillipsite andchabazite cluster in the silica—poor variety with dominant Ca in ultra—basic and basic rocks, whereas they belong to the silica—rich varietywith dominant alkalis in acidic tuffs in alkaline, saline lake deposits[16, 19].

Temperature is another important factor tb control zeolite speciesas tabulated in Table 3. Zeolites formed in deep—sea sediments at lowtemperature are phillipsite and clinoptilolite, both of which containdominant alkalis with K>Na, and possibly some analcime. Zeolites form-ed at or near the Earth's surface, viz., in alkali soils, in alkaline,saline lake deposits and in zeolitic tuffs altered by percolating

groundwater, are phillipsite, chabaite, clinoptilolite, erionite,mordenite, natrolite, gismondine, faujasite, gonnardite, and analcime.Zeolites formed by deep burial and high—temperature hydrothermal alter-ation at a temperature over approximately 100°C are low—water, calcianzeolites such as laumontite, wairakite and yugawaralite besides anal—cime. In particular, wairakite and yugawaralite are frequently foundin cores of geothermal wells at a temperature over 200°C. Analcimes insome igneous rocks are considered as the primary mineral of magma inlater crystallization [38]. Zeolites formed by shallow burial an4 low—temperature hydrothermal alteration below about 100°C are thomsonite,ferrierite, heulandite, mesolite, scolecite and stilbite in addition to

Geology of natural zeolites 2127

those which occur at or near the surface. Analcimes occur in all overthe temperature range from deep—sea sediments to magma. The Si:(Al+Fe)ratio in analcimes, varying from 1.7 to 2.9, is affected by variousfactors: it decreases with increasing temperature in synthetic anal—cimes [261, although it does not systematically change with increasingburial depth within the analcime zone of burial diagenesis [7, 1061; itis inherited from the precursor zeolites, viz., high—silica analcimereplacing high—silica clinoptilolite and mordenite while low—silicaone replacing low—silica phillipsite and chabazite [10, 75]. Analcimeand wairakite constitute a solid solution [1071, though detailed miner-

alogical study is a future problem [108].Ageing, also, influences zeolite species. Natural zeolites occur

in various rocks of varying geologic ages up to Late Precambrian time[3, 5, 7, 11]. The amount of zeolitic rocks in the Earth's crust seemsto be fairly common in the Cenozoic and upper Mesozoic strata, while it

Table 4. Number of zeolite species in geologic time. Vein—fillingsare excluded. See references [3, 5, 11, 13].

Era Number of zeolite species

Cenozoic 32Mesozoic 7

Late Paleozoic 4

Early Paleozoic 2

Late Precambrian 1

Early Precambrian 0

becomes rather uncommon in the lower Mesozoic and Upper Paleozoic,scarce in the Lower Paleozoic, and negligible in the Precambrian.Moreover, the number of zeolite species found in strata of each geolog-

ic period tends to decrease rapidlywith increasing age (Table 4). Theratio of analcime to all zeolites in alkaline, saline lake depositsincreases with ascending age [3, 4]. These facts may be due to decom-position and substitution of many unstable zeolites by a few stablezeolites, like analcime and laumontite, which are eventually replacedby alkali feldspars and other minerals. The phenomenon seems to beinterpreted as suggesting either ageing [3, 4] or deeper burial and soincreasing temperature of older strata [6, 7].

Chemical composition of pore waters such as pH, alkalinity and theconcentration and activities of chemical components essentially con-trols the formation of zeolites especially in saline lake deposits [10,62]. Also, Kirov et al. [35] experimentally show that both geothermaland chemical gradients play important roles on the vertical zeolitezones during burial diagenesis. However, informations of interstitialwater in a thick column of strata are very scarse and far from fully

understood.

REFERENCES

1. L.B. Sand and F.A. Mumpton (ed.), "Natural Zeolites, Occurrence,

Properties, Use", Pergamon Press, Oxford, (1978), p. 546.

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