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New Zealand Journal of Geology and Geophysics

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Zeolite equilibria in the system CaAI2Si208 ‐NaAISi3O8 ‐ SiO2 ‐ H2O

J. G. Liou , Christian de Capitani & Martin Frey

To cite this article: J. G. Liou , Christian de Capitani & Martin Frey (1991) Zeolite equilibria in thesystem CaAI2Si208 ‐ NaAISi3O8 ‐ SiO2 ‐ H2O, New Zealand Journal of Geology and Geophysics,34:3, 293-301, DOI: 10.1080/00288306.1991.9514467

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New Zealand Journal of Geology and Geophysics, 1991, Vol. 34: 293-3010028-8306/91/3403-0293 $2.50/0 © Crown copyright 1991

293

Zeolite equilibria in the system CaAI2Si208 - NaAISi3O8 - SiO2 - H2O

J. G. LIOUCHRISTIAN DE CAPITANI

Department of GeologyStanford UniversityStanford, CA 94305, U.S. A.

MARTIN FREY

Mineralogisch-Petrographisches InstitutUniversity of BaselPetersplatz 1,4003 Basel, Switzerland

Abstract On the basis of experimentally determinedstabilities of common zeolites, including analcime, stilbite,heulandite, yugawaralite, laumontite and wairakite, T - XQ,phase relations of these zeolites and plagioclase in thepresence of excess quartz and H2O were constructed for thesystem CaAl2Si208 - NaAlSi3O8 - SiO2 - H2O at Pfluid = 1000,500, and 300 bars. Extensive solid solutions exist forwairakite-analcime and for heulandite-clinoptilolite series,whereas very limited Na substitution occurs in laumontite,stilbite, and yugawaralite. Such compositional variabilityresults in co-existence of two zeolites in natural parageneses.Progressive changes both in assemblages and in compositionsof co-existing zeolites with increasing temperatures areschematically shown with many compositional loops. Cazeolites vary in composition, depending on the mineralassemblage. Progressive changes in zeolite assemblages aresensitive to bulk-rock composition. Some zeolites such asyugawaralite, stilbite, and laumontite may be restricted only tohigh Ca/Na rocks. For rocks with a low Ca/Na ratio, zeoliteparageneses (+ quartz) with increasing temperature, areanalcime + heulandite heulandite + albite (or laumontite +analcime) laumontite + albite albite + wairakitess.wairakites.s. + plagioclase. In low SiO2 metabasites, however,the progressive change could be stilbite + analcimelaumontite + albite wairakitess,+ plagioclase. Wairakite (orcalcian analcime) + plagioclase occurs and heulandite is notcommon. These paragenetic sequences are consistent with theobserved zeolite assemblages in hydrothermal, burial, andocean-floor metamorphism described in the literature.

Keywords zeolites; phase equilibria; paragenesis; low-Tmetamorphism

G90063Received 29 September 1990; accepted 2 April 1991

INTRODUCTION

This paper is dedicated to Professor Douglas Coombs who hasmade significant contributions to the study of zeolitemineralogy and low-temperature (low-T) metamorphism. Hehas inspired the authors to continue such studies. Combs(1954) first proposed the zeolite facies and later (Coombs1960), the prehnite-pumpellyite metagreywacke facies.Through subsequent research on paragenesis and crystalchemistry of zeolites and related minerals by himself and hisassociates (e.g., Coombs 1961, 1971; Coombs & Whetten1967; Boles & Coombs 1975, 1977; Coombs et al. 1959,1976), a framework for low-T metamorphism has beenestablished (e.g.,Liou et al. 1985a, 1987).

The index assemblages for low and high T zeolite faciesare, respectively, analcime + heulandite (or stilbite) + quartz +corrensite and laumontite + albite + quartz + corrensite (±pumpellyite, prehnite, or epidote). The disappearance ofzeolites from mafic rocks has been considered to be theboundary for the transition from the zeolite to prehnite-pumpellyite facies (e.g., Coombs et al. 1959). The multi-variant nature for such a transition has been recentlyemphasised (e.g., Frost 1980; Cho et al. 1986). The purposesof this paper are: (1) to review the occurrence and com-positional variations of common zeolites; (2) to determine theeffect of temperature on zeolite assemblages and compositionsin prograde metamorphism; and (3) to evaluate phaseequilibria of zeolite and feldspar minerals in the simple systemNaAlSi3O8 (Ab) - CaAl2Si208 (An) - SiO2 (Qz) - H2O. Thesignificance of continuous reactions for zeolite parageneses inthe zeolite facies, and its transition to the prehnite-pumpellyitefacies, should be better understood.

Compositions and abbreviations of common zeolites andtheir related phases in very low grade metamorphic rocks arelisted in Table 1.

PARAGENESES AND COMPOSITIONS OF Ca ANDNa ZEOLITES

Analcime and some Ca zeolites, including mordenite, stilbite,heulandite, laumontite, and wairakite, commonly occur inveins and amygdules of volcanic units, as well as in the glassytuffaceous matrix of greywackes and other volcanogenicsediments in low-grade metamorphic terranes (for review seeBoles 1977, 1988). Their regional distributions have beenextensively described, particularly from the circum-Pacificorogenic belts, after the establishment of the zeolite facies byFyfe et al. (1958) and Coombs et al. (1959). Depending on theP/T gradient, rock and fluid compositions, occurrence, anddepth, the distribution of zeolites and other hydrous Ca-Alsilicates varies from place to place. Miyashiro & Shido (1970)divided common zeolites in low-grade metamorphic rocksinto Ca and Na zeolites, according to four components:CaAl2Si208, Na2Al2Si20g , SiO2, and H2O. Steps andtemperatures for progressive dehydration of zeolite assem-

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Wages have been suggested for observed parageneses inzeolite facies metamorphism in Taringatura Hills (Coombs etal. 1959), Tanzawa Mountains (Seki et al. 1968), easternIceland (Walker 1960), Wairakei geothermal field (Coombs etal. 1959), and Onikobe geothermal field (Seki et al. 1969).

The occurrence of zeolites in low-T metamorphic rockscan be generally divided into two types: (1) hydrothermal and(2) burial. The former occurrence includes active and fossilgeothermal systems, and rocks hydrothermally altered bymagmatic intrusions. Zeolites developed on a regional scale inthick stratigraphic sections are commonly attributed to burialmetamorphism (Coombs 1960; Zen & Thompson 1974).Occurrence of zeolites in some metavolcanics, such as those indrilled oceanic basalts and in on-land ophiolite sequences, is acombination of burial, hydrothermal, and thermal meta-morphism. Some characteristic features for zeolite occurr-ences in low-grade metamorphic terranes are summarisedbelow.

(1) Zeolites most commonly occur as alteration products ofvolcanic glasses and calcic plagioclases.

(2) Zeolites commonly make up < 25 vol. % of the bulk rock,except in altered vitric tuffs where much higher con-centrations may be found.

(3) Depth distribution of individual zeolites commonly showconsiderable overlap, which indicates that factors otherthan pressure (P) and temperature (T) are also importantreaction controls.

(4) On a worldwide basis, laumontite, analcime, andheulandite-group zeolites are most abundant. Tertiary andyounger rocks may contain more diverse zeolitic assem-blages including mordenite, erionite, chabazite, andphillipsite.

(5) Zeolites are generally associated with authigenic quartz,albite, adularia, calcite, smectite, corrensite, and lesscommonly with titanite, prehnite, epidote, and pum-pellyite.

AnalcimeAnalcime occurs as an essential constituent in some under-saturated igneous rocks. It is also commonly found in quartz-

Table 1paper.

Compositions* and abbreviations of phases used in this

Am =WrNtThChLm =YuHuCp =StEp =PrPmAbAnPIQz =Fs.s. =

Analcime, Na(AlSi2O6) H2OWairakite, Ca(Al2Si4O12) 2H2ONatrolite, Na2(Al2Si3O10) 2H2OThomsonite, NaCa2(Al5Si5O20) 6H2OChabazite, Ca(Al2Si4O12) 6H2OLaumontite, Ca(Al2Si4O12) 4H2OYugawaralite, C a ^ S ^ O , , ; ) 4H2OHeulandite, (Na,K)Ca4(Al9Si27O72) 24H2OClinoptilolite, (Na,K)3(Al3Si15036) 10H2OStilbite, NaCa4(Al9Si27072) 30H2OEpidote, Ca2(Al,Fe)3Si3012(0H)Prehnite, Ca2(Al,Fe)AlSi3O10 (OH^Pumpellyite, Ca4(Al,Fe)5MgSi6O21(OH)2Albite, NaAlSi3O8

Anorthite, CaAl2Si2OgPlagioclase, (CaAl,NaSi)AlSi2Og

Quartz, SiO2Fluidsolid solution

•••Compositions of zeolites are modified from Gottardi & Galli (1985).

bearing diagenetic and low-grade metamorphic rocks. Theanalcime-quartz assemblage, along with heulandite-quartz,has been used as an indicator of the initial stage of zeolitefacies metamorphism (Coombs 1960). Analcime forms as areplacement product from volcanic glass of widely variablecompositions, and may constitute as much as 60 vol. % insome altered vitric tuffs. Two distinct chemical variations ofanalcime compositions occur: (1) Most analcimes are Na rich,but some may contain appreciable K (up to 10 mole% of totalNa + K + Ca + Mg) (Utada 1970). Moreover, Surdam (1973)and Seki (1973) reported extensive solid solution betweenanalcime and wairakite. (2) The variation of Si/Al, hence H2Ocontent in analcime, was suggested to accord with the depth ofburial by Nakajima & Koizumi (1966), who emphasised thatanalcime composition may be used as a geothermometer.However, later studies (e.g., Coombs & Whetten 1967;Wilkinson 1968; Utada 1970; Boles 1971) concluded that norelationship exists between analcime composition and depthof burial. Evarts & Schiffman (1983) reported that analcimeco-existing with quartz has nearly ideal stoichiometry, butanalcime replacing plagioclase in a quartz-free volcanicbreccia shows substantial solid solution toward wairakite.Analcimes from low-grade metamorphic rocks of NewZealand have Si/Al ratios of about 2.4, whereas those fromJapan are slightly more siliceous (av. 2.7). Kim & Burley(1971, 1980) experimentally determined the synthesis fieldsfor both high and low analcimes; both exhibit large Si/Al andCa/Na ratios while some low analcime even extends to albitecomposition. Nakajima (1986) recently synthesised bothcubic analcime (Analcime I) and noncubic analcime(Analcime II) and concluded that analcime II is restricted tointermediate composition between analcime and wairakite.Natural analcimes may exhibit some deviation from cubicsymmetry and possess tetragonal or orthorhombic structures.Monoclinic analcimes with very low Ca content have beenreported (e.g., Pechar 1988).

MordeniteMordenite commonly occurs at shallow depths in alteredandesitic to dacitic rocks in active geothermal systems, as oneof the earliest formed zeolites (Steiner 1955; Seki et al. 1969;Kristmannsdottir & Tomasson 1978; Liou et al. 1985b).Mordenite occurrences are restricted to Tertiary and youngersediments as replacements of volcanic glass and as radial fan-like or spherulitic aggregates of fine prismatic crystals in veinsor vesicles (Boles 1977). It is commonly associated withclinoptilolite. Optical distinction between them is difficult.The chemistry of mordenites has been reviewed by Passaglia(1975) and its stoichiometric formula can be written as(Na,K,Ca)8(AlgSi4O96).28H2O. Pure Ca and Na end-membermordenites have been synthesised in the laboratory. However,natural mordenites range in their Si/(Si + Al) ratio from 0.8 to0.85, Ca from 1.6 to 2.5, Na from 2.0 to 5.0, and K from 0.1 to0.8 per unit cell.

Stilbite and heulanditeStilbite and heulandite occur chiefly in amygdules and veins oflow-grade metabasites. They are often associated with otherzeolites, such as mordenite, chabazite, clinoptilolite, and otherminerals of metamorphic origin. Stilbite, or, more commonly,heulandite, is the first Ca zeolite formed in a progressivesequence of diagenesis and zeolite facies metamorphism.Heulandite is a diagnostic mineral of the zeolite zone in

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uppermost volcanic rocks of some ophiolite sequences,particularly those with andesitic composition (e.g., Del PuretoOphiolite in California, Evarts & Schiffman 1983).Heulandite, analcime, and laumontite together with quartzcomprise the index minerals for the zeolite facies (Coombs etal. 1959). Distinct zoning of stilbite -* laumontite -> prehnite(and pumpellyite) -> actinolite -» hornblende uponapproaching a hypabyssal intrusive contact in the TanzawaMountains, Japan (Seki et al. 1968), is well developed.Quantitative data on the chemistry of stilbite in low-grademetamorphic terranes are very few. These few analysesindicate that stilbite is dominantly Ca rich and has a Si/Al ratioranging from about 2.5 to 3.0 (Liou 1971c; Boles 1972).Stilbite contains higher H2O, and is less chemically variablethan heulandite, which exhibits continuous solid solutionbetween end-member heulandite, CaAl2Si7Oi8.6H2O, andclinoptilolite, (Na,K)6(Al6Si3o072).20H20 (Gottardi & Galli1985). Heulandites in zeolite facies rocks are very calcic andtheir Si/Al ratio varies according to that of precursor materials.For example, heulandite as pseudomorphs of plagioclase in abasaltic breccia contains lower Si/Al ratio than that replacingandesitic tuffs, and those in siliceous tuffs have the highest Si/Al ratio (e.g., Boles & Coombs 1975; Evarts & Schiffman1983).

LaumontiteLaumontite is the best index mineral for zeolite faciesmetamorphism. It is widespread in oceanic-ridge (e.g., Liou1979), burial, and subduction zone metamorphism (e.g., Ernst1975) of basaltic and associated sedimentary rocks. It iscommonly found in burial sequences (e.g., Coombs et al.1959) and in active geothermal areas with a distinct zonaldistribution (e.g., Liou et al. 1985b). It occurs extensively asvein and amygdaloidal mineral, as alteration products ofvolcanic glass and calcic plagioclase, and as replacement ofearlier formed zeolites such as stilbite and heulandite.Compositionally, laumontite is very close to the ideal formulaCaAl2Si4Oi2.4H2O, with only minor substitution of Na, K,and Fe3+ totalling less than 2 wt%. Laumontites from zeolitefacies Karmutsen metabasites display fairly restricted Sivalues of 3.96-4.11 and (Na + K) values of 0-0.22 in achemical formula based on 12 oxygens, and show nosystematic variation between Si and (Na + K) values (Cho etal. 1986). This relation indicates that natural laumontite mayexhibit limited cation substitutions of the type 2(Na,K) = Ca +• , where • represents a vacant site.

Wairakite

Wairakite is common in active geothermal systems and insome low-grade metamorphic rocks, particularly thosemetamorphic sequences related to rising temperaturesresulting from epizonal intrusion of magma. In geothermalareas, it is widely developed in amygdules and veins involcanic rocks (e.g., Steiner 1955; Seki 1966), as well as in thetuffaceous matrix of greywackes and as cementing material ofhydrothermally altered sandstones (e.g., Bird et al. 1984). Inthese occurrences, wairakite exhibits characteristic very lowbirefringence and often shows cross-hatched twinning.Wairakite was suggested to be a stable phase in prehnite-pumpellyite facies Karmutsen metabasites (Cho & Liou1987).

Wairakite was classified into two types according to theiroccurrences and compositions by Seki (1971). Wairakite in

metavolcanics, such as those in the Tanzawa Mountains (Sekiet al. 1968) and in the Karmutsen Group (Surdam 1973; Cho& Liou 1987; Starkey & Frost 1990), are characterised by highNa content (> 2 wt%) and by variations in the Si/Al ratio. Forexample, most analysed wairakites from Karmutsen meta-basites range in XQ, values [= 2nCa/(2nCa + nNa + nK)] from0.78 to 0.97. Some Karmutsen wairakites with lower Xca

values may not be associated with quartz, and others could benoncubic analcime. This relationship, which indicates acomplete solid solution between wairakite and analcimesuggested by Seki (1971), may exist in a quartz-deficientassemblage. In contrast, wairakites associated with quartzfrom active geothermal systems, such as in Wairakei (Steiner1955), and in Onikobe (Seki et al. 1969; Liou et al. 1985b),contain high Ca with only a very limited substitution of Fe3+

for Al in tetrahedral sites. Compositions of wairakite fromOnikobe show low Al/Si ratios relative to the wairakite-analcime solid solution, CaAl2Si4Oi2.2H2ONaAlSi2O6-H2O, and range in Xca value from 1 to 0.76. Seki& Oki (1969) suggested a possible compositional gap atintermediate compositions between the wairakite and anal-cime end-members. Thus, depending on the presence orabsence of quartz, wairakites may show significant variationin their X ^ values.

Aoki (1976) provided some experimental evidence for thepossible limit of the wairakite solid solution. He carried outsynthesis experiments in the temperature range of 250-340°Cand suggested a minimum CaO/(CaO + Na2O) value of0.824 for synthetic wairakites in the analcime-wairakiteseries. Recent experiments by Nakajima (1986) support thissuggestion: both noncubic calcian analcime (II) anddisordered wairakite (wairakite I) co-exist in a systemCaAl2Si4012 .2H20 - Na2Al2Si3O10.2H2ONa2Al2SigOi6.2H2O. However, co-existence of such dis-ordered wairakite and calcian analcime has not been con-firmed in natural parageneses. The wairakite-analcime solidsolution may be extensive if these phases possess thesuperstructure suggested by Takeuchi et al. (1979). Details forthe structural and compositional relations for the wairakite-analcime solid solution series remain to be investigated.

P-T STABILITIES OF Ca AND Na ZEOLITES

The stability relations of a number of Ca zeolites (Hu, St, Lm,Yu, and Wr; see Table 1 for abbreviations) and analcime in thepresence of excess quartz and fluid are summarised in Fig. 1.Stilbite, heulandite, and laumontite are confined to low-T andlow-P conditions, and wairakite and yugawaralite are stableonly along high-T metamorphic gradients. The stability fieldof analcime in the presence of excess quartz is bounded on thehigh-P side by a reaction involving the formation of jadeite,and on the high-T side by dehydration to albite. The stability ofanalcime (+ Qz) is restricted to very low temperatures andpressures corresponding to diagenetic and zeolite faciesenvironments. At temperatures above 200°C, Na-bearingwairakite + quartz occurs in high-T zeolite facies and inprehnite-actinolite facies rocks in active geothermal systems.Changes in composition within these end-member phaseswould modify the P-T stability ranges for these minerals.

The paragenetic depth sequence of the Ca zeolites is highlydependent on the imposed metamorphic gradient, thePH o / Ptot?1 ratio, and other factors such as fluid composition.Two invariant points among zeolite equilibria are related to

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2000 -

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1500 -

1000 -

50 100

Temperature (°C)

Fig. 1 Experimentally determined P-T relations among various Ca zeolites (St, Hu, Lm, Yu, Wr), analcime (Am), albite (Ab), and anorthite(An) in the presence of excess of quartz (Qz) and fluid (from Liou 1970,1971a, b, c; Zeng & Liou 1982; Cho et al .1987). Parageneses of Caand Na zeolites for two bulk compositions (X = plagioclase of An50; and Y = basaltic composition) are shown in terms of An, Ab', Qz, andexcess H2O components.

such sequences. One is the invariant point for the stable co-existence of stilbite, heulandite, and laumontite at c. 600 barsand 150°C (Cho et al. 1987). Heulandite, which is stablebetween the stability fields of stilbite and laumontite, canoccur only at pressures higher than that of the invariant point.These data are consistent with natural parageneses in low-grade metamorphic rocks recrystallised under conditions ofvery low X c o in the fluid. The direct zonal transition fromstilbite to laumontite without heulandite may be favored bylower pressure conditions, such as at Tanzawa Mountains,Japan (Seki et al. 1968) and the Horokani ophiolite, Japan(Ishizuka 1985), whereas regional distribution of heulanditeand its transformation to laumontite, such as in New Zealand,may be typical of a higher pressure burial sequence. Asdescribed below, variations in heulandite compositions,including substitution of alkalis for Ca and differences in SiQjand H2O contents, significantly affect the heulandite stability,and account for heulandite occurrences in submarine meta-basalts (e.g., Malley et al. 1983) and in active geothermalsystem (e.g., Mehegan et al. 1982).

Another invariant point at 230°C and 0.5 kbar is that forthe co-existence of laumontite, yugawaralite, and wairakite.Zeng & Liou (1982) showed that yugawaralite has a very

restricted P-T stability field. In geothermal systems wherePH o / Ptotal ratio is about 0.3, its occurrence is restricted todep2ths shallower than 500 m. In a geothermal system with arelatively high metamorphic gradient and a high PH o / P^^ratio, yugawaralite may be stable, and the sequence oT zeoliteswith depth could be mordenite -> laumontite -> yugawaralite—> wairakite. By contrast, in regions with a lower meta-morphic gradient and a lower PH 0 / PtoUl ratio, yugawaraliteis not stable and the observes zonation of Ca zeoliteswould be mordenite —» laumontite -> wairakite. Becausemany geologic, geochemical, and hydrologic conditions maycontrol both the PH 0 / Ptotal ratio and the metamorphicgradient, different dep2th zonation patterns of Ca zeolites mayoccur even within a single geothermal system or burialmetamorphic sequence. Such variations have been recorded inthe Onikobe geothermal area by Liou et al. (1985b).

The system Ab - An • Qz • H2O

P-T stability relations shown in Fig. 1 are for the end-membercompositions in the system CaO - A12O3- SiO2 - H2O for Cazeolites and Na2O - A12O"3 - SiO2 - H2O for analcime.However, most natural zeolites, as discussed in previoussections, vary in their Si/Al ratio and show significant

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P = 1000 bars Wr-Am complete solid solution P = 1000 bars

297

Wr-Am limited solid solution

100

Ca

Fig. 2 Isobaric T - Xca relations showing compositional variations Fig. 3 Isobaric T - XQ, relations similar to Fig. 2 except for theof zeolites for the system Ab - An - Qz - H2O at 1000 bars Pfluid assumption that analcime - wairakite solid solution is limited,under the assumptions that (1) quartz and H2O are present in excess,and (2) analcime - wairakite solid solution is complete after Seki &Oki (1969). One-phase fields are shown as stippled areas, and co-existence of two phases are indicated. Possible peristerite gap orcompositional loop (as thin dotted line) for Na-rich feldspar isshown.

P = 500 bars Wr-Am limited solid solution o P = 300 barsT ° C J Wr-Am limited solid solution

Fig. 4 Isobaric T - XCa relations of zeolite similar to Fig. 3 at Pfluid Fig. 5 Isobaric T - Xca relations of zeolite similar to Fig. 3 at PflUid= 500 bars. =300 bars.

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substitution of Ca by Na (or K or Ba or Sr). Therefore, theeffect of minor components should be considered when Fig. 1is applied for zeolite paragenesis. Figures 2-5 show T-XQ,relations involving various Ca zeolites (laumontite,yugawaralite, stilbite, heulandite, and wairakite), analcime,and plagioclase in the presence of quartz and H2O in the Ab -An - Qz - H2O system at P = 1000,500, and 300 bars. Stabilityfields for a single phase are shaded and those for two-phaseregions (+ Qz + H2O) are labelled.

The phase diagrams shown in Fig. 2-5 are schematicallydrawn. They were constructed according to Frost (1980) forthe reaction loop for analcime-wairakite solid solution, themaximum substitution of Na for Ca in Ca zeolites, andcompositional and paragenetic relations described in theprevious sections. Thermodynamic data for most zeolites ofthe end-member compositions are not available (e.g., deCapitani & Liou in review), and evaluation of mixing proper-ties for zeolites with intermediate compositions has not beenattempted.

The univariant reactions for end-member zeolites shown inFig. 1 become divariant continuous equilibria if an additionalcomponent is introduced. Each reaction in the pseudobinaryT-Xca projection of the Ab - An - Qz - H2O system will beshown as a compositional loop. Unfortunately, knowledge forcompositions of co-existing zeolites as a function of mineralassemblages and temperature is very limited, hence, thecompositional loops in Fig. 2-5 are only qualitative.Maximum substitutions of Na for various Ca zeolites are fromanalysed compositions from the literature. For example,extensive solid solution has been suggested for analcime-wairakite (Seki & Oki 1969; Seki 1971, 1973; Surdam 1973)and for heulandite-clinoptilolite (Boles 1972; Gottardi & Galli1985). Limited Na substitution occurs in laumontite, stilbite,and yugawaralite (e.g., Gottardi & Galli 1985).

To illustrate the phase relations shown on the pseudo-binary diagram of Fig. 2, a complete analcime-wairakite solidsolution is first considered. Equilibria Wr = An + Qz + 2H2Oand Am + Qz = Ab + H2O relate the stability of plagioclase tothat of analcime-wairakite solid solution and forms a reactionloop as suggested by Frost (1980). For sodic plagioclase, theco-existence of albite and oligoclase is shown as the peristeritegap (e.g., Maruyama et al. 1982) or compositional loop (e.g.,Smith 1983) in Fig. 2-5. Thus, depending on temperature, Cazeolite may co-exist with albite or plagioclase of differentcomposition.

Figure 2 shows the phase relation at 1000 bars where acomplete solid-solution between analcime and wairakite isassumed. A wide compositional field for heulandite-clino-ptilolite is also assumed at temperatures below 200°C, anddiagenetic clinoptilolite tends to contain abundant Na and K.This assumption needs to be verified as heulandite and stilbiteco-existing with analcime in low-T metavolcanics tend to bedepleted in Na. Furthermore, the existence of a wide T-X fieldfor heulandite with intermediate composition excludes thepossible occurrence of stilbite + analcime (+ Qz) in naturalparageneses. T - XQJ loops for two co-existing zeolites (Hu +Am, Hu + St, Hu + Lm and Lm + Wr) are schematicallyshown. In a progressive sequence with increasing temperature,laumontite may decrease its Ca content when it co-exists withheulandite, but increases its Ca content when laumontite co-exists with wairakite at higher temperatures. Hence, thecompositional trend for laumontite can be complex, and it ishighly dependent on temperature, effective composition, andmineral assemblage. Figure 2 shows thatcalcian analcime (or

wairakite) is stable with plagioclase of a wide compositionalrange, whereas Na-rich wairakite co-exists with albite at lowertemperatures. Intergrowth of calcian analcime-bytownite inbasalt has been recently reported by Livingstone (1989), whosuggested that Na-Ca analcime-plagioclase assemblages mayform over the entire analcime-wairakite range with corres-ponding equilibrium Ab-An compositions.

Figure 2 does not have a stability field for the assemblagelaumontite + albite (+ Qz), which has been commonlyrecorded in natural parageneses (e.g., Coombs et al. 1959;Ishizuka 1985; Cho et al. 1986). This in part may be due toother minor components which were not considered, and inpart may be due to the assumption of complete wairakite-analcime solid solution in the presence of excess quartz. Forquartz-rich rocks, such as andesitic tuffs of the Onikobesystem, limited solid solution of wairakite was found (Liou etal. 1985b). The phase relations ofFig. 2 for 1000 bars may alsoapply to zeolite parageneses at pressures of 2 kbar.

Figure 3 is constructed for the phase relations involving acompositional gap between analcime and wairakite, and thesame phases described above. In addition to the limited solidsolution between wairakite and analcime, a wide stability fieldfor the laumontite + albite (+ Qz) assemblage is shown. Bothlaumontite and albite of this assemblage may increase their Cacontent with increasing temperature. However, stable co-existence of laumontite and wairakite is rather restrictedcompared to that in Fig. 2. At pressure of 1000 bars,laumontite may not be stable with analcime, as no stabilityfield of laumontite and analcime (+ Qz) is shown in Fig. 2 and3. Moreover, for progressive metamorphism with increasingtemperature, wairakite co-exists either with laumontite oralbite, depending on the Na/Ca ratio of the host rocks. Attemperatures higher than about 270°C, the peristerite gap mayappear, and wairakite may be stable with plagioclase ofintermediate to calcic compositions in prehnite-actinolite orgreenschist facies rocks. At temperatures below 200°C albitemay co-exist with heulandite, even with analcime in dia-genetic environments.

Phase relations at temperatures above 170°C at 500 and300 bars shown in Fig. 4 and 5, respectively, are similar tothose in Fig. 3, except that all assemblages and reactions occurat lower temperatures. Moreover, laumontite may be stablewith analcime at a narrow temperature range. In progressiveparagenesis, laumontite co-exists with stilbite, heulandite,analcime, albite, and wairakite with increasing temperature.Although heulandite is not stable for the Ca end-membercomposition, as described by Cho et al. (1987), the addition ofNa to the system stabilises heulandite-clinoptilolite solidsolution and produces the assemblages Hu + Am, Hu + Lm,and Hu + St (all + Qz). The transition from stilbite tolaumontite may occur only in highly Ca rich compositions.When the amount of Na increases, stilbite is no longer stable,and heulandite (± analcime) may be the most abundant zeoliteassemblage in some incipiently metamorphosed rocks.Similarly, Fig. 5 shows relations with additional yugawaralite.Its stability field is confined to Ca-rich environments.Yugawaralite occurs in some geothermal fields and may co-exist with laumontite, analcime, albite, or wairakite.

DISCUSSION AND CONCLUSIONS

Schematic phase relations shown in Fig. 2-5 show progressivechanges in both mineral assemblages and compositions of co-existing zeolites with increasing temperatures. They were

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constructed according to the experimentally determined phaserelations in the presence of excess quartz and PH o = Pfluid.We have recently made modification of zeolite2stabilitiesaccording to Berman's databases (Berman 1988). Thecalculated results indicate that heulandite is the stable Cazeolite, which possesses a wedge-like stability field at lowerpressures, as indicated by Liou (1983). For the Ca end-member, heulandite is not stable at pressures higher than2 kbar. At lower pressures stilbite is successively replaced byheulandite, yugawaralite, laumontite, and wairakite withincreasing temperatures. The new data support the phase rela-tions shown in Fig. 3, but would modify those in Fig. 4 and 5.

The effect of SiC»2 on the phase relations described in theprevious sections should also be emphasised. In silica-deficient rocks, analcime is stable at much higher temper-atures (e.g., Liou 1971c), and may contain a substantialamount of wairakite. Calcian analcime is intergrown withbytownitic plagioclase (e.g., Livingstone 1989). Low-Si-bearing Ca zeolites, such as thomsonite, natrolite, andchabazite may occur, instead of heulandite or stilbite. Hence,the lower temperature phase relations shown in Fig. 2-5should be significantly modified. Progressive changes ofzeolite-bearing assemblages from Ch + Am + Th or Ch + Am+ St through Lm + Am + Th and Lm + Th + Ab, Lm + Ab +Qz, to Wr+Ab + Th and Wr + Ab + Qz, are well documentedin metabasites of the Horokanai ophiolite (Ishizuka 1985).Heulandite was not found, and the presence and absence ofquartz in local domains significantly control the zeoliteassemblages. A progressive sequence (+ Qz) from Hu + Amthrough Lm + Am to Lm + Ab and lack of stilbite-bearingassemblage, however, has been described for the uppermostandesitic tuffs of the Del Puerto ophiolite sequence inCalifornia (Evarts & Schiffman 1983).

Nevertheless, several conclusions are apparent fromFig. 2-5.(1) The extent of Na substitution for various Ca zeolites varies

significantly. Both wairakite and heulandite have largecompositional ranges. Laumontite, stilbite, and yuga-waralite show very limited substitution.

(2) Wairakite and analcime may form a complete solid-solution in the SiCVdeficient environment, and the extentof solid solution series may be dependent on the Si-Alorder-disorder or superstructure for these zeolites. Thephase relations of such a system are not discussed.

(3) In the presence of quartz, wairakite and analcime show alimited solid solution, and the laumontite + albiteassemblage has a distinct stability field.

(4) Compositions of Ca zeolites vary depending on the mineralassemblage. The compositional trend of zeolite para-geneses with increasing temperature can be complex. Ifsuch compositional loops and phase relations shown inFig. 2-5 can be established, compositions of the co-existing zeolites can be used as geothermometer.

(5) The compositional range of heulandite solid solution maybe sensitive to pressure and precursor composition. Atpressures higher than about 500 bars, Ca end-memberheulandite is stable at temperature ranges between stilbiteand laumontite. However, at low pressures (e.g., below500 bars), heulandite requires substantial Na to stabilise itsstructure, and Na-bearing heulandite (± analcime,laumontite, and stilbite) assemblages are common fordiagenetic sediments and low-grade metamorphic rocks.

(6) Progressive changes in zeolite and sodic plagioclaseparageneses are sensitive to bulk-rock composition. Somezeolites such as yugawaralite, stilbite, and laumontite maybe favorable only in Ca-rich host rocks. In environmentswith high Na/Ca ratio, zeolite parageneses (+ Qz) withincreasing temperature are Am + Hu -• Hu + Ab ( or Lm +Am) -+ Lm + Ab -» Ab + Wrs.s. -> Wrs.s. + PI. In low-SiC^metabasites, the progressive zeolite sequence will be Ch(or St) + Am -> St + Am -> Lm +Ab -> Ab + Wrss, andheulandite may not occur (e.g., the Horokanai ophiolite,Ishizuka 1985).

(7) The peristerite gap exists only in higher temperatureparageneses. Most low-T Ca zeolites such as laumontite,heulandite, stilbite, and yugawaralite co-exist with eitheralbite or analcime (+ Qz).The conclusions described above support the suggestions

made by Professor Coombs a long time ago. Zeoliteparageneses in very low grade metamorphic terranes arecharacterised by domain equilibrium. Depending on theprecursor materials, different co-existing zeolite pairs mayoccur in different domains, hence the same zeolite in differentdomains of a single thin section may vary considerably in itscomposition. As long as equilibrium can be demonstrated,compositions of co-existing zeolites should be analysed. Theymay provide significant information with regard to thephysical and chemical constraints for their genesis. Phaseequilibria of Fig. 2-5 are schematic. They require com-positional data of co-existing zeolites (or + plagioclase),experimental stabilities, and structural information of theanalcime-wairakite series to improve their utilities to naturalparageneses.

ACKNOWLEDGMENTS

The idea of this paper was from an informal discussion withProfessor Coombs while the senior author was spending sabbaticalmonths in the University of Otago, supported by the NSF INT-8600403. The manuscript was critically reviewed and materiallyimproved by Jim Boles, Nick Mortimer, Shige Maruyama, MoonsupCho, and Gary Ernst. We thank the above-named institutes andindividuals for their help.

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