Per. Mineral. (2008), 77, 79-91 doi:10.2451/2008PM0006 http://go.to/permin

PERIODICO di MINERALOGIAestablished in 1930

An International Journal ofMINERALOGY, CRYSTALLOGRAPHY, GEOCHEMISTRY,ORE DEPOSITS, PETROLOGY, VOLCANOLOGYandappliedtopicsonEnvironment,ArchaeometryandCultural Heritage

l

AbstrAct.—Twoclinoptilolite-bearingignimbritesbelongingtoTertiarySardinianvolcanismandout-cropping in different localities were treated with2MNaOH,3MNaOHand2MNaOH+Al(OH)3at100,80and70°Cinordertoenhancetheircationexchangecapacity.BronzeTeflon-linedautoclavesanda reactorequippedwithmagneticstirrerwereusedforhydrothermalinteractions,withinteractiontimes rangingfrom30minutes to36hours. Inallexperimentalcycles,clinoptilolitedissolutionwasfollowedbyNa-P1zeolitecrystallization.Thenewly-formed zeolite changes from typical ball-shapedmorphology to star-shapedgrainswith interactionincreasing.The best interacting solution was 2MNaOH+Al(OH)3. The cation exchange capacities(CEC)ofthereactedproductswerehigherthanthoseoftherawmaterials(upto2.5times,i.e., from88to268meq/100g).Theuseofareactorequippedwithmagneticstirrerstronglyreduces interaction timesfor complete clinoptilolite dissolution and Na-P1crystallization.(e.g.<6hoursat80°C).

riAssunto. — Alfinediaumentarnelacapacitàdiscambiocationico(CEC),duecampionidiignimbriti

Terziariepomiceo-cineritichedellaSardegna,ilcuivetroerastatotrasformatoinclinoptilolite,sonostatefatteinteragireconsoluzioni2MNaOH,3MNaOHand2MNaOH+Al(OH)3a100,80and70°C.Perleinterazioniidrotermalisonostateutilizzateautoclavirivestite in Teflon ed un reattore con agitatoremagnetico con tempi di interazione compresi tra30 minuti e 36 ore. In tutti i cicli sperimentalialla dissoluzione della clinoptilolite è seguita lacristallizzazione di una nuova zeolite ascrivibileallaNa-P1.Lazeolitedineoformazionesipresentainizialmenteinpiccolesfereche,all’aumentaredeitempi di interazione, si trasformano in individuiconunamorfologiastellata.Lamiglioresoluzioneinteragenteerisultataesserela2MNaOH+Al(OH)3.Lacapacitàdiscambiocationiconeiprodottisinteticièaumentataanchediduevolteemezzorispettoaquelladelmaterialenaturale(da88a268meq/100g).E’importantesottolineareche,utilizzandoilreattoreconagitatoremagnetico, lacompletadissoluzionedellaclinoptiloliteecristallizzazionedellaNa-P1sirealizzainmenodiseiorea80°C.

Key Words: Ignimbrite, clinoptilolite, cation exchange capacity, hydrothermal synthesis, Na-P1 zeolite.

Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing cation exchange capacity

AlessiA de FAzio1, Pietro brotzu1, MAriA rosAriA GhiArA* 1, MAriA luisA FerciA 2, roberto lonis 2 and Antonio sAu 2

1DipartimentodiScienzedellaTerra,UniversitàdegliStudidiNapoli“FedericoII”,ViaMezzocannone,8,80134Napoli,Italy

2PROGEMISAS.p.A.AgenziaGovernativaRegionale,ViaContivecchi,09100Cagliari,Italy

Submitted, October 2007 - Accepted, January 2008

*Correspondingauthor,E-mail:mghiara@unina.it

80 A.DeFazio,P.Brotzu,M.R.Ghiara,M.L.Fercia,R.LonisandA.Sau

introduction

The poorly welded ignimbrites and relateddeposits (e.g., epiclastites and base surges)belongingtoSardinianTertiaryvolcanism(Leccaet al.,1997)underwentpostdepositionalalterationprocesses,leadingtomoreorlessseverechangesin the glassy components (e.g. pumice, shardsandash)whichareoften replacedbyamosaicof authigenic minerals (Ghiara et al., 1997,1999,2000;Langellaet al., 1998;Morbidelliet al., 1999,2001;Cerriet al., 2001,2002).InthecentraltonorthernSardinianignimbrites,essentialauthigenic minerals, in order of decreasingabundance,are:clinoptilolite,smectite,opale-CTandquartz;mordenite,analcime,erionite,calcite,glauconite, chalcedony and adularia are alsolocallyobserved.Clinoptiloliteisanaturalzeolitecurrentlyminedallovertheworldandusedinthefieldsofagriculture,industryandpollutioncontrol(Mumpton,1984;Pansini,1996;Kazantsevaet al., 1997).

TexturalevidenceindicatesthattheauthigenicclinoptiloliteoftheSardinianignimbritesgrowsonglassycomponents(ash,shard)orfillsvugsandtubulartosubsphericalvesicles(Morbidelliet al., 1999).AccordingtoMorbidelliet al., (1999)andCerriet al., (2001),alterationprocessesaredrivenbycirculatingmineralizedhydrothermalfluids.

Theamountofclinoptiloliteinthepoorlyweldedignimbritesfromcentral-northernSardiniavariesgreatly,from32-81wt.%,withmanysamplesintherange45-69wt.%(Cerriet al., 2002).Notably,inseveralignimbriticflows,thereisevidenceofsuddenvariations inclinoptilolitecontent,evenwithin a single cooling unit (Morbidelli et al.,2001).Forinstance,intheCaseOddoraiignimbriticsequence(70mthick),theclinoptilolitecontentprogressivelyincreasesfromthelower(~30wt.%)totheintermediateportions(~60wt.%),andthandecreasestoafewwt.%towardtheupperportion(Morbidelliet al., 2001).Thisisamajorproblemforminingpurposes.Inspiteofthis,somethickdepositsfromtheBonorvaandRomanaareashavehighclinoptilolitecontentsuitableforlarge-scalemining.

According to the cross exchangemethod theclinoptilolite-bearing pyroclastic flows fromcentral-northernSardiniahaveacationexchangecapacity(CEC)intherange0.35-1.20meq/gwith

manysamplesintherange0.87-1.20meq/g(Cerriet al., 2002). If compared with other depositsminedintheworld,theSardinianzeolititesfromthecentral-northernSardiniahaveonaveragealowcationexchangecapacity.

It iswell-knownthathigh-powerzeolitescanbe synthesized throughhydrothermal treatmentwith strongly alkaline solutions (e.g., 2-3 MNaOH) starting fromvariousnaturalmaterials,including clinoptilolite-bearing ignimbrites.Asanexample,threeKoreanacidictuffscontainingdifferingamountsofclinoptiloliteandmordenite,treatedwitha2MNaOHsolutionat103°Cfortimes ranging from1 to 16hours, experiencedclinoptiloliteandmordenitecompletedissolution,forming zeolite Na-P aggregates (Kang andEgashira, 1997). The CEC of the synthesizedproductswerehigherbymorethan2timesthanthoseof thestartingmaterialsand ranged from175to418,129to286and108to271meq/100grespectively.Therefore,hydrothermalprocessesabletoenhancetheexchangecapacityofnaturalzeolite-bearingrocksinnorthernSardiniamustbeconsidered.

Inthiswork,thehydrothermaltreatmentatlowtemperature(i.e.,70,80and100°C)withNaOHandNaOH+Al(OH)3solutionsoftwodifferentclinoptilolite-bearingignimbrites(samples1-2aandPON16)outcroppingnearthevillagesofBonorvaand Romana (northern Sardinia) respectivelywasinvestigated.Theaimsofthisstudyweretoidentifythenewly-formedmineralsandtodefinetheexchangecapacityof syntheticproducts, inordertohighlightthepotentialuseofSardiniannaturalclinoptilolite-bearingignimbrites.

exPeriMentAl Methods And MAteriAls

Samples

Asrawmaterialstwopoorlyweldedignimbriteblockssized25x25x25cmweresamplednearCaseOddorai(sample1-2AIGMIF.480sez.IIForestaBurgos)andRomanavillage(samplePON16-IGMIF.479sez.IIMara).Bothpyroclasticrockshaveaeutaxitic textureandaporphyritic indexvariable from5 to30wt.%.Thephenocrysticassemblageisdominatedbyplagioclase,quartz,biotite,rareK-feldspar,andminorclinopyroxeneclinopyroxenegrains. The main lithic clasts (~5wt. %) are

Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing.... 81

representedbyangularorfragmentsof“andesitic”rocks and welded ignimbrites. A mosaic ofclinoptilolitegrainsalmostreplacescuspateshardsandtheashymatrix;euhedralcrystalsalsooccurwithinvugsandtubularorsubsphericalvesicles.Lastly,smectiteonglassycomponentsandchloriteonbiotitelathsmaybeobservedasnewlyformedphases;CTopallepispheres,minornewly-formedhematitegrainsandlatefibresofmordeniteoccurlocally. Quantitative diffractometric analysescarriedouton1-2AsampleusingthecombinedRietveld-RIR(ReferenceIntensityRatio)method(for major details see Morbidelli et al., 2001andreferencestherein)indicatedaclinoptilolitecontentof53±3wt.%andloweramountsofCTopal(~4wt.%),glassycomponents(~8wt.%)andclayminerals(~2wt.%).Ahigherclinoptilolitecontent(~62±2wt.%)withminoramountsofglass(~6wt.%)andmordenite(~5wt.%)occursinthePON16sample.

Thechemicalformula,determinatedbyelectronmicroprobeanalysis,ofclinoptilolitefromsample1-2Ais:(Na0.43K2.41Ca0.90Mg0.73)(Si29.72Al16.16O72)16.66H2O.

Blocks 1-2A and PON16 were crushed andreduced toparticleswithgrain sizebetween5-0.7mmand3-0.5mm,respectively.Onlyforthe1-2Asamplethefractionpassingthrougha0.25-mmsievewasalsocollected.Since thepresentstudyispartofaresearchprojectforagriculturalapplicationsofSardinianzeolitites(PONProjectn.12701),onlytheformergrainsizes(i.e.,5-0.7and3-0.5)wereusedinhydrothermalexperimentscarriedoutina4560Parrreactor(seebelow).

Mineralogical and chemical analyses

Rock mineralogy and newly-formed phasesinthesynthesizedproductswereinvestigatedbyX-ray powder diffraction in the 5°-75° 2θ range (SEIFERTPAD4diffractometerequippedwitha pyrolitic graphite analyzer crystal) using thefollowingoperativeconditions:unfilteredCuKα radiation, 40 KV, 30 mA, step size = 0.02°,countingtime18sforeachstep,0.5°divergenceslit, 0.1mm receiving slit, 0.5° antiscatter slit.Sideloadingwasusedforallsamplesinordertominimizepreferredorientationeffects.

Newly-formed phase morphology in thesynthesizedproductswasobservedbyscanning

electron microscopy (SEM LEO EVO 50VP,OxfordInstruments,ProgemisaS.p.A.Laboratory,Cagliari).

Al concentrations in the filtered interactingsolutionswasdeterminedbyinductivelycoupledplasmaopticalemissionspectrometry(ICP-OESPerkinElmerOptima2100DV).DetectionlimitforAlwas:0.20mg/l.

CEC analyses

Bulk-rock cation exchange capacity wasdetermined in theProgemisaS.p.A.LaboratoryfollowingtherecommendationsofMinatoet al., (1997).

Synthesis

Twoexperimentalapparatuswereemployed:a)bronzeTeflon-linedautoclaves,andb)a4560Parrreactorequippedwithmagneticstirrer.Asfora)5.00gofrawmaterialwereputintotheautoclavesanddispersedin50mlofthefollowinginteractingsolutions:2MNaOH,3MNaOHand2MNaOH+Al(OH)3.TheAl(OH)3concentrationinthelattersolutionwas10wt.%.Unstirredhydrothermalexperimentsintheautoclaveswascarriedoutat80±3°Cand100±3°Cforvarioustimesupto39hours(Table1).Inthefirstsixhours,thereactedproductswereanalysedhourlyandthaneverythreehours.Asregardstheb)hydrothermalapparatus,15.00gofrawmaterialwiththecoarsergrainsizewereputintothereactoranddispersedin150mlof2MNaOH+Al(OH)3solution.Asexpectedthehydrothermalreactionsismuchfasterinthestirredexperimentswithinteractiontimesrangingfrom½to6hours(Table2).Thehydrothermalexperimentswascarriedoutat70±3°C,80±3°Cand100±3°Candthereactedproductswereanalysedat½hourandthenhourly.Afterhydrothermalinteraction,thevesselwasquenchedtoroomtemperatureandthesolidproductwasfiltered,washedthreetimeswithdeionizedwater,driedatroomtemperature,andstoredinapolyethylenebottle.

results And discussion

Theoveralldata-setofhydrothermalexperimentsperformedwithbronzeTeflon-linedautoclavesand2-3MNaOHsolutionsshowedthatclinoptilolite

82 A.DeFazio,P.Brotzu,M.R.Ghiara,M.L.Fercia,R.LonisandA.Sau

progressivelydissolvesandthatanewzeolitestartstocrystallizeathigherinteractiontimes.Themostintensepeaksofthisnewlyformedzeolitewereat3.17,7.10,4.10,2.68and5.01Å,correspondingtotheNa-P1zeolite(Breck1974),ahighcapacitycation exchanger.The interaction times for the

appearanceofNa-P1zeoliteanddisappearanceofclinoptilolitearelistedinTable3.Theyaremainlyrelatedtothetemperatureofhydrothermalsystemandtothegrainsizeofthestartingmaterial,butareirrespectiveofNaOHconcentration.

Sample Temperature Sampleweight Interactingsolution Grainsize Interactiontimes Exp.50ml number

1-2A 100±3°C 5g 2MNaOH 5-0.7mm from1hto24h 1

1-2A 80±3°C 5g 2MNaOH 5-0.7mm from1hto39h 2

1-2A 100±3°C 5g 2MNaOH <0.25mm from1hto29h 3

1-2A 80±3°C 5g 2MNaOH <0.25mm from1hto27h 4

1-2A 100±3°C 5g 3MNaOH 5–0.7mm from1hto24h 5

1-2A 80±3°C 5g 3MNaOH 5-0.7mm from1hto24h 6

1-2A 100±3°C 5g 3MNaOH <0.25mm from1hto28h 7

1-2A 80±3°C 5g 3MNaOH <0.25mm from1hto27h 8

1-2A 100±3°C 5g 2MNaOH+Al(OH)3 5-0.7mm from1hto24h 9

PON16 100±3°C 5g 2MNaOH 3-0.5mm from1hto24h 10

PON16 100±3°C 5g 3MNaOH 3-0.5mm from1hto24h 11

PON16 100±3°C 5g 2MNaOH+Al(OH)3 3-0.5mm from1hto24h 12

tAble 1Bronze Teflon-lined autoclaves�� experimental conditions�� experimental conditions

Sample Temperature Sampleweight

Interactingsolution Grainsize Interactiontimes Exp.150ml number

1-2A 100±2°C 15g 2MNaOH+Al(OH)3 5-0.7mm from½hto6h 13

1-2A 80±2°C 15g 2MNaOH+Al(OH)3 5-0.7mm from½hto6h 14

1-2A 70±2°C 15g 2MNaOH+Al(OH)3 5-0.7mm from½hto6h 15

PON16 100±2°C 15g 2MNaOH+Al(OH)3 3–0.5mm from½hto6h 16

PON16 80±2°C 15g 2MNaOH+Al(OH)3 3–0.5mm from½hto6h 17

PON16 70±2°C 15g 2MNaOH+Al(OH)3 3–0.5mm from½hto6h 18

tAble 24560 Parr reactor�� experimental conditions

Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing.... 83

As expected, early crystallization of Na-P1zeolite occurs in the raw natural material withlowergrainsize(i.e.,

84 A.DeFazio,P.Brotzu,M.R.Ghiara,M.L.Fercia,R.LonisandA.Sau

Fig.1–Experimentalrun1.Scanningelectronmicrographof:a)clinoptilolitecrystalsandopal-CTlepispheresinthestartingmaterial;b)largecavitiesintheclinoptilolitecrystalsafter5hofinteractionandNa-P1spherules;c)clinoptiloliterelictsandclustersofball-shapedNa-P1zeoliteat6hofinteraction.

Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing.... 85

above (Table 4).As expected, the appearanceof Na-P1 and disappearance of clinoptilolitein sample1-2Ashift slightly toward lowerandhighertimesofinteraction,respectively.InsamplePON16nodifferenceswereobservedwithrespecttotheexperimentalcyclecarriedoutusingNa(OH)solution.

The XRD patterns, including progressivechangesinpeakintensitiesandSEMmicrographsofthereactedproducts,areverysimilartothosedescribedaboveforexperimentalcycleswith2MNa(OH)solutions.

Hydrothermal experiments in a 4560 Parrreactor equippedwithmagnetic stirrer and2MNaOH+Al(OH)3 solutions show a sharp fall ininteractiontimesforbothNa-P1appearanceandclinoptilolitedissolution(Table5).NotethatNa-P1zeolitealsoformsatrelativelylowtemperatures(e.g., 80°C)andlowinteractiontimes(e.g.,2h).Inaddition,withincreasinginteractionat100°CanalcimestartstocrystallizeandNa-P1tendstobeconsumed.Eitherintheseexperimentalruns,quartzandfeldsparsarenotaltered.

TheXRDpatternsofclinoptiloliteshowasharpdecreaseinpeakintensityuntilthefirsttimesof

Fig.2–Experimentalrun1.Scanningelectronmicrographof:a)clusterofbladedrosettesofNa-P1zeoliteafter15hofinteraction;b)clustersofstar-shapedNa-P1zeoliteafter16hofinteraction.

86 A.DeFazio,P.Brotzu,M.R.Ghiara,M.L.Fercia,R.LonisandA.Sau

hydrothermal interaction, whereas a very goodsettlementofthoseofNa-P1zeolitealreadyoccursafter1hofinteraction,evenat80°C.

SEMobservationsonsynthesizedproductsfromexperimentalcycle13showearlyNa-P1spherules

after½hofinteraction.Notethat,inexperimentalcycles14,manyNa-P1spherulesoflessthan2µm(Fig.4a)alreadyappearafter1hofhydrothermalinteraction. Also in the later experimentalcycles,Na-P1zeoliteprogressivelychanges its

Fig.3–Alluminiumvariationsintheinteractingsolutions

Sample Grainsize Interactingsolution T Na-P1 Clinoptilolite Exp.appearance disappearance number

1-2A 5–0.7mm 2MNaOH+Al(OH)3 100±3°C 5h 18h 9PON16 3–0.5mm 2MNaOH+Al(OH)3 100±3°C 6h 10h 12

tAble 4Times of clinoptilolite disappearance and Na-P1 appearance in the Teflon-lined bronze autoclaves. Teflon-lined bronze autoclaves.

Interacting solutions�� 2M NaOH + Al(OH)3

Sample Grainsize Interactingsolution T°C Na-P1 Clinoptilolite Exp.appearance disappearance number

1-2A 5-0.7mm 2MNaOH+Al(OH)3 100±2°C ½h 3h 131-2A 5-0.7mm 2MNaOH+Al(OH)3 80±2°C 1h 4h 141-2A 5-0.7mm 2MNaOH+Al(OH)3 70±2°C 2h >6h 15PON16 3-0.5mm 2MNaOH+Al(OH)3 100±2°C ½h 2h 16PON16 3-0.5mm 2MNaOH+Al(OH)3 80±2°C 1h 3h 17PON16 3–0.5mm 2MNaOH+Al(OH)3 70±2°C 2h >6h 18

tAble 5Times of clinoptilolite disappearance and Na-P1 appearance in the 4560 Parr reactor. Interacting solutions��

2M NaOH + Al(OH)3

Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing.... 87

Fig.4–Experimentalrun13.Scanningelectronmicrographof:a)clustersofball-shapedNa-P1zeoliteat1hofinteraction;notethestronglycorrodedrelictofclinoptilolite;b)clustersofbladedrosettesofNa-P1zeoliteat2hofinteraction;c)clustersofstar-shapedNa-P1zeoliteat6hofinteraction.Analcimestarttocrystallize.

88 A.DeFazio,P.Brotzu,M.R.Ghiara,M.L.Fercia,R.LonisandA.Sau

morphology, from spherical aggregates to star-shapedNa-P1zeolitewithincreasinginteractionwithoutanychangesintheXRDpatterns(Figs.4b-c).Theappearanceofafewanalcimecrystalsathigherinteractiontimesinexperimentalcycle13isclearlyrevealedbySEMstudies(Fig.4c).

Cation Exchange Capacity (CEC)

In order to evaluate the cation exchangecapacity (CEC) of the synthesized products,severalsampleswithhighinteractiontimesandcontainingbladedorstar-shapedNa-PaggregateswereanalysedfollowingthetechniquedescribedbyMinato(1997).Asregardsproductssynthesizedinautoclaves(Table6),theCECvaluesareoftenmorethantwiceashighasthoseoftherespectivestartingmaterials. Inparticular, theCECvalue

ofreactedproductsofsamples1-2AandPON16areintherange183-217meq/100gand189-267meq/100grespectivelyafter15hofinteraction.ThehighCECvaluesofsamplePON16areduetothehigherclinoptiloliteamountsinthestartingmaterials.

OnthebasisofthetheoreticalCEC(460meq/100g)ofpurezeoliteNa–P1,themaximumcontentofzeoliteNa–P1after thealkaline treatmentofSardinian clinoptilolite-bearing ignimbrites isestimatedtobeabout60%.

NotethathigherCECvaluesareobtainedusing2MNaOH+Al(OH)3interactingsolutions.

The products synthesized in the Parr reactor(Table7)clearlyshowthattheirCECprogressivelyincreaseswithinteractiontime(Table7).Thesereactedproductsreachveryhighexchangecapacityinashorttime.

Sample Interactingsolution Times T CEC Exp.meq/100gr number

1-2AStartingmaterial 88

1-2A 2MNAOH 21h 100±3°C 183 11-2A 3MNaOH 20h 100±3°C 201 71-2A 3MNaOH 22h 100±3°C 197 71-2A 2MNAOH+Al(OH)3 18h 100±3°C 212 91-2A 2MNAOH+Al(OH)3 20h 100±3°C 217 9PON16Startingmaterial 127

PON16 2MNAOH 24h 100±3°C 189 10PON16 3MNAOH 21h 100±3°C 211 11PON16 2MNAOH+Al(OH)3 15h 100±3°C 260 12PON16 2MNAOH+Al(OH)3 18h 100±3°C 267 12

tAble 6 Exchange capacity (meq/100g) of synthesized products with the Teflon-lined bronze autoclaves.with the Teflon-lined bronze autoclaves. Teflon-lined bronze autoclaves.

Hydrothermal treatment at low temperature of Sardinian clinoptilolite-bearing ignimbrites for increasing.... 89

conclusiVe reMArKs

Zeolites belonging to the Na-P group weresynthesizedwithhydrothermaltreatmentstartingfrom various materials, including sodiumaluminosilicate gels, kaolinite and halloysite,illite-smectite,flyash,volcanicglassandnaturalzeolitites (Mondragomet al.,1990;Amrheinet al.,1996;BerkgautandSinger,1996;Baccoucheet al., 1998;Kanget al., 1998;FaghihiamandKazemian,2000;Gualtieri,2001;Mimuraet al.,2001;Morenoet al.,2001a, b; Querolet al.,1997,2001,2002;Juanet al.,2002).

Theoveralldata-setofhydrothermaltreatmentatlowtemperatures(

90 A.DeFazio,P.Brotzu,M.R.Ghiara,M.L.Fercia,R.LonisandA.Sau

lowtemperatures(~70°C);thiswasthemainresultofthepresentresearch;

•thecationexchangecapacityofthesynthesizedproductsisbyfarhigherthanthatofthestartingmaterials(upto2.5times).Itisimportanttonotethat,inthehydrothermalexperimentscarriedoutat80°Cusingareactorequippedwithmagneticstirrers, the cation exchange capacity of 1-2Asampleincreasesfrom88to268meq/100gafter6hofinteraction.

OuressentialresultsfitthoseobtainedbyKang.andEsagashira(1997)onKoreannaturalzeolite-bearingrocks.

Inconclusion,andtakingintoaccounttheaimsofthepresentwork,hydrothermaltreatmentwithalkalinesolutionsallowstoproducehigh-powerzeolites, starting from clinoptilolite-bearingignimbrites outcropping in central-northernSardiniaandoftenformingwideplateaux.Asaconsequence,thepotentialapplicationsforvariouscation-exchangeprocessesofSardinianpoorly-welded ignimbritic rocks should be carefullyreconsidered.

AcKnoWledGeMents.

Thisworkispartofaprojectentitled“Applicazionidi zeoliti naturali per lo sviluppo di tecnicheagronomicheinnovativeeperilmiglioramentodellacompatibilità ambientale” financed by the ItalianMinistry of the University and the Scientific andTechnological Research “Programma OperativoNazionale(PON)-Ricerca,SviluppoTecnologicoeAltaFormazione2000-2006”.

M.deGennaroiskindlyacknowledgedforusefuldiscussionsandreadingofthemanuscript.SpecialthankstoA.Gualtieriforhisusefulcomments.

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