American Mineralogist, Volume 83, pages 577-589, 1998

Crvstal chemistrv of the zeoliteserionite and offretite E. Plssac"ro,l a. ARTroLr,2'*,lNn A. Guar.rrnnrr

'Dipartimento di Scienzedella Terra, Universith di Modena, via S. Eufemia 19, I-41 100 Modena, Italy 'zDipartimentodi Scienzedella Terra, Universith di Milano, via Botticelli 23,1-20133 Milano, Italy

Ansrnq,cr Many known occurrencesof the erionite and offretite have been characterized by electron probe microanalysis, X-ray powder diffraction, and optical microscopy. For the first time, a substantialamount of experimentallyconsistent and homogeneouschemical and crystallographic data have been evaluatedfor these natural zeolites. Systematicanal- ysis of the data, performed by statistical multivariate analysis,leads to the following con- clusions: (1) the two zeolites have well-defined compositional fields in the chemical space describing the extraframework cation content, best illustrated in a Mg-Ca(+Na)- K(+Sr+Ba) diagram; (2) no discrimination is possible on the basis of the framework Si/Al ratio becauseof the extensive compositional overlap between the two species,however the Si-Al content in the framework tetrahedrais the major control on the unit-cell volume dimensions,particularly in erionite; (3) the crystal chemistry of the Mg cations is a major factor in controlling the crystallization of the mineral species;(4) cation compositions at the boundary of the recognized compositional fields might be due to chemical averaging of two-phaseintergrowths, although thesemixed-phase occurrences are much less common than previously thought; (5) the sign of optical elongation is not a distinctive characterof the two phases,it is related to the SilAl ratio in the framework tetrahedraof each zeohte type and cannot be used for identification purposes;(6) the mineral speciesepitax- ially overgrown on levyne in all casesis identified as erionite; in a few casesoffretite was found to be overgrown on chabazite;(7) erionite samplesepitaxially overgrown on levyne are substantiallymore Al-rich and Mg-poor than the erionite samplesassociated with other zeolites.

INrnonucrroN of the two mineral zeolites,whereas offretite occurrences Erionite and offretite are natural zeolites havine differ- are scarce.Typical occurences, morphological and opti- f915e1' and earlier crystal chemical studies are well ent topologies (tERIl- and lOFF]{opological coies fol- 9l lowing vteler anA Oison 1992). BotL zeolites are found described in the literature (Sheppard and Gude 1969; in vugi of volcanic massiverocks, and availableliterature Wise and Tschernich 1976; Gottardi and Galli 1985; descriptionsinclude: one-phaseoccuffences and epitaxial Tschernich 1992). The presentstudy stemsfrom two ma- intergiowths of the two spicies (Pongiluppi 1976; Rinaldi jor problems commonly encounteredin the characteriza- 1976; Wise and Tschernich 1916; Hentscheland Schricke tion of erionite-offretite mineral samples:(l) The identi- 1976;Betz and Hentschel 1918; Rychlf et al. 1982), ep- fication of mineral species is troublesome, due to the itaxial overgrowth of both erionite and offretite on levyne structural and crystal chemical similarities of the two ze- (Shimazu and Mizota 1972; Passagliaet al. 1974; Shep- olites; and (2) the literature descriptionsavailable to date pard et al. 1974; Wise and Tschernich 1976; England and do not provide clear discriminatory parametersfor the Ostwald 1979; Birch 1989; Kile and Modreski 1988), and definition and the distinction of the two minerals, unless epitaxial overgrowth of offretite on chabazite (Passaglia a complete structural study is performed. and Thgliavini 1994; Passagliaet al. 1996). Only erionite The first point is readily justified: In the literature it is is also found as an authigenic mineral in volcanoclastic possible to find several cases where the minerals were silicic layers and tuffs diagenetically altered in continen- misidentified by simple routine mineralogical analysis. tal (Staples and Gard 1959; Sheppard and Gude 1969; An example is the sample from Beech Creek, Oregon, Sheppard et al. 1965; Gude and Sheppard 1981; Boles which was originally identified as an offretite overgrowth and Surdam 1979; Surdam and Eugster 1976) and marine on levyne on the basis of optical elongation sign and X- (Shameshima 1978) environments.Given the wider range ray -subsequentlypowder diffraction data (Sheppardet al. 1974), but for conditions of formation, erionite is the more common was redefined as erionite on levyne on the basis of thorough X-ray and electron diffraction analysis * E-mail:[email protected] and adsorptioncapacity measurements (Berurett and Grose 0003-004x/98/0506*0577$05.00 577 578 PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE

DeAloSiaoOzz MoAloSisoOzz

DoAlrzSiznOzz MrzAhzSizqOzz Frcunr 1. Compositional diagram showing the reliable (1965); Sheppardand Gude (1969); Staplesand Gard (1959); chemical analysesof erionite (open circles) and offretite (solid Surdamand Eugster(1976), Wise and Tschernich(1976). About circles) from the literature. Sourcesof the data: Batiashvili and half of the erionites are reported to be "hydrothermal," and half Gvakhariya(1968); Belitskiy and Bukin (1968); Birch (1988, are sedimentary.M : (Na + K); D : (Ca + Mg + Sr + Ba). 1889);Boles and Surdam(1979); England and Ostwald(1979); All data normalized to 72 framework O atoms. Horizontal dis- Gude and Sheppard(1981); Haradaet aI. (1967); Kile and Mod- crimination lines based on Si/Al ratio are after Sheppard and reski (1988);Noh and Kim (1986);Passaglia er al. (1974);Pas- Gude (1969) [-.-.-], Wise and Tschernich(1976) t- - - -1, and saglia and Tagliavini (1994, 1995); Pongiluppi (1976); Rinaldi Rinaldi (1976) [...... 1. Sub-vertical line (D : M) is the discrim- (1976);Rychlj et al. (1982);Sameshima (1978); Sheppard er al. inant after Sheppardand Gude (1969)

1978). Several of the samples obtained for the present natory diagram based on the above chemical parameters study were also misidentified by the original authors:The (Fig. 1), it is evident that none among the proposed cri- close relationship between the crystal structures of the teria satisfactorily defines appropriate compositional two minerals (Bennett and Gard 1967) is the cause for fields apt to describethe literature information. Chemical such frequent misidentifications. Although both crystal analysesare consideredto be reliable if (Si + Al) - 36, structures are now reasonably well defined, at least in on the basis of 72 O atoms; and balanceenor E < l}Va, terms of major crystallographic features (Staples and where EVo (Passaglia1970) is Gard 1959; Kawahara and Curien 1969; Gard and Tait 1972, 1973; Alberti et al. 1996; Gualrieri et al. 1998), the 100 x [(Al + Fe).0-Al,n]/Al,n (1) available crystal chemical characterizations are lncom- Al* : 1gu+ K + 2 x (Ca+ Mg + Sr + Ba). (2) plete and often inconsistent.This is the causeof the sec- ond problem listed above:Based on the presentliterature, The situation is even more confused if we try to plot it is impossible to define universal discrimination criteria the available data describing the extraframework cation applicable to all reported erionite and offretite samples. content of the two zeolites, using the same literature Problems exist becauseearlier discrimination parame- sources(Fig. 2). We assumethat the lack of discrimina- ters were based on a limited number of samples.Shep- tory compositional fields for the two zeolites is due to pard and Gude (1969) proposedspecies distinction based several factors: First of all, several minerals in the liter- on the optical sign of elongation(positive in erionite, neg- ature could be simply misidentified; second, several of ative in offretite, a widely used identification parameter the analysesmay have been carried out on mixed-phase in subsequentstudies) and on chemical parameters[eri- samples. onite: Sil(Al + Fe) > 2.9 and (Na + K) > (Ca + Mg)1. The present work attempts to redefine the crystal Subsequently,Wise and Tschernich (1976) and Rinaldi chemistry of erionite and offretite minerals. We collected (1976) proposed a distinction based on the SilAl ratio, as many available samplesas possible from different lo- the former defining as erionite the mineral having SilAl calities, identified the associatedminerals, and eventually > 3.0, the latter defining as erionite the mineral having separatedpure erionite and offretite samples. For each sl\l > 2.4. sample, we performed electron microprobe analysis If all reliable chemical analysesof erionites and offre- (EMPA), thermal gravimetric analysis,X-ray powder dif- tites available in the literature are inserted in a discrimi- fraction, and optical microscopy. Statistical analysis of PASSAGLIA ET AL: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE 579

Ca (+116;

M9(c^*Nt': o

Mg K (+S1+t|3)

Frcunr 2. Compositional diagram showing the extraframework cation content of reliable chemical analysesof erionite and offretite minerals in the literature. Sourcesof the data and svmbols as in Fisure 1. the internally consistentand homogeneouschemical and rated for preliminary X-ray diffraction analysis by long crystallographic data was used to evaluate and interpret exposures(up to U h) using a Gandolfi camera.The non- possible discrimination parametersbetween the two zeo- destructive technique allowed identification of the sepa- lites. A companion study (Gualtieri et al. 1998) presents rated crystals and guaranteedintegrity of the crystals for the full structural characteization of a limited number of the subsequentEMPA. samplesto establisha relationship betweencrystal chem- Erionite and offretite have very similar X-ray diffrac- istry and crystal structure in the two closely related tion patterns,due to their similar structuresand unit-cell zeolites. parameters(Kerr et al. 1910; Bennett and Grose 1978). Indeed, careful analysis of the X-ray diffraction patterns SIupT,B PREPARATIoNAND IDENTIFICATIoN is necessaryfor proper identification of the phases.Most After preliminary screening,25 erionite and 13 offre- of the Bragg peaks of the two zeolites coincide exactly tite samples were selected,mainly on the basis of our in the diffraction pattern, apart from a few relatively ability to separatea sufficient amount of pure material for strong diffraction lines due to the unit-cell doubling in all subsequentanalyses. Localities and available literature erionite(namely the 101, 2Ol,2Il,2l3,3Il reflections). descriptionsare reported in Table 1. This is commonly consideredan appropriatetest only for Each sample was first checked by optical microscopy the presenceof erionite: That is if the lines are absent, to describe the morphological habit and mineral associ- then the sample can be consideredpure offretite; if the ation of the erionite and offretite candidate crystals. A lines are present, this is no guaranteeof the absenceof few crystals or crystal aggregateswere then hand sepa- offretite in the erionite sample. However, simulation 580 PASSAGLIA ET AL: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE

TaeLe 1. Localitiesof the erioniteand offretitesamples The only ambiguous case is the sample from Araules, studiedin the Dresentwork France, see note in Table 1. The method can be readily Locality Reference applied to various powder diffraction patterns,including those obtainedby conventionalparafocusing diffractome- erionite ,| ters if abundantmaterial is available or those obtained Bog Hill Quarry,Northern lreland Foy, pers.comm. by Dunseverick,Northernlreland ibid optical scanningof Gandolfi films (Lutterotti et al. 1996), Ballyclare,Northern lreland ibid in caseavailable material is scarce.Both techniqueswere Lady Hill Quarry,Northern lreland ibid 5 Agate Beach,Oregon, U.S A Wise and Tschernich1976 used in the present study. Simulations and preliminary 6- Westwold,British Columbia, Can- ibid semi-quantitative analysis were performed using the aoa GSAS program system (Larson and Von Dreele 1996). 7- Jindivick,Australia Birch 1987 8- Philliplsland. Australia Birch 1988 Sampleswere carefully screenedfor phase purity and 9. Philliolsland. Australia ibid then prepared for subsequent analyses. Usually 4-5 10- Merriwa,Australia Englandand Ostwald1979 grains or fiber aggregateswere enclosed 11- CairnsBay, Australia Birch 1988 in epoxy resins 12 Shourdo,Georgia Batiashviliand Gvakhariya and polished for electron microprobe analysis; 5 mg of 1968 powder were used for thermal gravimetric analysis; and 13 NizhnyayaTunguska, Russia Belitskiand Bukin 1968 14- BeechCreek, Oregon, U-S A. Sheppard el al 1974 about 20-30 mg of powder were mixed with reagent 15. MontecchioMaggiore, ltaly Giovagnoliand Boscardin1979 grade Pb(NO.) as an internal standardfor X-ray diffrac- 16 MontecchioMaggiore, ltaly Saccardo, pers. comm tion unit-cell parametermeasurement. For some samples, 17" Montresta,Nuoro, ltaly Passagliaet al 1974 18 Faedo,Vicenza, ltaly Passagliaand Tagliavini1995 especially those showing erionite-levyne intergrowths, it 19t Araules,Ht Loire,France Pongiluppi1976 was impossible to separatesubstantial quantities of pure 20 Ortenberg,Germany Hentschel 1986 material. In such cases the pure hand-separatedgrains 21" lslandof Skye,Scotland Rinaldi,pers. comm 22 CampbellGlacier. Antarctica Yezzalini et al 1994 were used for chemical analyses,and the impure material 23 Ml- Adamson.Antarctica ibid was used for the X-ray powder data collection. A multi- 24" Mt. Admason,Antarctica ibid phase 25 Durkee,Oregon, U-S.A. Sheppardand Gude 1969 Rietveld analysiswas used to extract lattice param- eters from these offretite mixed-phasesamples. 1 Germany Gabelica,pers comm 2 ContradaRe, Vicenza,ltaly Boscardin,pers. comm. ExpnnrlrnNrAt, METHODS 3 Puntadel Hattaral.Soain Gabelica,pers comm. 4 Sasbach,Germany Rinaldi1976 Chemical analysis 5 HorseshoeDam, Arizona,U.S.A Wise and Tschernich1976 6 Vinarice,Czech Republic Gabelica,pers comm. EMPA were performed on an ARL-SEMQ instrument 7 Herbstein,Germany Hentschel 1980 using wavelength dispersive mode of operation, 30 pm 8 Gedern,Germany Betz and Hentschel1978 diameter electron beam size, 15 kV accelerating 9 Mont Semiol,France Passagliaand Tagliavini1994 voltage, 10 Adamello,ltaly ibid and 20 nA probe sample current. Reference standards 11 Fittel,Soave, ltaly Passagliaet al. 1996 were microcline (K), anorthite (Ca), albite (Na, Si, Al), 12 Fita, Soave,ltaly ibid 13 MontorsoVicentino. ltalv Boscardin,pers. comm olivine (Fe), diopside (Mg), Sr-anorthite(Sr), and celsian (Ba). Nofe.'Available referenceinformation is reDorted * Erionite overgroMh on levyne Individual point analysesfor each sampleranged from t This samplewas originallydefined as oflretite(Pongiluppi 1976), pre- 6 to 21, dependingon the number of single grains avail- liminarily interpretedhere as erionite on the basis of X-ray powder diffrac- able in the epoxy mount. In elongatedcrystals or crystal tion and chemicalanalysis (which showed that the sample lies on the boundaryof the erionitefield), and finallyshown to be an offretitewith a aggregates,at least 3-4 point analyses were performed high densityof erionitefaults by transmissionelectron microscopy (Gual- along the fiber to check for chemical homogeneity. The tieri et al 1998) point analyses of each sample were highly consistent, showing a variation of major elementswithin 3% of the estimatedinstrumental errors and indicating a high degree methodsbased on the structuremodels of the two phases of chemical homogeneity within each sample. Most show that the intensity ratio of Bragg peaks containing EMPA results also proved very reliable in terms of for- only the erionite scatteringcontribution to those contain- mula stoichiometry (Si + Al : O/2) and charge balance ing both the erionite and the offretite contributions can (E = * lOVo).The small variance and the good internal be used to determine the offretite content in erionite. Fig- consistencyof the single analysessupport the assumption ure 3 shows a portion of the simulated X-ray powder that the unit formulae reported in Tables 2 and 3, and diffraction pattern for CuKct radiation in the angular resulting from the average of the single point analyses, range 18-22'20. The patterns have been normalized to are representativeof the crystal chemistry of each zeolite the intensity of the 210 reflection, to clearly show the sample. change in the 2l0l2l1 intensity ratio as a function of the The HrO content has been independentlymeasured by erionite and offretite content of the mineral mixture. thermal gravimetric analysis (TGA) on all samples for This technique,employing Rietveld analysis based on which 5 mg of material were available.When the material established structure models, is more than adequate to was insufficient, the HrO content was assumed as the discriminate among the phasespresent in the specimen. weighted average of the measured estimates on other PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE 581

'-a c c) .= c o .N 6 E o 100 N c 75 i 50 q€ L9 a)

z ?e/o)- rt

Frcunr 3, Simulation of a portion of the X-ray powder diffraction pattern producedby a mixture of erionite and offretite, as a function of the varying composition of the multi-phasemixture Intensitieswere normalized to the 210 Bragg peak. samplesof the samephase. TGA measurementswere per- photometer.The powder patterns were correctedfor fllm formed in air on a Seiko SSC/5200thermoanalyzer, using saturation and non-linear responsebefore Rietveld anal- a heatingrate of l0'C/min. ysis (Lutterotti et al. 1996). The chemical analysesreported in Tables 2 and 3 are In all instances, the erionite or offretite sample was the EMPA results,renormalized by the HrO content mea- mixed with reagent grade Pb(NO.), as an internal stan- sured (or estimated)by the TGA analysis.The tables also dard, in the proportion of about 8 wt%oof the final mix- list the observed optical sign of elongation, as observed ture. The unit-cell parameterof the lead nitrate standard with polychromatic polarized light using a gypsum retar- was externally calibrated against the NIST stan- dation slab (lst order red shift : 550 pm). dard (referencematerial 640a) to be a : 7.8581(l) A. This value was held constant in all subsequentRietveld Unit-cell parametersmeasurement refinements,allowing an appropriatecorrection for zero- If more than 15-20 mg of samplewere available,pow- point shift and sampledisplacement in each data set. The der diffraction data were collected on a Philips diffrac- Rietveld analyseswere carried out assumingthe structural tometer in flat-plate Bragg-Brentanogeometry, equipped models of erionite and offretite from Gard and Tait (1973\ with a diffracted-beam graphite crystal analyzer. Data and Alberti et al. (1996), respectively.A Chebyshevpoly- were collected using CuKct radiation in the 5-80'20 an- nomial function with 6-9 coefficients was used to model gular range,fixed stepsof 0.03 '20 and 3 s/stepmeasuring the background.Bragg peak profiles were modeled by an time. asymmetric pseudo-Voigt function with three refinable If the available material was less than a few milli- coefficients. Independent peak profile coefficients were grams, data collection was performed in Debye-Scherrer refined for each phase,including the internal standard.In geometry using a Gandolfi camera and Ni-filtered CuKct several experiments, the hand-separatedmaterial was radiation. The films were exposedfor about 24h andthen found to contain a significant amount of impurity phases, scanned by a computer-interfacedOET AS-880 micro- mainly levyne. When the impurity level was higher than 582 PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE

Teale 2. Chemicalcomposition, unit-cell parameters, and opticalsign of erionites

SampleNo. 1 2 3 4 5 sio, 54.86 50.46 50.54 48.82 56.56 50 83 49 50 48 99 49 83 Al,o3 15.21 18.96 18.50 18.30 15.75 1847 1961 1905 1939 traf\ o21 004 IT tr. 0.07 Ir. Ir. IT tr. Mgo 056 0.35 o47 0.04 115 0.14 0.32 005 041 CaO c+o 1.68 748 7.75 470 6.99 077 686 SrO 0.03 0.13 0.03 006 008 0.16 0.04 Ir. 010 BaO 0.o2 0.15 0.05 o.o7 001 006 011 008 003 N4o 0.03 5.40 0.10 0.21 0.19 020 078 o.J6 080 KrO 2.78 4.30 343 289 334 319 3 Zl J tz '19 H.o 20 84 18.53 1946 21 32 1860 19 47', 19.47- 1947- 47' Si 27 18 25 07 25 14 24 97 27 18 25.27 24.67 24.83 24 79 AI 888 1111 1085 11.03 892 1082 1152 11.37 11.37 Fe 008 001 o02 Mg 041 0.26 UJC 003 o82 010 0.24 0.04 031 Ca 290 089 399 4.25 242 390 373 o.42 365 5r 001 0.04 001 002 o02 0.05 001 003 Ba 003 001 0.01 001 o02 0.02 001 Na 003 520 010 o21 018 019 0.75 6.27 o77 K 1.76 273 2.14 2.24 177 212 203 3.37 198 HrO 34 43 30 71 3228 36 36 29 82 32.29 JZ JO 32.90 32.31 Numberof points 969 13 8 6 778 E/o +5.3 +7 1 -1.0 -o 4 +5.3 +37 +6 8 +7.5 +5 9 R 0.75 0 69 0.70 0.69 0.75 0 70 0 68 0.69 0.69 M/(M+D) 035 087 034 0.36 0.37 0.36 0 41 0.95 0.41 CalD 087 0.73 0 92 0.99 0.74 0.96 0.93 0 88 0.91 Na/M o.o2 0 66 0.04 0.09 0.09 0.08 0 27 0.65 0.28 a (a) 13.310(1) 13320(1) 13.333(1) 1333s(1) 1328e(1) 13336(1) 1s338(1) 13321(1) 13.330(1) c (4) 15070(3) 1s 186(2) 15 091(2) 15 112(2\ 15 07e(2) 15.084(3) 15 126(2) 15 194(2) 15 174(4) Y (A") 23121(5) 23334(4\ 2323.3(s) 2328.8(4) 2306.2(4) 2323.0(5) 2330.4(4) 2334.8(4) 2335 1(6) Opticalsign t-

Nofe:EMPA analyseswere renormalizedusing the watercontent measured by thermalgravimetric analysis or usinga theoreticalestimate based on the grand mean valuesof erionitesamples (-) Atomic ratiosare based on the cell o{ 72 trameworkO atoms.The balanceerror (E/"\ is definedin Equation1as 100x (Al + Fe).b- AlthyAlth,whereAlrh- Na+ K+ 2 x (Ca+ Mg + Sr+ Ba)(Passaglia1970) M: Na+ K; D: Ca + Mg + Sr +Ba

Teete 2-Continued

SampleNo. 10 11 to 14 15 16 17 18 sio, 49 29 50.44 57 75 53 82 49 00 51 96 54.99 5072 53 51 Al2o3 19.44 1953 15.12 1660 19.48 1720 tc oo 18.61 to Jo Fe.O" tr 043 004 010 IT 003 010 o02 IT Mgo 068 050 074 010 055 020 010 057 0.05 CaO 770 2.55 472 7.17 815 ozc 2.41 7.06 655 SrO 008 IT Ir. 0.03 0.07 058 026 005 o12 BaO 0.08 004 0.04 0.09 0.09 006 007 0.02 002 Naro 0.1s 0.16 006 o.14 026 314 0.29 0.31 KrO 310 391 332 270 3.04 3.06 410 3.18 2.56 H"O 1947. 16.83 1811 19.33 19.47" 20.40 19.47- 19.47" 20.52

Si 24 58 24.63 ZI JV 26 41 24 47 26.00 27 15 zc to 26.57 AI 11.42 1124 8.51 9.60 11.47 10.14 894 10.88 957 Fe 016 001 0.04 001 004 0.01 Mg 051 036 UCJ 007 o.41 015 o07 0.42 004 Ca 411 IJJ 242 377 4.36 JJC 127 348 ST 002 001 0.02 o.17 008 0.01 004 Ba o.o2 001 001 o.o2 0.02 0.01 0.01 Na 0.15 546 015 0.06 0.14 o.25 301 0.28 030 K 1.97 244 2.O2 1.69 1.94 10q 258 201 1.62 HrO 32.37 27 41 28.86 31.63 32.43 34 04 32.06 32.21 33 98 'l Numberof points 87 12 24 1 98 10 9 Eo/" o 2 -0.5 +52 +1.2 2.0 +6.1 +6.1 +2 0 +5.9 R 0.68 0.69 0 76 0.73 0.68 0.72 0.75 o 70 0.74 M(M+D) 0.31 0.82 o 42 0.31 0.30 0.37 0.80 0 35 0.35 Ca/D 0.88 0.78 o.a2 0.97 0.91 0.91 0.89 0.90 0.98 Na/M o.o7 0.69 0.07 0.03 o.o7 0.11 054 0.12 0.16 a(+) 13344(1) 13331(1) 13264(1) 13.304(1) 13 345(1) 13316(1) \s277(1) 13340(1) 1331 1(1) c (4) 1s.128(4) 15220(3) 1s067(1) 15.078(3) 15 124(3) 150ss(2) 15.124(3) 15 110(2) 15067(2) v (A1 2332.8(6) 23425(5) 22s5.7(3) 2311.0(6) 2332.6(5) 2317 8(4) 23087(4) 2328.s(4) 2311 8(2) Opticalsign PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE 583

TABLE2-Continued

19 20 21 22 zo 24 25 sio, 49.08 56 84 51.67 55.26 53.54 5031 Jd JJ '19 Al,03 37 1387 17.97 14.60 I o./o 1752 1332 Fe.O" IT 004 0.02 0.81 0.06 o02 016 Mgo 1.12 0.08 o.47 0.78 010 008 083 CaO 695 187 o-Iz 2.71 427 4.63 4.11 Sro 035 tr' Ir. Ir. 003 004 tr. BaO o21 012 0.05 0.04 006 0.03 0.68 Na"O 003 2.42 0.30 2.83 279 339 036 KrO 342 528 J.JZ 3.37 346 3.31 2.72 HrO 1947- 1947- 1947" 19.60 18.93 2067 1947-

5l 24 54 28 01 25 57 27.21 26.32 zJ +v 28 31 AI 1141 805 1048 8.48 971 1045 761 Fe 0.02 0.01 030 o02 001 006 Mg 083 006 0.35 057 o07 0.06 0.60 372 099 143 lZC 251 2.14 Sr 010 0.01 001 Ba 004 0.02 0.01 0.01 0.01 0.01 013 Na 003 231 0.29 270 200 333 0.34 K 218 2.10 2.17 214 168 HrO 32 47 31 99 32 14 32.1I 31.04 34 93 31 50 Numberof points 8 10 7 I I I 8 '1 E/" 7 +38 t z-a -4.1 +2 1 -1 8 -0.9 R 0.68 078 o.71 0.76 073 071 079 M(M+D) o32 0.84 0.38 0.71 o67 068 041 Ca/D 079 092 0.91 o.71 096 097 0.75 Na/M 001 0.41 0.12 0.56 055 061 017 a(+) no. 13227(11 13.338(1 ) n.d 13.290(1) 13312(1) 13.233(1 ) c (4) no. 15 075(3) 15.1 00(2) n.d 15 132(2) 15.162(2) 15.055(4) v (A") nd. 2284.O(41 2326 2(4) n.d 23147(3\ 2326 8(4\ 2283 1(6) Opticalsign + +

TeaLe3. Chemicalcomposition, unit-cell parameters, and opticalsign of offretites

sio, 51 42 CU.OJ 52.96 51.30 c4.cv cJ ou 50 62 51.32 51.92 Al,o3 19.54 18.57 1723 18.56 16.20 17 00 1925 18.49 1809 Fe,O" 1r 0.40 006 Ir. o 02 0.05 003 002 o02 Mgo J IJ 278 1.97 2.89 205 232 J Zl 2.69 2.88 CaO 3.80 422 260 3.49 3 56 3.90 401 417 367 Sro 016 0.10 IT 0.38 002 tr 0.03 IT 008 BaO 005 o.77 0.04 0.12 0 11 0.09 0.08 002 007 Naro 004 tr 1.52 0.08 o24 0 07 o02 0.04 Ir. K"O 298 268 o. /o 3.34 3.36 312 291 341 279 HrO 18.88 19.85- 1985. 19.85- 19.85. 19.85- 1985. 19.85. 20 48

Si 1249 12.55 13.05 tzoo I J.JO 1312 12.46 12.65 1281 AI 560 5.43 5.01 5.40 4.67 4.91 cco 537 5.26 Fe 0.08 0.01 0.01 0.01 Mg 1 13 103 0.73 1.06 0.75 0.85 1 18 0.99 1.06 099 1.12 0.69 0.92 0.93 1.02 106 1.10 o97 Sr o02 0.01 0.05 001 Ba 0.01 0.08 0.01 0.01 0.01 0.01 001 Na o02 0.73 004 0.11 0.03 0.01 o.o2 K 092 u.oc 1.18 105 | .uc 0.97 0.91 1.07 088 H"o IC JU 16.41 16.32 16.33 16.20 16.21 16.29 16.32 1685 Numberof points 5b 10 13 11 12 10 11 7 Ev" +6 I +3.5 +5.8 +4 1 +2.7 +3.2 +3.3 +2.1 +5.6 R 0 69 0.70 0.72 0 70 0.74 0 73 0.69 0.70 0.71 M/(M+D) 0 30 0.2a 0.57 0.35 0.41 0.35 o.29 0.34 0.30 CalD 0 46 0.50 0.49 0.45 0.55 0.54 0.47 0.53 0.47 Na/M 0 02 0.00 0.38 0.04 0.09 0.03 0.01 0.02 0.00 a(+) 13 315(2) 13.28e(3) 13.267(2) 13 294(4) 13277(3) 13278(2) r3 31e(3) 13.300(3) 13.2s3(2) c (4) 7.5s8(2) 7 s88(2) 7 583(2) 7.571(3) 7 5e2(2) 7.587(2) 7.604(2) 7594(2) 7608(3) v (A1 1166 7(4) 1160.6(6) 11 s6.1 (4) 11 58 9(9) 115e.0(5) 1158.4(5) 1168.3(6) 1163 4(s) 1164.3(8)

Opticalsign f=

ivote.Results of the EMPAanalyses have been renormalizedusing the watercontenl measured by thermalgravimetric analysis or usinga theoretical estimate based on the grand mean values of offretite samples (-) Atomic ratios are based on the cell of 36 framework O atoms. The balance error (E%)isdefinedasin Equationl M: Na + K; D: Ca + Mo + Sr + Ba, 584 PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE

Ttete 3-Continued believed to be highly consistent, due to the use of the 'I 1 '13 internal standard.A few sampleswere measuredin both the Bragg-Brentanoand Debye-Scherrergeometries, as a sio, 51.21 51.10 50.77 50.59 for systematicerrors: The refined unit-cell param- Al2o3 18.94 18.57 18 84 18.77 check Fe,O" 0.02 0.09 022 0 04 eters always agreed within 3 esd. Final results are re- Mgo 2.90 2.77 1 89 297 ported in Tables 2 and 3. CaO 4.09 4 26 5 66 4.06 Sro o.24 0 07 O21 tr BaO 009 003 0.03 0.47 Multivariate statistical analysis Naro tr. 0 01 002 006 The resulting crystallochemical and unit-cell parame- tqo 2 68 2.90 2 50 3.20 HrO 1985- 2020 1985- 1985. ters were statistically analyzed by multiple regression procedures parame- Si 1258 12 62 1254 1252 to discriminate distinctive chemical AI 5.49 c4l 5.49 547 ters for the two zeolitesand their possiblecorrelation with Fe o02 0.04 001 the unit-cell parameters.The multivariate analysis was Mg 106 102 0.70 109 108 1 13 1.50 108 carried out with the widely used Statistical Packagefor Sr 003 001 003 the Social Sciences(SPSS) and a revised version of the Ba 001 0.05 stepwise regressionprogram BMDO2R (Health Sciences Na 0.01 001 003 K 084 0.91 079 101 Computing Facility, UCLA). Besides simple chemical '16.27 HrO 16.64 1635 1639 variables,the following combined parameterswere tested Numberof poinls 927 97 for significant correlation with each other and with the E% +5.6 +3.2 +5.2 +O2 unit-cell parameters:R : Si/(Si + Al); M : Na + K; D R 0.70 0 70 0 70 0.70 M(M+D) 0.28 0 30 o 26 0.32 : Ca + Mg + Sr + Ba; Vl : M/D; V2 : M/(M + D); CaJD 0.50 0.52 0 67 0.49 V3 : CalD;V4 : Na/M; V5 : (CalD) + (Na,/M).Some Na/M 0.00 0 01 0 01 0.03 parameters a (4) 13 311(3) 13.30e(3) r3 308(5) 13.301(3) of these are reported in Tables 2 and 3. The c (4) 7 604(2) 7 5e8(2) 7 5e7(4\ 7 594(3) Araules sample was excluded from the statistical com- v (A1 11668(5) 1165.6(7) 11647(e) 11635(4) putations becauseof its ambiguousdefinition. Opticalsign Rnsur-rs AND DrscussroN Figure 4 shows the presentchemical analyseson a di- about 4-5 wt%o,as visually estimatedfrom the diffraction agram similar to the representationof the literature data powder patterns, the impurity phase was inserted as an in Figure 1. The vertical distribution of plotted data rep- additional phasein the refinement. resentsthe variation in the Si/Al ratio of the samples[i.e., Unit-cell parametersrefined by the aboveprocedure are R : Si/(Si + Al) increasesfrom 0.61 at the bottom to

DeAloSisoOzz MoAloSisoOzz

R=S7/(Si+Al)

DoAlrzSiz+Ozz MtzAhzSizaOtz Frcunn 4. Compositionaldiagram showing the mean chemicalanalyses ofthe studiederionite (opencircles : erionitesassociated with levyne; open squares: isolated erionites) and offretite (solid diamonds) samples.The sample numbers correspondto those listed in Table 1. Discrimination lines as in Figure I Optical sign of elongationis adjacentto each symbol: No mark meansnegative elongation sign, (+) means positive elongation sign, (+) means sample with crystals having positive or negative elongation sign (see text for details). PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE 585

0.83 at the topl, whereasthe horizontal distribution rep- severalreports of offretitelevyne intergrowths in the lit- resentsthe variation in D/IVI cation content (i.e., divalent erature,e.g., the samplesfrom Douglas Lake Rd. (no. 6), cations only at the left; monovalent cations only at the Beech Creek (no. l4), and all of the Australian occur- right). Erionite samplesassociated with levyne have sym- rences (nos. 7-11) reported in Table 1, have been proven bols distinct from those of isolated erionites. to be misidentified and erionite is the speciespresent. In Although the overall distribution of points in Figures this light, the other occrurencesof offretite on levyne I and 4 is broadly comparable,the compositional field of (Wise and Tschernich 1976; Ridko5il and Danek 1983; the two zeolite speciesdiffers in the two diagrams. One Kile and Modreski 1988) should be considereddubious. major difference is the vertical distribution (i.e., the Si/Al Instead offretite is found as an epitaxial overgrowth on ratio) of the samples.The literature data (Fig. l) seemto chabazite(Passaglia and Tagliavini 1994; Passagliaet al. indicate a neat division (at about R : 0.73) with all of- 1996). fretite sampleshaving R < 0.73, and all but one erionite Erionite and offretite are not distinguishableon the ba- sampleshaving R > 0.73. This limit is about the average sis of the optical elongation sign, as originally proposed of those proposedfor erionite-offretite discrimination by by Sheppardand Gude (1969), although this method has Wise and Tschernich(1976: SilAl = 3.0 or R : 0.75) become a routine tool for identification. All erionite and and Rinaldi (1976: SilAl : 2.4 or R : 0.71). The dis- offretite samplesin the lowermost part of Figure 4 essen- crimination is not supportedby the new chemical analy- tially have a negative elongation sign, independentof the ses(Fig. 4): Both the grand mean value and the observed mineral species,whereas most sampleshaving R > 0.74 range of R in erionite and offretite are similar. The sta- have a positive elongation sign. In the case of erionite, tisticalmean R valueis 0.12(3) and0.7I(2), andobserved the elongation sign is negative in sampleshaving R = 0.14, rangeis 0.68 < R < 0.79 and 0.69 < R < 0.74 in erionite positive in sampleshaving R = 0.76, and positive or neg- and offretite, respectively. ative depending on the particular crystal investigated in Two results are appa"rent:(l) in Si,Al compositional sampleshaving intermediateR values. In the case of of- spacethe crystal-chemical fields of erionite and offretite fretite, the elongation sign is negative in sampleshaving largely overlap, and there is no discrimination between R < 0.70,positive in sampleshaving R= O.72,and again the two species;(2) The observedR range is extendedto positive or negative depending on the particular crystal larger R values for offretite and to lower R values for investigatedin sampleshaving R values close to 0.71. erionite, than the data reported in the literature. Conse- Therefore,optical investigationis not conclusivefor iden- quently, compositional limits based on framework Si/Al tification purposesbecause the observed sign of elonga- ratio cannot be used to discriminate species. tion in erionite and offretite dependson the SilAl content Furthermore,Figure 4 clearly indicates that among the in the zeolite framework. The earlier observations of analyzederionite samples,differences exist in the frame- Sheppardand Gude (1969) were biased by the presence work Si/Al content between the levyne-associated(open of many Si-rich "sedimentary" erionites among the an- circles) and the levyne-free (open squares)erionites: The alyzed samples.Erionites diageneticallyformed by alter- erionite samplesepitaxially overgrown on levyne are sub- ation of volcanic glass are invariably Si- and Na-rich, and stantially more Al-rich. Becausethe averageR value of therefore commonly show positive optical sign of elon- levyne is 0.65(2) (Galli et al. 1981),it is clear thar erio- gation. Optical properties alone, when not supportedby nite can grow on levyne only by developing a tetrahedral adequateX-ray or electron diffraction characterization, framework with a SilAl ratio as close as possible to le- cannot be used for erionite-offretite discrimination. The vyne and with an R factor lower than about 0.71. Erionite purely optical method is the origin of many misdefined samplesnot associatedwith levyne (Table 1), both iso- samplesin the literature. lated (nos. I, 12, 13, 18, 20) and associatedwith other The proposed discrimination based on the D/IVI cation zeolites such as analcime (no. 16), phillipsite (no. 23), or ratio (Sheppardand Gude 1969: D = M compositional heulandite (nos. 5, 22, 25), are invariably more Si-rich. limit plotted as a dotted sub-vertical line in Figs. I and The observed difference is particularly evident in a few 4) is also not applicable.Figure 4 clearly showsthat most samples.In the specimensfrom nearby localities at Mon- of the analyzedsamples, whether erionite or offretite, plot tecchio Maggiore, the erionite associatedwith levyne (no. on the leftmost part of the diagram, that is in most sam- 15) has a substantiallylower SilAl ratio that the isolated ples the divalent cation content is significantly larger than erionite (no. 16). The same is observed in the samples the monovalent cation content. from Mt. Adamson, Antarctica (nos. 23, 24). All samples The compositional diagram based on the extraframe- but one (no. 1) from Northern Ireland are intimately as- work cation content of the studied samplesis shown in sociated with levyne. No. t has a very high SilAl ratio, Figure 5. Although the analogousdiagram showing the whereasother samplesfrom nearby localities are all very literature data presentsa rather confusedpicture (Fig. 2), Si-poor (nos.2, 3, and4). the present erionite and offretite samples plot in well- It is remarkable that no offretite-levyne intergrowth defined compositional fields. All points are located in a was observed during the present investigation. In con- limited region in terms of (K + Sr + Ba) variation, trast, erionite was commonly found as an epitaxial over- whereasthere is a large distribution (and discrimination) growth on levyne lamellae. On the basis of our results, in terms of Mg/(Ca + Na) ratio. 586 PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE

Ca (+113;

M\cu*Na1

Mg K (+$p+$s;

Frcunn 5. Compositionaldiagram showing the extraframeworkcation content of studied erionite and offretite samples.Symbols as in Fisure 4.

The narrow (K + Sr + Ba) distribution is due to the onite cage), whereastwo different cavities are presentin nearly invariant K content and negligible Sr and Ba con- offretite (the l4-hedron or gmelinite cage; and the large tents in most samples.As shown by the available struc- channelsformed by l2-membered rings of tetrahedra).A ture models, this restriction is clearly due to the fact that detailed structural study carried out to elucidatethe prob- the K atoms in both structuresare located in the cancrin- lem (Gualtieri et al. 1998) seemto indicate that the ability ite cage, and the averageK content shown by the chem- of offretite to host a large amount of octahedrally coor- ical analysis is very close to the one required for full dinated Mg cations is related to the presenceof several occupancyof the crystallographic site at the center ofthe structurally different large openingsand thereforerelated cancnmte cage. to the availability of various extraframeworkcation sites. The most significant discrimination between erionite Clear distinction of the erionite and offretite composi- and offretite is based on the Mg/(Ca + Na) cation ratio, tional fields shown in Figure 5 also indicatesthat erionite- with offretites showing values close to 1.0, and erionites offretite intergrowths are very uncommon in natural sam- showing values significantly less than 0.3. It is not ples, at least in large amounts. Faulted sequences straightforward to interpret the possible crystal chemical containing domains of the two zeolites or high stacking- role of Mg on the basis of the crystal structural models. defect densitieshave been reported in the literature (Ko- However, the observed chemical discrimination clearly kotailo et al. l9l2: Bennett and Grose 1976; Rinaldi shows that the Mg/(Ca * Na) cation ratio might well be 1916) and are certainly possible. However, if most oc- a major controlling factor influencing the crystallization clurences consistedof a mixture of the two phases,it is of offretite in place of erionite, in the presenceof similar to be expectedthat many sampleswould fall betweenthe Si,Al activities in the circulating fluids. All Mg, Ca, and two describedcompositional fields. Rarity of erionite-of- Na cations, and at times excessK cations, are located in fretite intergrowths is also supportedby the fact that all the large zeolitic cages of the two structures.Only one the investigated samplespresent a high degree of chem- type of cage is present in erionite (the 23-hedron or eri- ical homogeneity. This is clearly impossible should the PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE 587

a erionite b offretite

1170

1168 x\.. I too 1164 :\s\\ o 2320 a\ o E E 1162 J \ = 6 2310 6 1160 \ .'.e .\--r-- o \ o 1158 T' o 2300 C) .\'.. 1156 It... 2290 S 1154 2280 1152

2270 1150 0.66 0.68 0.70 0.72 0.74 0 76 0 78 0 68 0.69 0.70 0.71 0.72 073 074 0.75 R=Si/(Si+Al) R=Si(Si+Al)

Frcunn 6. Correlation between the unit-cell volume and the R : Sii(Si + Al) ratio in the framework of (a) erionite and (b) offretite. The regressioncurve fit is marked with a thick line, and the dashedlines show the 957oregression confidence level. individual crystals present different amounts of the two axis (as shown by the good a vs. volume correlation),the phases. Al atoms in erionite are expectedto preferentially enter Presenceof a small quantity of intergrowths,below the the T2 framework tetrahedralsite, forming the single six- detectionlimit of the X-ray powder diffraction technique, memberedrings that connect the columns formed by the might be present in the sampleslocated at the boundary cancrinite cagesand the double hexagonalrings. The cor- of the compositional fields, i.e., in Mg-poor offretites or relation between the Al content and the unit-cell volume in Mg-rich erionites. This possibility was thoroughly (or the a unit-cell parameter) in erionite is statistically checked in several samples using transmission electron significant, and it is possible to reliably estimate the R microscopy (Gualtieri et al. 1998), and the results were ratio in the erionite framework from the measuredcrys- negative in all cases except for the anomalous erionite tallographic lattice parameters,on the basis of the regres- sample from Araules (no. 19). sron equatlon: Statistical analysis using multivariate regressiontech- R : Sil(Si + Al) : - 0.00213 x V + 5.65149 (3) niques indicates that no significant correlations exist among the chemical parameterspertaining to each zeolite, The same correlation is presentin offretite although it and the only significant correlation among the crystallo- is much less significant, leading to the speculation that graphic parametersis the expectedlarge influence of the the Al atoms should be essentially disordered on the unit-cell parametera on the unit-cell volume, both in er- framework sites in offretite, in agreementwith the con- ionite and in offretite (correlation factors 0.92 and 0.95, clusions of Alberti et al. (1996\. respectively). In comparison, the correlation factors be- tween the unit-cell parameterc and the unit-cell volume CoNcr-usroNs are below 0.8 in both zeolites,showing that the structures The resulting average crystal-chemical formulae rep- are relatively rigid along the major symmetry axis. resentativeof the two zeolites: A good correlation was also found between the com- position of the framework tetrahedra(expressed in terms erionite Ca.KrNar[Al,nsiruorr].30HrO Z: I of the Si/Al or the R ratios) and the unit-cell volume. The offretite caKMg[AlrSi,.o,u].16H,o Z:I correlation is excellent in erionite (Fig. 6a: correlation factor 0.96), and poorer in offretite (Fig. 6b: correlation are different from thosecommonly acceptedand reported, factor 0.72), in part becausethe erionite R compositional for example, in Gottardi and Galli (1985). range is much broader. Both zeolites show large variation in framework Si/Al Becausethe framework expansionproduced by the in- ratio and extraframework cation content. Erionite shows troduction of the Al atoms has almost no influence on the a wider range in Si/Al ratio than offretite, especiallywhen c axis of erionite and is linked to the eloneation of the a it is associatedwith levyne. Large cation variations in s88 PASSAGLIA ET AL.: CRYSTAL CHEMISTRY OF ERIONITE AND OFFRETITE erionite are best describedusing the CalNa ratio. The cat- -(1986) Paulingit und andereSeltene Zeolithe in einem gefritteten ion content of offretite has very limited variation, and the Sandsteineinschlussim von Ortenberg(Vogelsberg) Geologisch- es Hessen,1 14. 249-256 Cal1vlgratio is always very close to 1.0. Jahrbuch Hentschel,G and Schricke, W. (1976) Offretit von Geilshausen(Vogels- berg, Hessen) GeologischesJahrbuch Hessen, 104, 173-176. AcrNowr,nocMENTS Kawahara,A. and Curien, H (1969) La structurecristalline de l'6rionite Italian MURST and CNR are acknowledgedfor financial support WD Bulletin de la Soci6t6franEaise de Min6ralogie et de Cristallographie, Birch, M Boscardin, H Foy, Z Gabelica,and F Saccardokindly made 92.250 256 samplesavailable for the investigation G. Vezzalini and S Bigi performed Ken, IS, Gard,JA, Baner,RM, and Galabova,IM (1970)Crystal- the chemical analyseson the ARL-SEMQ electron microprobe analyzer lographic aspectsof the co-crystallization of zeolite L, offretite, and at the Universiti di Modena M Bertacchini helped with the graphicsof erionite. American Mineralogist, 55, 441-454 chemicaldiagrams D L Bish, R. Downs, and an anonymousreviewer are Kile, D.E and Modreski, PJ. (1988) Zeolites and related minerals from thanked for detailed suggestionsand improvementof the manuscript. the Table Mountain lava flows near Golden. Colorado The Mineral- ogicalRecord, 19, 153-184 Rnpnnrncns crrED Kokotailo,G T, Sawruck,S., and Lawton,S.L. (1972)Direct observation of stacking faults in the zeolite erionite American Mineralogist, 57, Alberti, A, Cruciani,G, Galli, E, and Vezzalini,G (1996)A reexami- 439-444. nation of the cryslal structureof the zeolite offretite Zeoliles,17,457- Larson, AC and Von Dreele, RB (1996) GSAS, generalizedstructure 461 analysis system Los Alamos National Laboratory, document LAUR Batiashvili, T.V and Gvakhariya, GV (1968) Erionite found for the first 86-748 time in Georgia. Doklady of the RussianAcademy of Sciences:Earth (1996) ScienceSections, 179, 122-124 Lutterotti, L., Gualtieri, A., and Aldrighetti, S Rietveld refinement using Debye-Scherrerfilm techniques Materials ScienceForum, 228- Belitskiy, I A md Bukin, G V. (1968) First find of erionite in the U S.SR ) I 1 ?q_11 Doklady of the RussianAcademy of Sciences:Earth ScienceSections, 178.103-106. Meier, WM. and Olson, D.H. 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