Cora, Ildikó

Dallos, Zsolt

Dódony, István

Kristály, Ferenc

Ormándi, Szilvia

Szilvia Ormándi

Ildikó Cora

Zsolt Dallos

Ferenc Kristály

István Dódony

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Resolution and Discovery

Structural study of mordenite from Mátra Mts. (N-Hungary): dachiardite moduls reduce channel size in mordenite

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Original ArticleStructural Study of Mordenite from Mátra Mts. (N-Hungary):Dachiardite Moduls Reduce Channel Size in MordeniteSzilvia Ormándi1*, Ildikó Cora2, Zsolt Dallos1, Ferenc Kristály3 and István Dódony11Department of Mineralogy, Eötvös Loránd University, Pázmány Péter sétány. 1/CBudapest, 1117 Hungary2Institute for Technical Physics and Materials Science, Centre for Energy Research, Hungarian Academy of Sciences,P.O. Box 49, H-1525 Budapest, Hungary3Institute of Mineralogy and Geology, University of Miskolc, Miskolc, Egyetem út, 3515 HungaryReceived: 07 June 2017; accepted: 01 September 2017* Author forFigure 1. Ta = 18.1, ba = 18.6, bof puckeredThe puckereat the lowerDOI: 10.155© 2017 TheZeolites are crystalline microporous alumosilicates whose unique pore and channel systems are the reason for theirimportant role in catalysis, separation, and ion exchange. This work focuses on the morphology and structure of anatural zeolite, mordenite. Our samples were collected at Lengyendi-galya (Gyökeres-tető) in the Mátra Mountains(NE-Hungary). Zeolite samples were investigated by X-ray powder diffraction, scanning, and transmission electronmicroscopy. Mordenite has typically fibrous appearance and occurs in association with other zeolites such as heu-landite and chabazite. Based on intense streaks and superlattice reflections in selected area electron diffraction pat-terns, we identified planar faults in the structure. A single fault produces a dachiardite-type structural slab within themordenite lattice, reducing its channel size.Keywords: Natural mordenite, structure, diffraction, faults1. IntroductionZeolites have complex alumosilicate frameworks. These frame-works contain large channels and interconnected cages. We use zeo-lites in gas and petroleum industry, water softening, sewagetreatment, agriculture, paper production, and radioactive waste treat-ment, as well as in construction materials. The structural channelsand voids are occupied by loosely bound cations and watercorrespondence: ormszi12@gmail.comhe mordenite framework projected along the c axis (Cmcm,= 20.5, c = 7.5 Å) and dachiardite along the b axis (C2/m,= 7.5, c = 10.4, β = 108.9°) [10]. The structure is composed(100) sheets (black tetrahedra at the upper half of the figure).d tetrahedral sheets of mordenite and dachiardite are shownpart of the figure6/2051.2017.00045Author(s). This article is published with Open Access at www.akademolecules that can be removed and replaced without disrupting thetetrahedral framework. This unique – so-called, zeolitic – propertymakes a zeolite structure the best candidate to perform cation ex-change, adsorption, and molecular sieving (passing a gas or liquidthrough a zeolite). Zeolites can be used in dehydration and rehydra-tion processes and can be resistant to high energy radiation [1].The mordenite group includes the minerals mordenite, dachiar-dite, epistilbite, ferrierite, and bikitaite [2, 3]. Zeolites in the mor-denite group are built by five-membered rings of tetrahedra – thisis called the secondary building unit – and puckered sheets madeof six-membered rings of tetrahedra parallel to (100), which ensurea perfect prismatic cleavage (Figure 1). Parallel to the c axis, thereare channels formed by twelve-membered rings (12MRc: aperture,6.5 × 7.0 Å) and compressed eight-membered rings (8MRc: aper-ture, 5.7 × 2.6 Å). There are another eight-membered rings, andchannels also occur parallel to b (8MRb: aperture, 3.4 × 4.8 Å)(Figure 2). In dachiardite, ten-membered rings are parallel to b(Figure 1) and there are also compressed eight-membered ringsigure 2. The channel system of the mordenite structure with the threeain tubes, bordered by 12-, 8-, and 8-membered rings of tetrahedralong c, c, and b, respectively (12MRc, 8MRc, and 8MRb). These tubesrm only one-dimensional passageways [4]. The locations of the extra-Fmafoframework A, D, and E cations are labeled in redmiai.comResolution and DiscoveryFigure 3. Stereomicroscopy images of zeolites from Gyökeres-tető. A — heulandite pseudo-hexagonal plates. B and C — heulandite and morde-nite crsytals. D — chabazite rhombohedra (the width of images in mm: A, 10.9; B, 13.6; C, 6.8; D, 6.9)Structural Study of Mordeniteparallel to the c axis [4]. Extraframework cations in mordenite oc-cupy mainly three sites. Two of these sites are close to the four-membered Al-enriched rings (light-gray shaded tetrahedra inFigure 1) and are located in the connecting channels parallel tothe b axis; the A site is in the center of the compressed eight-membered ring channels, whereas the D site is near the centerof the eight-membered ring, connecting to the wide channel.The E site is in the large channels (Figure 2).The ideal mordenite composition is (Na2,K2,Ca)4[Al8Si40O96]·28H2O; the structure is orthorhombic with space groups ofCmcm (a = 18.11, b = 20.46, c = 7.52 Å, Z = 1) [5]. The sym-metry lowering from Cmcm to Cmc21 occurs when locations ofextraframework cations are asymmetric [6].Mordenite is one of the zeolites which have very good ab-sorption and separation properties. For this reason, Barrer [7]examined this type of zeolite framework and synthesized mor-denite with a small port (“port” meaning the channels in theFigure 4. SE images of mordenite and heulandite from Gyökeres-tető. A anD — typical mordenite fibers2structure). Therefore, two different structural varieties are nowdistinguished – large-port and small-port mordenite – by the ac-cessibility of 12-membered ring channels (Figure 1). In thesmall-port mordenite, molecules fit with sizes less than 4.2 Å indiameter in the channels, and in the large-port mordenite, thechannels are accessible for molecules larger than 4.2 Å [8]. Mor-denite as a mineral typically has the properties of small-port mor-denite. The widely accepted explanation for the different portsizes is that stacking faults disrupt the large port structure, result-ing in channels of smaller diameter [9]. In the present study, weexamine a mordenite mineral specimen, in order to understand thestructural difference between the large- and small-port variants.2. ExperimentalThe locality of the measured samples is Lengyendi-galya(Gyökeres-tető), Mátra Mountains (NE-Hungary). The mordenited B — heulandite overgrows the first formed mordenite fibers. C andFigure 5. SAED patterns of mordenite. h + l odd reflections appear which are forbidden in the Cmc21 space groupFigure 6. SAED patterns obtained from mordenite. The red arrow indicates intense streaks along the lines of hkl reflections where l = 2n + 1S. Ormándi et al.bundles occur in hydrothermally highly altered andesite. Puremordenite sample was prepared from the andesite.X-ray diffraction measurements were carried out using BrukerD8 Advance diffractometer (Cu-Kα source) with Göbbel-mirror,parallel beam geometry, equipped with Vantec1 position sensitivedetector, with symmetrical 2.5° axial Soller slits. Measurementswere recorded in the 2°–100° (2Θ) angular range, with 0.007°(2Θ)/140 s goniometer speed. XRD patterns were evaluated bySearch/Match in DiffracPlus EVA using ICDD PDF2 (2005) forcrystalline components. Quantitative evaluation was performed inTOPAS4 with fundamental parameters approach, using crystalstructures from ICSD database.Morphological and textural observations were made using aWild M400 stereomicroscope and an AMRAY 1830 scanningelectron microscope (SEM). The crystal sturcture of mordenitewas investigated by selected area electron diffraction (SAED),using a Philips CM20 transmission electron microscope operatedat 200 kV. The composition was measured using NanoprobeX-ray microanalysis (EDS) with a Bruker system.Figure 7. Representation of lattice displacement by a ~1/4 [010] vectorin the mordenite framework, projected along [001]. If the turquoised partof the structure is translated by 1/4b, a dachiardite-type slab results (seeFigure 8B)3. Results and Discussion3.1. Appearance of mordenite and associated minerals.Using X-ray powder diffractometry, we measured the hydro-thermally altered andesite, composed dominantly by mor-denite and heulandite; other minerals include quartz, andesine,orthoclase, hematite, and smectite.X-ray diffraction patterns recorded on 1 mg powder sample.Rietveld measurements were carried out in TOPAS softvare in or-der to verify the powder purity. In the next step, peak search wasdone to prepare indexing data. Based on the measured reflections,we identified the Laue-group 2/m2/m2/m with C-centering. Weobserved forbidden reflections in the Cmc21 space group, so a re-duced symmetry is evident for the bulk material.Stereomicroscopy and secondary electron (SE) images re-vealed that the first product of hydrothermal alteration is morde-nite, followed by heulandite (Figures 3 and 4). Secondaryelectron images in Figure 4A and B show heulandite crystalsspeared by mordenite fibers. Apparently, mordenite formed asthe first zeolite and then heulandite grew around it.3.2. Transmission electron microscopy. Features in SAEDpatterns from the mordenite fibers suggest a reducedsymmetry compared to the Cmc21 space group and thepresence of planar faults. We observed forbidden h + l oddreflections in the Cmc21 space group. For example, in the [010]projection, 10l, 30l, and 50l reflections are present (red arrowsshow some h0l lines in Figure 5A). Another example is the3Figure 8. Changes in the mordenite structure as a result of planar faults. A) The original mordenite framework; B) mordenite modified by an ar-ray of 1/4 [010] planar defects; and C) the new dachiardite-type part of the mordenite framework, after structure relaxationStructural Study of Mordenite[1 10 ] projection in which 00l (l = 2n + 1) reflections arealso visible (Figure 5B).Intense streaks and superlattice reflections in SAED patternsof mordenite suggest the presence of planar faults. Figure 6shows [510] and [110] projected SAED patterns of mordenite, inwhich the lines of hkl reflections with l = 2n + 1 are intercon-nected by intense streaks, some of them marked with a redarrows.The composition of mordenite was measured with EDS as(Na1.25K0.42Ca1.67Mg0.21[H2O]28.9)[Al8.38Si40O96]. The cor-rect k factors were confirmed successfully on quartz and co-rundum earlier.4. DiscussionSong et al. [11] observed different defect types in mordenitesuch as dislocations, planar defects, and local intergrowths usinghigh-resolution electron microscopy (HREM) [11]. Planar faultswith a ~1/4 [010] displacement vector result in the presence ofdachiardite moduls in the structure. Based on intense streaks andsuperlattice reflections in SAED patterns, we also identified pla-nar faults. Figure 6 shows examples of the mordenite reciprocallattice in which the rows of hkl reflections with l = 2n + 1 areconnected by intense streaks. This diffuse scattering is interpretedas an evidence of faults, where a fault produces a dachiardite-type structural slab within the mordenite lattice.As shown by Figure 1, the puckered sheets are the same inmordenite and dachiardite, but the connection of these puckeredsheets by four-membered rings (light shading) differs. The pres-ence of the dachiardite domains affect the mordenite channelcapacity by disrupting the system. Figure 7 shows the morde-nite framework along [001]. With a displacement by a ~1/4[010] vector (marked with a black arrow), dachiardite cages ap-pear in the mordenite framework. This lattice displacementcauses a surplus and a deficit of bonds which will connect byflipping (Figure 8B). The modified bonds are then relaxed inorder to transform into a more stable structure (Figure 8C).45. ConclusionsSpace group determination from SAED patterns obtainedfrom mordenite fibers suggested a symmetry reduction fromCmc21 to 2/m2/m2/m Laue group by the forbidden h + l oddreflections and the evident C-centering. Possible space groupsinclude C222, Cmm2, C2mm, Cm2m, and Cmmm. This symme-try is also confirmed by powder diffraction data of mordenite.Acknowledgment. The authors gratefully thank IstvánGatter for his kind help in the fieldwork. The quality of thepaper has been significantly improved by the valuablecomments by M. Pósfai.Open Access. This is an open-access article distributedunder the terms of the Creative Commons Attribution-NonCommercial 4.0 International License (https://creativecommons.org/licenses/by-nc/4.0/), which permitsunrestricted use, distribution, and reproduction in any mediumfor non-commercial purposes, provided the original author andsource are credited, a link to the CC License is provided, andchanges - if any - are indicated.References1. Tschernich, R. W. Geosience Press Inc. USA, 1992.2. Meier, W. M. Natural Zeolites, Occurence, Properties, Use, Sand, L. B.;Mumpton, F. A. (eds) Pergamon Press, Oxford, 1978, 99–103.3. Armbruster, T.; Gunter, M. E. Natural Zeolites: Occurrence, properties,applications in Bish, D. L.; Ming, D. W. editors. Reviews in Mineralogy &Geochemistry, 2001, Vol. 45.4. Simoncic, P.; Armbruster, T. Am. Mineral. 2004, 89, 421–431.5. Meier, W. M. Z. Kristallogr. 1961, 115, 439–450.6. Alberti, A.; Davoli, P.; Vezzalini, G. Z. Kristallogr., Kristallgeom.,Kristallphys., Kristallchem. 1986, 175, 249–256.7. Barrer, R. M. J. Chem. Soc. 1948, 2158.8. Sand, L. B. Molecular Sieves Conference Papers, Meeting Date Societyof Chemical Industry, London, 1967, 71–77.9. Raatz, F.; Marcilly, C.; Freund, E. Zeolites 1985, 5, 329–333.10. http://www.iza-structure.org/.11. Song, S. G. J. Mater. Res. 1999, 14, 2616–2620.

Structural study of mordenite from Mátra Mts. (N-Hungary): dachiardite moduls reduce channel size in Mordenite

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Structural study of mordenite from Mátra Mts. (N-Hungary): dachiardite moduls reduce channel size in Mordenite

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