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Pentasil zeolites from Antarctica: from mineralogy to zeolite science and technology
Studies in Surface Science and Catalysis 135 A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (Editors) 9 2001 Elsevier Science B.V. All rights reserved.
83
Pentasil zeolites from Antarctica" from mineralogy to zeolite science and technology A. Alberti a, G. Cruciani a, E.Galli b, S. Merlino c, R. Millini d, S. Quartieri e, G.Vezzalini b, S. Zanardi a aDipartimento di Scienze della Terra, Universith di Ferrara, Italy. bDipartimento di Scienze della Terra. Universith di Modena e Reggio Emilia, Italy. CDipartimento di Scienze della Terra, Universit/l di Pisa, Italy. dEniTecnologie S.p.A., S. Donato Milanese, Italy. eDipartimento di Scienze della Terra, Universit/l di Messina, Italy.
SUMMARY In the course of a systematic investigation of zeolites from Northern Victoria Land, Antarctica, a large number of zeolitic species was identified in the Jurassic Ferrar Dolerites of Mt. Adamson. Noteworthy was the presence of three new zeolites: gottardiite, the natural counterpart of the synthetic NU-87, terranovaite, and mutinaite, the analogue of ZSM-5, as well as the two very rare zeolites tschernichite, the counterpart of zeolite beta, and boggsite. The chemical and crystallographic properties of these natural materials were compared with those of their synthetic analogues. The tetragonal and monoclinic polymorphic phases, intergrown in the beta zeolite, were isolated and structurally refined in tschernichite crystals, which differ by crystal size, morphology and chemistry. The occurrence of these natural zeolites demonstrates that the chemical existence field of their synthetic counterparts is larger than that argued up to now, and that their synthesis can be obtained in the absence of an organic template.
1. INTRODUCTION It is well known that zeolites containing a high proportion of five-membered rings of tetrahedra in their framework are widely used in heterogeneous catalysis; these include synthetic ZSM-5, ZSM-11, beta, theta-1, NU-87, the synthetic analogues of natural mordenite and ferrierite and the natural zeolite heulandite. During an investigation of zeolites from Northern Victoria Land, Antarctica, numerous zeolitic species, among which the pentasils are predominant, were found in the Jurassic Ferrar Dolerites of Mt. Adamson: heulandite, stellerite, stilbite, mordenite, erionite, levyne,
84 cowlesite, phillipsite, chabazite, epistilbite, ferrierite, analcime and, particularly interesting, the rare zeolites boggsite and tschernichite, the natural counterpart of zeolite beta [1]. Noteworthy is the presence of three new pentasil zeolites: gottardiite [2], the natural counterpart of NU-87, terranovaite [3] and mutinaite [4], the natural counterpart of ZSM-5. Very striking is the occurrence at Mt. Adamson of so many natural analogues of important synthetic zeolites and of two minerals (boggsite and terranovaite) still lacking their synthetic counterpart. The aim of this contribution is to highlight the impact that the study of mineral zeolites can have on zeolite knowledge. In particular: a) natural zeolites usually occur in crystals which are large enough for single-crystal X-ray diffraction studies; these investigations allow structural information to be obtained which is far more detailed and accurate than that gathered by powder diffraction data or other experimental techniques; b) the counterparts of synthetic zeolites found up to now in nature usually have significantly different Si/A1 ratios and extraframework contents. This shows that the chemical existence field of these topologies is wider than that up to now deduced from the compositions of the synthesised phases; it should also be borne in mind the extent to which chemical characteristics can influence the technological properties of the materials, and how much detailed structural information is essential for their comprehension. Below we describe the crystal-chemistry of the new and rarest zeolites from Mt. Adamson, with a particular emphasis on the most recent results conceming the structural features of the tschernichite-type mineral and a comparison with its synthetic analogue beta.
2. GOTTARDIITE The first of the new natural zeolites found at Mt. Adamson was gottardiite [2]. The crystals occur as thin lamellae, pseudo-hexagonal in shape or elongated along the a axis. The crystals, transparent and colorless, rarely occur in isolation; more frequently they form aggregates of a few individual crystals. The chemistry of gottardiite (unit cell content: Na2.sK0.zMg3.1Ca4.9Al18.8Sil17.zOzvz'93H20) is characterized by a high magnesium content and a very high Si/A1 ratio (6.2) compared with other natural zeolites. Gottardiite shows a high thermal stability and very high re-hydration capacity; the mineral quickly and completely regains its weight loss at temperatures of up to 800~ whereas at 1100~ its rehydration capacity becomes zero, probably due to the framework destruction occurring in this temperature range [2]. Its fast and complete rehydration suggests that no T-O-T bridge breaking occurs during dehydration [5]. The mineral is orthorhombic (a=13.698(2), b=25.213(3), c-22.660(2) A), with topological symmetry Fmmm and real symmetry Cmca [6]. In the Fmmm symmetry there are five non-symmetry-related sites on inversion centers. In this topological symmetry two oxygen atoms lie on two of these 1, causing energetically unfavourable T-O-T angles of 180 ~ In the Cmca sp.gr, these two inversion centers disappear, while the other three remain. This situation is common to all zeolites where, in the topological symmetry, framework oxygens lie on centers of symmetry [7].
85 The topology of gottardiite, which has not been found in other natural zeolites, is the same as that of synthetic zeolite NU-87 [8]. This is more evident if we describe the NU-87 unit cell not on the basis of the conventional monoclinic unit cell P21/c (a = 14.324 A, b = 22.376 A, c= 25.092 A, 13=151.52~ ), but in the pseudo-orthorhombic unit cell C l l 2 / / b (a = 13.663 A, b = 25.092 A, c = 22.376 A, ~/=90.37~ ). The framework of gottardiite can be described by the interconnection of the polyhedral subunits 5262, 5462 and 54. 5262 and 5462 units have also been found in other zeolites, whereas the unit 54 has been found here for the first time in zeolites. By interconnecting the 5462 and 54 units, a chain is generated which develops along the b axis. These chains are connected to form an impermeable sheet parallel to the ab plane. Each sheet is bonded to other parallel sheets through 4-rings of tetrahedra. The crystal structure is characterized by a two-dimensional channel system. Straight 10-ring channels run parallel to a, whereas 12-ring channels develop along b. These 12-ring channels are interrupted every 25 A (the value of parameter b) by the 4-ring between the sheets, and are connected to the 10-ring parallel to the a axis by a 10-ring window. Therefore these 12-ring channels are not straight, but "snake" in the b direction. 3. TERRANOVAITE The second new natural zeolite found at Mt. Adamson was terranovaite [3]. This mineral is very rare and frequently occurs in globular masses, sometimes in tabular, transparent, bluish crystals, closely associated with heulandite, from which it is barely distinguishable. Yerranovaite [(Na4.zK0.2Mg0.2Ca3.7)tot=8.3(Al12.3Si67.7)tot=80.0O160">29H20]is rich in sodium and calcium and has quite a high Si/A1 ratio (about 5.5). Its topological symmetry, hitherto unknown in either natural or synthetic materials, is orthorhombic, space group C m c m (a = 9.747(1), b = 23.880(2), c=20.068(2)A). However, the presence of a framework oxygen on an inversion center, with an unfavorable T-O-T angle of 180 ~ and the strong anisotropy of some framework oxygen atoms, indicate that the real symmetry is probably described by the acentric sp.gr. C2cm. The framework of terranovaite (Fig. 1), characterized by a pentasil chain, can be described by the interconnection of the polyhedral subunits 4264, 4254 and 5462. The 4264 unit has been found in laumontite and boggsite; the 4254 unit has been found in brewsterite, heulandite group zeolites and in synthetic SSZ-23 and SSZ-33; the 5462 unit has been found in gottardiite, boggsite and in synthetic EU-1. The net of terranovaite projected onto the bc plane (Fig. 1) is equivalent to that of many other pentasil zeolites (ferrierite, boggsite, ZSM5, ZSM-11, theta-1), while the net projected onto the ab plane is equivalent to that of A1PO441 [9]. A two-dimensional channel system parallel to the (010) plane is present in the terranovaite framework. Straight ten-membered ring channels run along [100] and [001]; the former is about circular in section (5.5 x 5.1 A), while the latter is strongly elliptic (7.0 x 4.3 A) (Fig. 1). These channels are connected through a 10-ring window.
86
Fig. 1. Perspective projection of terranovaite framework along [ 100].
4. M U T I N A I T E The third new natural zeolite found in the Ferrar Dolerites of Mt. Adamson is mutinaite [4], the natural counterpart of synthetic ZSM-5. The mineral [(Naz.76K0.11Mg0.21 Ca3.78)(All 1.20Si84.91)O192"60H20] occurs as subspherical aggregates of tiny radiating lath-like fibers or as aggregates of transparent tiny tabular crystals, with good (001) cleavage. This zeolite, very rich in calcium, has a Si/A1 ratio equal to 7.6, the highest found in natural zeolites; however, it is far lower than that of ZSM-5, where this ratio is always greater than 12. Moreover, mutinaite is characterized by a very high thermal stability and a high rehydration capacity. The mineral quickly regains more than 95% of its weight loss at temperatures up to 900~ [4]. The single-crystal structure refinement of mutinaite [ 10] was performed on a microcrystal of 0.03x0.03x0.015mm 3, collecting the data at the beamline ID 11 of the synchrotron radiation source of the European Synchrotron Radiation Facility (ESRF) of Grenoble. The mineral resulted orthorhombic with space group Pnma (a--20.201(2), b-19.991(2), c=13.469(2)A, V=5439 A3). This symmetry is consistent with the high aluminum percentage, and with the content and distribution of the extra-framework species. The structural refinement of mutinaite revealed the absence of order in the Si,AI distribution in the framework; this result is consistent with the conclusions of Toby et al. [ 11 ], who report the absence of highly occupied Br6nsted sites in the high-alumina ZSM-5. When mutinaite is compared with synthetic ZSM-5 phases (with Pnma symmetry) loaded with different molecules, we observe that the mean T-O-T angle is similar: 154 ~ in mutinaite, 155 ~ in TPA-ZSM-5 [12], and 154 ~ in PDCB (p-diclorobenzene-ZSM-5), PNAN (p-nitroaniline-ZSM-5) and NAPH (naphthalene-ZSM-5) [13]. On the contrary, many of the
87 single T-O-T angles of mutinaite strongly differ from the corresponding angles in the synthetic phases (by up to 19~ for T1-O1-T2 of mutinaite with respect to NAPH). These differences mainly affect the shape of the straight ring channel: in mutinaite it is strongly elliptical and, above all, the directions of minimum and maximum elongation are interchanged with respect to those of the synthetic phases.
5. BOGGSITE Boggsite was first described by Howard et al. [14]. This pentasil zeolite occurs in close association with tschernichite in Eocene basalts near Goble, Columbia County (Oregon), and was found for the second time at Mt. Adamson. Boggsite topology [ 15] was hitherto unknown in either natural or synthetic materials. The framework (topological and real symmetry Imma, a=20.25(2), b=23.82(1), c=12.78(1)A) can be described by the interconnection of the polyhedral subunits 4254, 4264, 5462 (found also in terranovaite), 5262 (present with 5462 in gottardiite) and 4262. A straight 12-membered ring channel runs along [100], and a straight 10-ring channel develops in the [010] direction. These channels are connected by a 10-ring window [ 15]. The chemical analyses of boggsite from Goble and Mt.Adamson indicate a constant value of the Si/A1 ratio (about 4.3), which is a usual value for the already known pentasil zeolites, but rather low when compared with that of the other pentasil zeolites from Mt. Adamson. Ca is always the most abundant extraframework cation, whereas Na is rather variable and can reach a content nearly equal to that of Ca. Minor quantities of K and Mg are present.
6. T S C H E R N I C H I T E Tschernichite was structurally defined as the natural counterpart of synthetic zeolite beta [ 16]. At Mr. Adamson the mineral occurs either as large, steep tetragonal dipyramids terminating in a basal pinacoid, or as radiating hemispherical groups of small crystals. Large and small drusy crystals were also reported from Goble tschernichite [17]. Microprobe chemical analyses [1] of large and small tschernichite crystals clearly show that large crystals are richer in A1 than the small ones (Si/A1 ratios 2.66 and 3.94, respectively), as applies also to tschernichite from Goble [17]. Due to the paucity of materials it was not possible to determine the thermal behaviour of tschemichite from Mt. Adamson, but a study on tschernichite from Goble [18] showed that its ammonium form is thermally stable to a temperature as high as 900~ It is known that synthetic zeolite beta can be regarded as a close intergrowth of two distinct, but related, structures [ 19] which can be described as consisting of (001) tetragonal layer-like building units [20]. According to the OD theory, these two structures represent the two maximum degree of order (MDO) topologies. The X-ray powder diffraction pattern of the large crystals of tschernichite-type mineral from Antarctica shows significant discrepancies, mainly in the low 0 region, with respect to those of Goble tschernichite [17] and beta zeolite [19]. These discrepancies, together with the different Si/A1 ratios between large and small crystals of tschernichite,
88
Fig. 2. Projection along [110] of the monoclinic polytype of tschernichite.
suggest that a different ratio of the two polytypes may be present in the crystals of this mineral, depending on their dimensions. We have recently used single crystal X-ray diffraction to study the structure of the two different morphologies of tschemichite from Antarctica, in order to verify if they are characterized by different structural features. Intensity data were collected on a fragment of a large crystal and on a small crystal, using an automatic four-circle Nonius KappaCCD diffractometer equipped with a CCD detector (radiation MoKot). A data collection performed on a large crystal indicated a monoclinic unit cell with a=17.983(3)A, b=17.966(2)A, c=14.625(2)A, [3=114.31(1) ~ V=4306.1A 3 and sp.gr. C2/c. A similar investigation on a small single crystal indicated a tetragonal unit cell with a=12.622(1)A, c=26.674(3)A, V=4249.6A 3 and sp.gr. P4122. The structure refinements of both samples were carried out starting from the DLS atomic coordinates of Higgins et al. [21]. Extraframework sites were located using Fo and AF Fourier maps. The diffraction patterns of both tetragonal- and monoclinic-dominant crystals have in common a set of sharp reflections, with h (and k) = 3n, which are related to the superposition structure. Due to layer stacking disorder, reflections with h (and k) = 3n + 1 show continuous streaks elongated in the c* direction. A detailed structural analysis of each polytype requires a 3-dimensional analysis of the diffuse peaks and an accurate intensity measurement, which can be obtained with an area-detector based diffractometer. For the two tschernichite crystals, the real symmetry was checked with the help of synthetic precession images constructed from the collected flames.
89 Figures 2 and 3 report the projection along [110] and [ 100] of the two polytype structures. The main results of the structure refinements are the following: a) regular T-O distances and partial Si/A1 ordering in both frameworks; b) identification of two Ca sites in the monoclinic structure; c) identification of two Ca sites in comparable positions in the tetragonal structure, but with lower occupancy; d) a further cation site probably occupied by Mg in tetragonaldominant crystals; e) the presence of many other extraframework sites characterized by low electron densities and large distances from the framework oxygens.
Fig. 3. Projection along [ 100] of the tetragonal polytype of tschemichite
90 7. CONCLUSIONS The discovery at Mt. Adamson of so many new and rare high-silica pentasil zeolites, most of which being natural counterparts of synthetic phases largely used in many technological applications, is of great interest as: a) it implies that organic templates, used as directing agents, may not be essential for their synthesis; b) the finding of the natural zeolites discussed above, with a Si/A1 ratio lower than that of the corresponding synthetic phases, suggests that the range of chemical composition required for the crystallization of their structural type is greater than that believed up to now; c) gottardiite, mutinaite and the ammonium form of tschernichite from Goble are stables to temperatures as high as 900~ We can argue that also terranovaite and boggsite are characterized by a similar, very high thermal stability, d) terranovaite and boggsite are interesting additions to the pentasil family, and the synthesis of their analogues should be of great interest to all those who work in the field of microporous materials. All the above described zeolites from Mt. Adamson are characterized by the dispersion of the extraframework ions over a large number of sites; they are usually characterized by weak electronic density and large distances from the framework oxygens which prevent (with the exception of tschernichite) an unambiguous site assignment of cations and water molecules. These features, together with the crystal growth structures of tschernichite, could suggest that these minerals grew very quickly, possibly during a rapid environment cooling, and that they could be metastable at room conditions. The defining of the genetic conditions of these phases, which are potentially useful as molecular sieves and catalysts, is the aim of our future research work. In conclusion, we believe that the results of this research well demonstrate how much natural materials can contribute to the knowledge of microporous materials. To stress this point again, we remind the reader of the recent occurrence of two natural zeolites analogous to previously synthesized phases, and two others lacking their synthetic counterparts: a) gaultite [22], a framework silicate unique in nature with zinc in tetrahedral sites, chemically and structurally analogous to VPI-7; b) pahasapaite [23], a berylloposphate with the same topology as the synthetic aluminosilicate RHO; c) maricopaite [24], an interrupted framework aluminosilicate with lead as dominant extraframework cation, forming Pba(O,OH)4 clusters; and d) tsch[]rnerite [25], characterized by a super-cage with 96 tetrahedra and 50 faces and by CuZ+12(OH)24-bearing clusters. ACKNOWLEDGEMENTS Italian PNRA, CNR and MURST ("Transformations, reactions, ordering in minerals" COFIN 1999) are acknowledged for financial support. REFERENCES
[1] E. Galli, S. Quartieri, G. Vezzalini and A. Alberti, Eur. J. Mineral., 7 (1995) 1029.
91 [2] E. Oalli, S. Quartieri, G. Vezzalini and A. Alberti, Eur. J. Mineral., 8 (1996) 687. [3] E. Galli, S. Quartieri, G. Vezzalini, A. Alberti and M. Franzini, Amer. Mineral., 82 (1997a) 423. [4] E. Galli, G. Vezzalini, S. Quartieri, A. Alberti and M. Franzini, Zeolites, 19 (1997b) 318. [5] A. Alberti and G. Vezzalini, in: Proceeding of the Sixth International Zeolite Conference, D. Olson and A. Bisio (eds.),Butterworth & Co., Guildford, UK, (1984) 834. [6] A. Alberti, G.Vezzalini, E. Galli and S. Quartieri, Eur. J. Mineral., 8 (1996) 69. [7] A. Alberti, in: New developments in zeolite science and technology. Y. Murakami, A. Iijima and J.W. Ward (eds.), Proc. 7 th Int. Zeolite Conf. Kodansha, Tokio, (1986) 437. [8] M.D. Shannon, J.L. Casci, P.A. Cox and S.J. Andrews, Nature, 353 (1991) 417. [9] R.M. Kirchner and J.M. Bennett, Zeolites, 14 (1994) 523. [ 10] G. Vezzalini, S. Quartieri, E. Galli, A. Alberti, G. Cruciani and ,4,. Kvick, Zeolites, 19 (1997) 323. [11] B. Toby, S. Purnell, R. Hu, A. Peters and D.H. Olson, in: Proceeding of the 12th International Zeolite Conference. Treacy, B.K. Marcus, M.E. Bisher and J.B. Higgins (eds.), Materials Research Society, (1999), 2413. [12] H. Van Koningsveld, H. van Bekkum and J.C. Jansen, Acta Cryst., B43 (1987) 127. [13] H. Van Koningsveld, and J.H. Koegler, Microporous Materials, 9 (1997) 71. [14] D.G. Howard, R.W. Tschernich, J.V. Smith and G.L. Klein, Amer. Mineral., 75 (1990) 1200. [15] J.J. Pluth and J.V. Smith, Amer. Mineral., 75 (1990) 501. [16] J.V. Smith, J.J. Pluth, R.C. Boggs and D.G. Howard, J. Chem. Soc., Chem. Commun., (1991) 363. [17] R.C. Boggs, D.G. Howard, J.V. Smith and G.L. Klein, Amer. Mineral., 78 (1993) 822. [18] R. Szostak, K.P. Lillerud and M. St6cker, J. Catal., 148 (1994) 91. [19] J.M. Newsam, M.M.J. Treaty, W.T. Koetsier and C.B. De Gruyter, Proc. Roy. Soc. London, A420 (1988) 375. [20] B. Marler, R. B6hme and H. Gies, in: Proceeding of the 9th International Zeolite Conference, Montreal 1992, R. von Ballmoos, J.B. Higgins and M.M.J. Treacy eds, Butterworth-Heinemann, (1993) 425. [21] J.B. Higgins, R.B. LaPierre, J.L. Schlenker, A.C. Rohrman, J.D. Wood, G.T. Kerr and W.J. Rohrbaugh, Zeolites, 8 (1988) 446. [22] T.S. Ercit and J. Van Velthuizen, Canad. Mineral., 32 (1994) 855. [23] R.C. Rouse, D.R. Peacor and S. Merlino, Amer. Mineral., 74 (1989) 1195. [24] R.C. Rouse and D.R. Peacor, Amer. Mineral., 79 (1994) 175. [25] H. Effenberger, G. Giester, W. Krause and H.J. Bernhardt, Amer. Mineral., 83 (1998) 607.
83
Pentasil zeolites from Antarctica" from mineralogy to zeolite science and technology A. Alberti a, G. Cruciani a, E.Galli b, S. Merlino c, R. Millini d, S. Quartieri e, G.Vezzalini b, S. Zanardi a aDipartimento di Scienze della Terra, Universith di Ferrara, Italy. bDipartimento di Scienze della Terra. Universith di Modena e Reggio Emilia, Italy. CDipartimento di Scienze della Terra, Universit/l di Pisa, Italy. dEniTecnologie S.p.A., S. Donato Milanese, Italy. eDipartimento di Scienze della Terra, Universit/l di Messina, Italy.
SUMMARY In the course of a systematic investigation of zeolites from Northern Victoria Land, Antarctica, a large number of zeolitic species was identified in the Jurassic Ferrar Dolerites of Mt. Adamson. Noteworthy was the presence of three new zeolites: gottardiite, the natural counterpart of the synthetic NU-87, terranovaite, and mutinaite, the analogue of ZSM-5, as well as the two very rare zeolites tschernichite, the counterpart of zeolite beta, and boggsite. The chemical and crystallographic properties of these natural materials were compared with those of their synthetic analogues. The tetragonal and monoclinic polymorphic phases, intergrown in the beta zeolite, were isolated and structurally refined in tschernichite crystals, which differ by crystal size, morphology and chemistry. The occurrence of these natural zeolites demonstrates that the chemical existence field of their synthetic counterparts is larger than that argued up to now, and that their synthesis can be obtained in the absence of an organic template.
1. INTRODUCTION It is well known that zeolites containing a high proportion of five-membered rings of tetrahedra in their framework are widely used in heterogeneous catalysis; these include synthetic ZSM-5, ZSM-11, beta, theta-1, NU-87, the synthetic analogues of natural mordenite and ferrierite and the natural zeolite heulandite. During an investigation of zeolites from Northern Victoria Land, Antarctica, numerous zeolitic species, among which the pentasils are predominant, were found in the Jurassic Ferrar Dolerites of Mt. Adamson: heulandite, stellerite, stilbite, mordenite, erionite, levyne,
84 cowlesite, phillipsite, chabazite, epistilbite, ferrierite, analcime and, particularly interesting, the rare zeolites boggsite and tschernichite, the natural counterpart of zeolite beta [1]. Noteworthy is the presence of three new pentasil zeolites: gottardiite [2], the natural counterpart of NU-87, terranovaite [3] and mutinaite [4], the natural counterpart of ZSM-5. Very striking is the occurrence at Mt. Adamson of so many natural analogues of important synthetic zeolites and of two minerals (boggsite and terranovaite) still lacking their synthetic counterpart. The aim of this contribution is to highlight the impact that the study of mineral zeolites can have on zeolite knowledge. In particular: a) natural zeolites usually occur in crystals which are large enough for single-crystal X-ray diffraction studies; these investigations allow structural information to be obtained which is far more detailed and accurate than that gathered by powder diffraction data or other experimental techniques; b) the counterparts of synthetic zeolites found up to now in nature usually have significantly different Si/A1 ratios and extraframework contents. This shows that the chemical existence field of these topologies is wider than that up to now deduced from the compositions of the synthesised phases; it should also be borne in mind the extent to which chemical characteristics can influence the technological properties of the materials, and how much detailed structural information is essential for their comprehension. Below we describe the crystal-chemistry of the new and rarest zeolites from Mt. Adamson, with a particular emphasis on the most recent results conceming the structural features of the tschernichite-type mineral and a comparison with its synthetic analogue beta.
2. GOTTARDIITE The first of the new natural zeolites found at Mt. Adamson was gottardiite [2]. The crystals occur as thin lamellae, pseudo-hexagonal in shape or elongated along the a axis. The crystals, transparent and colorless, rarely occur in isolation; more frequently they form aggregates of a few individual crystals. The chemistry of gottardiite (unit cell content: Na2.sK0.zMg3.1Ca4.9Al18.8Sil17.zOzvz'93H20) is characterized by a high magnesium content and a very high Si/A1 ratio (6.2) compared with other natural zeolites. Gottardiite shows a high thermal stability and very high re-hydration capacity; the mineral quickly and completely regains its weight loss at temperatures of up to 800~ whereas at 1100~ its rehydration capacity becomes zero, probably due to the framework destruction occurring in this temperature range [2]. Its fast and complete rehydration suggests that no T-O-T bridge breaking occurs during dehydration [5]. The mineral is orthorhombic (a=13.698(2), b=25.213(3), c-22.660(2) A), with topological symmetry Fmmm and real symmetry Cmca [6]. In the Fmmm symmetry there are five non-symmetry-related sites on inversion centers. In this topological symmetry two oxygen atoms lie on two of these 1, causing energetically unfavourable T-O-T angles of 180 ~ In the Cmca sp.gr, these two inversion centers disappear, while the other three remain. This situation is common to all zeolites where, in the topological symmetry, framework oxygens lie on centers of symmetry [7].
85 The topology of gottardiite, which has not been found in other natural zeolites, is the same as that of synthetic zeolite NU-87 [8]. This is more evident if we describe the NU-87 unit cell not on the basis of the conventional monoclinic unit cell P21/c (a = 14.324 A, b = 22.376 A, c= 25.092 A, 13=151.52~ ), but in the pseudo-orthorhombic unit cell C l l 2 / / b (a = 13.663 A, b = 25.092 A, c = 22.376 A, ~/=90.37~ ). The framework of gottardiite can be described by the interconnection of the polyhedral subunits 5262, 5462 and 54. 5262 and 5462 units have also been found in other zeolites, whereas the unit 54 has been found here for the first time in zeolites. By interconnecting the 5462 and 54 units, a chain is generated which develops along the b axis. These chains are connected to form an impermeable sheet parallel to the ab plane. Each sheet is bonded to other parallel sheets through 4-rings of tetrahedra. The crystal structure is characterized by a two-dimensional channel system. Straight 10-ring channels run parallel to a, whereas 12-ring channels develop along b. These 12-ring channels are interrupted every 25 A (the value of parameter b) by the 4-ring between the sheets, and are connected to the 10-ring parallel to the a axis by a 10-ring window. Therefore these 12-ring channels are not straight, but "snake" in the b direction. 3. TERRANOVAITE The second new natural zeolite found at Mt. Adamson was terranovaite [3]. This mineral is very rare and frequently occurs in globular masses, sometimes in tabular, transparent, bluish crystals, closely associated with heulandite, from which it is barely distinguishable. Yerranovaite [(Na4.zK0.2Mg0.2Ca3.7)tot=8.3(Al12.3Si67.7)tot=80.0O160">29H20]is rich in sodium and calcium and has quite a high Si/A1 ratio (about 5.5). Its topological symmetry, hitherto unknown in either natural or synthetic materials, is orthorhombic, space group C m c m (a = 9.747(1), b = 23.880(2), c=20.068(2)A). However, the presence of a framework oxygen on an inversion center, with an unfavorable T-O-T angle of 180 ~ and the strong anisotropy of some framework oxygen atoms, indicate that the real symmetry is probably described by the acentric sp.gr. C2cm. The framework of terranovaite (Fig. 1), characterized by a pentasil chain, can be described by the interconnection of the polyhedral subunits 4264, 4254 and 5462. The 4264 unit has been found in laumontite and boggsite; the 4254 unit has been found in brewsterite, heulandite group zeolites and in synthetic SSZ-23 and SSZ-33; the 5462 unit has been found in gottardiite, boggsite and in synthetic EU-1. The net of terranovaite projected onto the bc plane (Fig. 1) is equivalent to that of many other pentasil zeolites (ferrierite, boggsite, ZSM5, ZSM-11, theta-1), while the net projected onto the ab plane is equivalent to that of A1PO441 [9]. A two-dimensional channel system parallel to the (010) plane is present in the terranovaite framework. Straight ten-membered ring channels run along [100] and [001]; the former is about circular in section (5.5 x 5.1 A), while the latter is strongly elliptic (7.0 x 4.3 A) (Fig. 1). These channels are connected through a 10-ring window.
86
Fig. 1. Perspective projection of terranovaite framework along [ 100].
4. M U T I N A I T E The third new natural zeolite found in the Ferrar Dolerites of Mt. Adamson is mutinaite [4], the natural counterpart of synthetic ZSM-5. The mineral [(Naz.76K0.11Mg0.21 Ca3.78)(All 1.20Si84.91)O192"60H20] occurs as subspherical aggregates of tiny radiating lath-like fibers or as aggregates of transparent tiny tabular crystals, with good (001) cleavage. This zeolite, very rich in calcium, has a Si/A1 ratio equal to 7.6, the highest found in natural zeolites; however, it is far lower than that of ZSM-5, where this ratio is always greater than 12. Moreover, mutinaite is characterized by a very high thermal stability and a high rehydration capacity. The mineral quickly regains more than 95% of its weight loss at temperatures up to 900~ [4]. The single-crystal structure refinement of mutinaite [ 10] was performed on a microcrystal of 0.03x0.03x0.015mm 3, collecting the data at the beamline ID 11 of the synchrotron radiation source of the European Synchrotron Radiation Facility (ESRF) of Grenoble. The mineral resulted orthorhombic with space group Pnma (a--20.201(2), b-19.991(2), c=13.469(2)A, V=5439 A3). This symmetry is consistent with the high aluminum percentage, and with the content and distribution of the extra-framework species. The structural refinement of mutinaite revealed the absence of order in the Si,AI distribution in the framework; this result is consistent with the conclusions of Toby et al. [ 11 ], who report the absence of highly occupied Br6nsted sites in the high-alumina ZSM-5. When mutinaite is compared with synthetic ZSM-5 phases (with Pnma symmetry) loaded with different molecules, we observe that the mean T-O-T angle is similar: 154 ~ in mutinaite, 155 ~ in TPA-ZSM-5 [12], and 154 ~ in PDCB (p-diclorobenzene-ZSM-5), PNAN (p-nitroaniline-ZSM-5) and NAPH (naphthalene-ZSM-5) [13]. On the contrary, many of the
87 single T-O-T angles of mutinaite strongly differ from the corresponding angles in the synthetic phases (by up to 19~ for T1-O1-T2 of mutinaite with respect to NAPH). These differences mainly affect the shape of the straight ring channel: in mutinaite it is strongly elliptical and, above all, the directions of minimum and maximum elongation are interchanged with respect to those of the synthetic phases.
5. BOGGSITE Boggsite was first described by Howard et al. [14]. This pentasil zeolite occurs in close association with tschernichite in Eocene basalts near Goble, Columbia County (Oregon), and was found for the second time at Mt. Adamson. Boggsite topology [ 15] was hitherto unknown in either natural or synthetic materials. The framework (topological and real symmetry Imma, a=20.25(2), b=23.82(1), c=12.78(1)A) can be described by the interconnection of the polyhedral subunits 4254, 4264, 5462 (found also in terranovaite), 5262 (present with 5462 in gottardiite) and 4262. A straight 12-membered ring channel runs along [100], and a straight 10-ring channel develops in the [010] direction. These channels are connected by a 10-ring window [ 15]. The chemical analyses of boggsite from Goble and Mt.Adamson indicate a constant value of the Si/A1 ratio (about 4.3), which is a usual value for the already known pentasil zeolites, but rather low when compared with that of the other pentasil zeolites from Mt. Adamson. Ca is always the most abundant extraframework cation, whereas Na is rather variable and can reach a content nearly equal to that of Ca. Minor quantities of K and Mg are present.
6. T S C H E R N I C H I T E Tschernichite was structurally defined as the natural counterpart of synthetic zeolite beta [ 16]. At Mr. Adamson the mineral occurs either as large, steep tetragonal dipyramids terminating in a basal pinacoid, or as radiating hemispherical groups of small crystals. Large and small drusy crystals were also reported from Goble tschernichite [17]. Microprobe chemical analyses [1] of large and small tschernichite crystals clearly show that large crystals are richer in A1 than the small ones (Si/A1 ratios 2.66 and 3.94, respectively), as applies also to tschernichite from Goble [17]. Due to the paucity of materials it was not possible to determine the thermal behaviour of tschemichite from Mt. Adamson, but a study on tschernichite from Goble [18] showed that its ammonium form is thermally stable to a temperature as high as 900~ It is known that synthetic zeolite beta can be regarded as a close intergrowth of two distinct, but related, structures [ 19] which can be described as consisting of (001) tetragonal layer-like building units [20]. According to the OD theory, these two structures represent the two maximum degree of order (MDO) topologies. The X-ray powder diffraction pattern of the large crystals of tschernichite-type mineral from Antarctica shows significant discrepancies, mainly in the low 0 region, with respect to those of Goble tschernichite [17] and beta zeolite [19]. These discrepancies, together with the different Si/A1 ratios between large and small crystals of tschernichite,
88
Fig. 2. Projection along [110] of the monoclinic polytype of tschernichite.
suggest that a different ratio of the two polytypes may be present in the crystals of this mineral, depending on their dimensions. We have recently used single crystal X-ray diffraction to study the structure of the two different morphologies of tschemichite from Antarctica, in order to verify if they are characterized by different structural features. Intensity data were collected on a fragment of a large crystal and on a small crystal, using an automatic four-circle Nonius KappaCCD diffractometer equipped with a CCD detector (radiation MoKot). A data collection performed on a large crystal indicated a monoclinic unit cell with a=17.983(3)A, b=17.966(2)A, c=14.625(2)A, [3=114.31(1) ~ V=4306.1A 3 and sp.gr. C2/c. A similar investigation on a small single crystal indicated a tetragonal unit cell with a=12.622(1)A, c=26.674(3)A, V=4249.6A 3 and sp.gr. P4122. The structure refinements of both samples were carried out starting from the DLS atomic coordinates of Higgins et al. [21]. Extraframework sites were located using Fo and AF Fourier maps. The diffraction patterns of both tetragonal- and monoclinic-dominant crystals have in common a set of sharp reflections, with h (and k) = 3n, which are related to the superposition structure. Due to layer stacking disorder, reflections with h (and k) = 3n + 1 show continuous streaks elongated in the c* direction. A detailed structural analysis of each polytype requires a 3-dimensional analysis of the diffuse peaks and an accurate intensity measurement, which can be obtained with an area-detector based diffractometer. For the two tschernichite crystals, the real symmetry was checked with the help of synthetic precession images constructed from the collected flames.
89 Figures 2 and 3 report the projection along [110] and [ 100] of the two polytype structures. The main results of the structure refinements are the following: a) regular T-O distances and partial Si/A1 ordering in both frameworks; b) identification of two Ca sites in the monoclinic structure; c) identification of two Ca sites in comparable positions in the tetragonal structure, but with lower occupancy; d) a further cation site probably occupied by Mg in tetragonaldominant crystals; e) the presence of many other extraframework sites characterized by low electron densities and large distances from the framework oxygens.
Fig. 3. Projection along [ 100] of the tetragonal polytype of tschemichite
90 7. CONCLUSIONS The discovery at Mt. Adamson of so many new and rare high-silica pentasil zeolites, most of which being natural counterparts of synthetic phases largely used in many technological applications, is of great interest as: a) it implies that organic templates, used as directing agents, may not be essential for their synthesis; b) the finding of the natural zeolites discussed above, with a Si/A1 ratio lower than that of the corresponding synthetic phases, suggests that the range of chemical composition required for the crystallization of their structural type is greater than that believed up to now; c) gottardiite, mutinaite and the ammonium form of tschernichite from Goble are stables to temperatures as high as 900~ We can argue that also terranovaite and boggsite are characterized by a similar, very high thermal stability, d) terranovaite and boggsite are interesting additions to the pentasil family, and the synthesis of their analogues should be of great interest to all those who work in the field of microporous materials. All the above described zeolites from Mt. Adamson are characterized by the dispersion of the extraframework ions over a large number of sites; they are usually characterized by weak electronic density and large distances from the framework oxygens which prevent (with the exception of tschernichite) an unambiguous site assignment of cations and water molecules. These features, together with the crystal growth structures of tschernichite, could suggest that these minerals grew very quickly, possibly during a rapid environment cooling, and that they could be metastable at room conditions. The defining of the genetic conditions of these phases, which are potentially useful as molecular sieves and catalysts, is the aim of our future research work. In conclusion, we believe that the results of this research well demonstrate how much natural materials can contribute to the knowledge of microporous materials. To stress this point again, we remind the reader of the recent occurrence of two natural zeolites analogous to previously synthesized phases, and two others lacking their synthetic counterparts: a) gaultite [22], a framework silicate unique in nature with zinc in tetrahedral sites, chemically and structurally analogous to VPI-7; b) pahasapaite [23], a berylloposphate with the same topology as the synthetic aluminosilicate RHO; c) maricopaite [24], an interrupted framework aluminosilicate with lead as dominant extraframework cation, forming Pba(O,OH)4 clusters; and d) tsch[]rnerite [25], characterized by a super-cage with 96 tetrahedra and 50 faces and by CuZ+12(OH)24-bearing clusters. ACKNOWLEDGEMENTS Italian PNRA, CNR and MURST ("Transformations, reactions, ordering in minerals" COFIN 1999) are acknowledged for financial support. REFERENCES
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