1,721,064 research outputs found
Elastic behavior and high pressure-induced phase transition in chabazite: new data from a sample from Nova Scotia
No full textRecently, the high pressure (HP) behavior of a natural chabazite from Vallerano [1] (VALL), and on the synthetic
phases SAPO-34 [1] and ALPO-34 [2], was investigated in the frame of a wider project aimed at understanding
the role of the framework/extraframework content on the compressibility of CHA-type porous materials.
In this work, further structural information is obtained studying the response to HP of another natural chabazite
sample from Nova Scotia (Canada) (NS) (s.g. R-3mR [3]), characterized by a different chemical composition with
respect to VALL. The study was performed by means of in situ synchrotron X-ray powder diffraction (XRPD) and
silicone oil as non-penetrating P-transmitting medium. XRPD experiments were performed in DAC at the BM01
beamline at ESRF (Grenoble, France) with a fixed =0.7355 Å. Powder patterns were collected from Pambup to 8.6
GPa and upon decompression. All the features of the Pamb pattern and the unit cell parameters are well recovered
upon P release. Below 2.1 GPa, a and cell parameters slightly decrease and increase respectively with a resulting
volume reduction of 3.6 %. Above 2.1 GPa, a transition to a triclinic P-1 pseudo-rhombohedral phase is observed.
The rhombohedral to triclinic phase transition is accompanied by an abrupt decrease in the unit cell parameters and
in the unit cell volume (V=-4.0%). Between 2.5 and 5.9 GPa, the triclinic/pseudo-rhombohedral cell parameters
decrease regularly and the unit cell volume variation (V=-3.0%) indicates a lower compressibility with respect to
that observed before the transition. In the highest P regime (5.9-7.2 GPa), a further slope change, with an increase
in compressibility, is observed. As a whole, V between Pamb and 7.2 GPa is -12.6%. The elastic parameters,
calculated with a second order BM-EoS, are V0 = 826 (1) Å3, K0 = 54(3) GPa and V0 = 784(2) Å3, K0 = 91(5)
GPa, for the rhombohedral and triclinic phase, respectively.
Preliminary results from Rietveld refinements up to about 1 GPa, suggest that the deformation mechanism acting
in the low-P regime is a cooperative tilting of the tetrahedra belonging to the double 6-ring – resulting in a decrease
of its thickness – accompanied by a simultaneous di-trigonalization of the two 6-rings. A similar mechanism was
previously observed during compression of levyne [4].
The HP-induced cell volume contraction of NS (12.6 %) is higher than that of VALL (10.3%) in the same P range.
This is congruent with the lower content in large extraframework potassium cations of NS, which contribute to
sustain the porous structure in VALL.
[1] L. Leardini, S. Quartieri, G. Vezzalini, Micropor. Mesopor. Mater. 127 (2010) 219-227
[2] L. Leardini, S. Quartieri, A. Martucci, G. Vezzalini, V. Dmitriev, Z. Kristallogr. (2012), doi:
10.1524/zkri.2012.1477.
[3] A. F. Gualtieri, E. Passaglia, Eur. J. Mineral. 18 (2006) 351–359.
[4] G. D. Gatta, P. Comodi, P. F. Zanazzi, T. F. Ballaran, Am. Miner. 90 (2005) 645-652
Crystal chemistry of stilbites: structure refinements of one normal and four chemically anomalous samples
No full textStructural refinements were performed on 4 stilbites with anomalous chem. compns. as regards the monovalent/divalent cation ratio, and a normal stilbite. The 4 anomalous samples showed, moreover, a monoclinic angle lower than that expected from an empirical correlation between monovalent cation content and β angle. In all samples the 2 cationic sites characteristic of stilbite, were found; in only 1 sample was a new cationic site localized. One site is fully occupied by Ca alone, or, with some Na, whereas the other sites are fully or partially occupied by monovalent cations only. In the 5 stilbites, different nos. of water mol. sites were localized. The frameworks, however, seem to be unaffected by the no. and compn. of extra framework sites. The small β angle of the anomalous samples was only partially explained by structural data
Crystal structure of a potassium-rich natural gmelinite and comparison with the other refined gmelinite samples
No full textA complete crystallochem. characterization of a gmelinite sample from Fara Vicentina (Vicenza, Italy) is given. The mineral is anomalous for the prevalence of K (2.72 atoms per unit cell) among the exchangeable cations. The lattice consts. are a = 13.621(3) and c = 10.254(1) Å. Its crystal structure was refined down to Rw = 5.92% and compared with the structures reported for Na-, Ca- and K-rich gmelinites in the literature; the sample studied is characterized by a very low occupancy (3%) of 1 (C2) of the 2 exchangeable cation sites and by water mols. distributed among 8 partially occupied sites. A correlation involving the C2 site and the geometry of the tetrahedral framework was found between the bond strengths: with decrease in the bond strength, the shape of the 8-ring of the gmelinite cage becomes more circular, the double 6-ring becomes more hexagonal and, consequently, the c parameter increases and the a parameter decreases. Post-crystn. cation exchanges explain the present chem. compn. of the mineral
Thermal behavior of zeolites. Single crystal x-ray study of dehydration and rehydration mechanism in yugawaralite
No full textDehydration and rehydration processes and phase transformations of yugawaralite (I) were studied by single crystal x-ray diffraction. I kept in vacuum at room temp. does not undergo any noticeable structural modifications. When I is heated at 100° in vacuum for 24 h, only the water mol. not coordinated with the Ca atom is lost (≈ 6%), while the 2 water mols. occupying alternative positions in the hydrated sample move into only one fully occupied site placed in an intermediate position: the Ca coordination remains virtually unchanged. At 150° a water loss of 53% is indicated, and the residual water mols. are localized in only 2 fully occupied sites. The Ca atoms are 6-coordinated by 4 framework O and the 2 residual water mols. The most relevant modifications of the framework are the increased difference between the Al-OCa-bonded and AI-OCa-nonbonded distances, and a lengthening (by ≈ 0.04 Å) and a narrowing (by ≈ 0.04 Å) of the structural units of I formed by a pair of 5-membered rings sharing an edge when compared with hydrated samples. At 180°, a sudden decrease of the I unit cell vol. occurs (≈ 12%), the cell parameter a is doubled, and the space group changes from Pc to Pn. The structure refinement of a crystal heated at 200° shows a further 50% loss of the residual water present at 150°, a collapsing of the channel system, and strong squashing of the framework. Owing to the doubling of parameter a there are 2 sym. independent Ca sites, both 6-coordinated with five framework O and 1 water mol. At 250°, I does not cause any significant modifications with respect to the structure at 200°. When I heated at 150° is brought back to room conditions for 2 wk, complete rehydration occurs; the water mols. are in the same positions and have the same occupancies as in the hydrated sample
Dachiardite from Hokiya-Dake: Evidence of a new topology
No full textA refinement of the crystal structure of dachiardite from Hokiya-dak, Japan, shows the presence (16% modal abundance) of domains of the hypothetical structure called "modified" dachiardite by S. Merlino (1975, 1976) assocd. with domains of "normal" dachiardite. As in the Elba dachiardite, 2 acentric framework configurations are present with the same frequency in the Hokiya-dake dachiardite, so that the statistical symmetry C2/m is maintained. This feature is interpreted as a tendency (common to all zeolites) to avoid T-O-T angles of 180° in the topol. C2/m symmetry of dachiardite. As with the related structures of mordenite and epistilbite, a remarkable Al-enrichment is found in the tetrahydrons of the 4-membered ring
Spectroscopic studies of coexisting prehnite-pumpellyite and epidote-pumpellyite
No full textX-ray absorption spectra-in the Fe K near-edge.region (XANES) and 57Fe Mossbauer spectra were collected on two Fe-rich
pumpellyite specimens from low-grade metarnorphosed spilitic rocks from the Keweenaw Peninsula, northern Michigan, and
on prehnite and epidote coexisting with pumpellyite, Spectroscopic data for prehnite, presented for the first rime, are discussed
in Iight of the pseudosymmetry of its structure, The data clearly show the prevailing role of Fe3+ in the crystal chemìstry of
the paragenetically assooiated Ca-silicates, This 1Sin agreement with the overall oxidizing conditions typical of the low-grade
metamorphic environment and the mìneralassemblages characteristic of alterationzones i.n basaltic rocks as described by
petrological analyses
Structural modifications induced by dehydration in the zeolite gismondine
No full textGismondine from Ontalto di Castro, Italy [Ca3.91Al7.77Si8.22O32.17.57 H2O] with a 10.0199(4), b 10.6773(5), c 9.8316(1) Å, β 92.561(6)°, space group P21/c, dehydrated in vacuum for 1 and 24 h transformed into 2 phases, here called gismondine (1 h) and gismondine (24 h), resp. Gismondine (1 h) is characterized by 9.5% H2O loss and by a small decrease in the cell vol. (ΔV = 0.6%) with a 9.989(3), b 10.616(3), c 9.820(3) Å, and β 92.57(2)°, Rw = 7.6%. The framework is almost undistorted, but a rearrangement of H2O mols. causes a change in space group to P21, with formation of a more regular 6-coordinated Ca polyhedron. Gismondine (24 h) is characterized by the orthorhombic space group P212121 and a unit cell doubled with respect to the nondehydrated sample; a 13.902(9), b 8.892(4), and c 13.952(5) Å, Rw = 7.6%. More than 50% of H2O is lost, the framework is highly distorted, and the channels are strongly squashed. Residual H2O sites are fully occupied. Ca polyhedra are 7-fold coordinated and are linked by vertexes to form infinite chains
Occurrence and crystal structure of a Ca-pure willhendersonite
No full textThe paper reports the occurrence and the crystal-chem. data of willhendersonite from the breccia ridge of a melilitite plug at Colle Fabbri, Terni, Italy. The chem. data suggests extensive chem. variability, ranging from the compn. of the holotype sample (CaKAl3Si3O12: 5 H2O) through intermediate compns., to a Ca-pure term; thus it becomes necessary for a chem. redefinition of the zeolite. The structural refinement of a Ca-pure willhendersonite single crystal exhibits the same space group and Si/Al ordering as the holotype willhendersonite, but it displays significant differences in the distribution of the extra-framework species. These structural features are discussed in the light of a comparison with the structures of the holotype willhendersonite and natural and exchanged chabazites
Compressibility of microporous materials with CHA topology: 1. Natural chabazite and SAPO-34
No full textThe HP behaviors of a natural chabazite and of SAPO-34 as-synthesized were investigated by means of in situ synchrotron X-ray powder diffraction, with the aim of understanding the role of the framework/extraframework content on the response to pressure. Compressibility was determined with silicon oil, as non-penetrating P-transmitting medium, up to 7.6 GPa and 4.5 GPa for chabazite and SAPO-34, respectively. The isothermal Equation of States (EoS), refined with a second-order Birch–Murnagan EoS, yielded the following parameters: V0 = 842(2) Å3, K0 = 35(5) GPa for chabazite at P 1.4 GPa, and V0 = 2439(4) Å3, K0 = 29(1) GPa, for SAPO-34. These results indicate that chabazite is definitely less compressible than SAPO-34. This is mainly ascribed to the more complex and hindering extraframework content of the natural sample. No complete structural refinements of the powder patterns were performed, but the compressibility of these materials with CHA topology was tentatively interpreted on the basis of the deformation mechanism observed in the structurally related zeolite levyne. The comparison between the volume decrease induced on chabazite by high pressure and by high temperature indicates that this zeolite is much more flexible under compression than upon dehydration
Crystal-fluid interactions in open-framework materials at high pressure
Full text linkThe number of experiments on the high-pressure behavior of open-framework materials increased significantly in the last decade [1,2]. The framework topology, the chemical composition and the so-called “host-guest” interactions (between the framework and the extraframework components) were found to significantly influence the response of zeolites to the applied pressure. However, the HP-behavior of zeolites may also be influenced by crystal-fluid interactions when P-transmitting fluids (PTF) are used to generate hydrostatic compression, and in particular when the PTF is “pore-penetrating”. In such a case, the P-induced penetration of PTF molecules into the zeolite structural voids leads to a change of the physical-chemical properties of the studied material, for example inducing a stiffening of the elastic behavior or leading to the hyperconfinement of supramolecular aggregates (in the zeolite channels) with functional properties [3].
In this study, we describe the HP-behavior and the crystal-fluid interactions of two synthetic zeolites with empty channels and cages, i.e. all-silica ferrierite (Si-FER) and ALPO4-5 (AlPO4), compressed with non-penetrating (silicone oil, s.o.) and potentially pore-penetrating PTF. The compression of Si-FER in s.o. evidences the remarkable flexibility of this framework: a first displacive phase transition was observed from the Pmnn to the P121/n1 space group at ~ 0.7 GPa. A second displacive phase transition, involving a significant unit-cell volume contraction, was observed at ~ 1.24 GPa from the P121/n1 to the P21/n11 space group (through an intermediate P-1 structure, “type-II” transition according to Christy [4]). The high-P P21/n11 polymorph was found to be stable at least up to 3.00(7) GPa, whereas - upon pressure release - the starting Pmnn structure was fully recovered. The three polymorphs were found to share a virtually identical bulk elastic behavior, being their average volume compressibility βV: 0.051(4), 0.056(9) and 0.055(3) GPa-1, respectively. The compression of Si-FER and ALPO-5 in potentially pore-penetrating PTF showed a lower bulk compressibility, different phase-transition paths (for Si-FER) and diverse atomic-scale deformation mechanisms with respect to the compression in silicone oil, suggesting the onset of significant crystal-fluid interactions, likely due to the P-induced penetration of PTF molecules. In addition, the HPbehavior of Si-FER is strongly influenced by the process kinetics, which was found to control the P-induced molecules intrusion phenomena and, as consequence, the P-induced phase transitions in this material.
The authors acknowledge the Italian Ministry of Education, MIUR-Project: “Futuro in Ricerca 2012 - ImPACTRBFR12CLQD”.
[1] G.D. Gatta, Y. Lee Mineral. Mag. 2014, 78, 267-291. [2] G. Vezzalini, R. Arletti, S. Quartieri Acta Cryst. 2014, B70,
444-471. [3] M. Santoro, F.A. Gorelli, R. Bini, J. Haines, A. Van der Lee Nat. Commun. 2013, 4, 1557-1563. [4] A.G.
Christy Acta Cryst. 1993, B49, 987-996
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