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This paper reports the results of an in situ high-pressure synchrotron X-ray powder diffraction investigation on the natural zeolite boggsite [([K.sub.0.06][Na.sub.0.36][Sr.sub.0.01] [Ca.sub.7.00] [Mg.sub.1.20])([Al.sub.17.52][Si.sub.78.62][Fe.sub.0.05][O.sub.192]) x 82.3 [H.sub.2]O]. The study was performed using both a (16:3:1) methanol:ethanol:water mixture (m.e.w.) as a nominally 'penetrating' hydrostatic P-transmitting medium and silicon oil (s.o.) as a'non-penetrating' medium. The studied pressure ranges are: [P.sub.amb] -7.6 and [P.sub.amb] -5.9 GPa in m.e.w and s.o., respectively. No complete X-ray amorphization is observed up to the highest investigated pressures, and the original unit-cell parameters are almost completely recovered upon decompression in both media. The reductions of a, b, c, and V, within the pressure-ranges investigated, are 5.3, 4.2, 4.0, and 13.0% in s.o. and 4.1, 4.1, 3.8, and 11.5% in m.e.w. The Rietveld structural refinements of the powder patterns of the experiments in m.e.w, converged successfully up to 3.6 GPa and demonstrated the penetration of 13 additional water molecules between 0.3 and 2.9 GPa. This over-hydration occurs without any unit-cell volume expansion and can be explained by the fact that no new extraframework sites arise during compression and that water penetration is the only factor to increase the occupancy of already existing sites. Boggsite compressibility is higher in s.o. than in m.e.w. In particular, compressibility in m.e.w. is lower below 3 GPa, whereas above this pressure, the P-V trend becomes similar in the two media. This can be ascribed to the fact that, during water molecule penetration (0.3 < P < 3 GPa), the effect of the P-transmitting medium is directed to compress the system as well as to penetrate the channels. Keywords: Zeolite, boggsite, high pressure, compressibility, over-hydration, crystal structure, synchrotron XRPD data DOI: 10.2138/am.2010.3482

The thermally induced structural modifications of the natural zeolite barrerite [[Na.sub.16] [Al.sub.16][Si.sub.56][O.sub.144] x 52[H.sub.2]O, a = 13.6239(4) [Angstrom], b = 18.2033(5) [Angstrom], c = 17.8317(7) [Angstrom], V = 4422.3(3) [[Angstrom].sup.3], space group Amma, framework type STI] were studied in a temperature-resolved X-ray powder diffraction experiment, using synchrotron radiation, in the temperature range 339-973 K. In the initial stage of heating, up to 508 K, barrerite Phase A (space group Amma) is stable, the unit-cell volume decreases by about 4% and a water release of about 66% is observed. Between 521 and 598 K, a phase transition to the collapsed so-caned barrerite Phase B (space group Amma) is observed. During the transition, the rotation of the [4.sup.2][5.sup.4] secondary building units causes a large decrease in cell volume and deformation of the channel system. Phase B, at 611 K, shows the statistical breaking of T-O-T bridges in the 4-rings and the migration of the involved tetrahedral atoms to new 'face-sharing' tetrahedra, with a consequent reduction of the free volume of the channels parallel to [100]. The new structure is stable up to 741 K and the total volume decrease is about 16%. A new phase appears from 754 K with cell parameters similar to those reported for the highly deformed barrerite Phase D and is stable up to 910 K, which is the temperature at which the total volume decrease is 22.5%. The material does not undergo amorphization up to the highest temperature investigated. Keywords: Zeolite, barrerite, dehydration, X-ray powder diffraction, synchrotron radiation, crystal structure

This paper reports the results of an in situ HP synchrotron X-ray powder diffraction investigation on the natural zeolite gismondine (ideal chemical formula [Ca.sub.4][Al.sub.8][Si.sub.8][O.sub.32] x 16[H.sub.2]O, space group P[2.sub.1]/c). The study was performed from [P.sub.amb] to 7.9 GPa, and upon decompression, using methanol:ethanol:water (16:3:1) mixture (m.e.w.) as a nominally penetrating hydrostatic P-transmitting medium. No complete X-ray amorphization is observed up to the highest investigated pressure, and the original unit-cell parameters are almost completely recovered upon decompression. From 0.6 GPa, the water content is slightly higher than at ambient pressure, as a result of a moderate over-hydration. Moreover, at about 2 GPa, a significant water molecule system re-arrangement occurs, characterized by an ordering of part of the water molecules from four partially occupied sites to only two fully occupied ones. The over-hydration, but not the water ordering, is substantially irreversible upon pressure release. The Rietveld structural refinements of the powder patterns converged successfully up to 2.8 GPa; above this pressure, a phase transition to triclinic symmetry was observed and only the unit-cell parameters were refined. The comparison of the overall cell volume reductions and of the bulk modula of gismondine compressed between [P.sub.amb] and 7.9 GPa in m.e.w, and in silicone oil, reveals that this is the unique zeolite with a higher compressibility in penetrating vs. non-penetrating P-transmitting media. This is ascribed to the re-organization of the water molecule system upon compression in m.e.w., which leaves a larger free volume inside the pores with respect to the phase compressed in silicone oil. Keywords: Zeolite, gismondine, high pressure, compressibility, over-hydration, phase transition, crystal structure, synchrotron XRPD data

The elastic behavior and the high-pressure structural evolution of the zeolite wairakite [ideal chemical formula Ca([A[.sub.2][Si.sub.4][O.sub.12]).2[H.sub.2]O, space group I2/a], the Ca-analog of analcime, have been investigated by means of in-situ synchrotron X-ray powder diffraction from ambient pressure to 7.8 GPa, and upon decompression. No complete X-ray amorphization is observed up to the highest investigated pressure, and the original unit-cell parameters are recovered upon decompression. The Rietveld structural refinements of the powder patterns converged successfully up to 2.5 GPa; above this pressure, a phase transition to triclinic symmetry is observed and only the unit-cell parameters were refined. An overall reduction of about 14% of the unit-cell volume between 0.0001 and 7.0 GPa is observed, demonstrating that wairakite is much more flexible upon compression than upon heating. The pressure dependence of the cell parameters is strongly anisotropic and larger after the transition to the triclinic space group. The elastic parameters refined with a second-order Birch-Murnaghan Equation of State are [V.sub.0] = 2536(4) [Angstrom].sup.3], [K.sub.0] = 39(3) GPa, and [V.sub.0] = 2632(38) [[Angstrom].sup.3], [K.sub.0] = 24(3) GPa, for the monoclinic and triclinic phases, respectively. The structure distortion of monoclinic wairakite, proceeding via tetrahedral tilting, induces deformations in the 4-, 6-, and 8-membered rings and an increase in the extra-framework Ca coordination number. A comparative discussion of the compressibility behavior of wairakite and analcime is reported. Keywords: HP studies, XRD data, crystal structure, wairakite, phase transition, compressibility measurements

The microscopic behavior of the Ca-zeolite yugawaralite has been studied by ab initio molecular dynamics simulations adopting experimental cell parameters obtained at pressures up to ~9 GPa. Pressure-induced volume contraction occurs via rotations of quasi-rigid T[O.sub.4] tetrahedra that reduce the size of the channels in which the extra framework species are located. Such rotations are governed by deformation of the coordination polyhedron of Ca, which is made up of water and framework O atoms. Contraction of the Ca-[H.sub.2]O distances is favored at moderate pressure: at higher pressure the shortening of Ca-framework O atom distances becomes prevalent. The hydrogen bond network plays a fundamental role in fine overall response to pressure. Our results indicate that the high-P-induced deformation of the framework structure is strictly correlated to the extra-framework species that act as 'templates' in the compression process.

High-temperature behavior and the process of thermal dehydration in the natural zeolite bikitaite have been studied by ab initio molecular dynamics simulations, and favorably compared with the X-ray powder diffraction data presented in Part I of this work (Ferro et al. 2003). The microscopic dynamical behavior of the extraframework species (water molecules and Li cations) has been characterized as a function of temperature. Two regimes have been detected, and the transition is characterized by the breaking of the one-dimensional water chain typical of bikitaite at room temperature. The elementary steps for the diffusion of water inside the bikitaite channels have been studied by means of a rare-events-sampling technique (Bluemoon Ensemble). The activation free-energy for a site-to-site water jump has been calculated and a mechanism for the dehydration process is proposed.

The thermal dehydration process of the natural Li zeolite bikitaite has been studied in situ by synchrotron radiation powder diffraction. The temperature-resolved experiments were performed using a translating imaging plate system. Rietveld refinements were carried out on 42 powder patterns in the temperature range from room temperature to 800 [degrees]C. Bikitaite is stable at least up to 800 [degrees]C, the temperature at which the phase transition to [gamma]-spodumene begins. The dehydration process begins at about 200 [degrees]C, affecting the two water sites in a similar way, and is complete at 468 [degrees]C. Such a process induces only very minor structural distortions in the framework, due to the anti-rotation of the internal T-O-T hinges. In this sense, bikitaite can be defined as a flexible but non-collapsible framework, and it is the zeolite that undergoes the lowest heating-induced distortion among those studied up to now, The high thermal and structural stability suggest that anhydrous bikitaite could be used as a solid porous matrix for embedding nanosized materials in its one-dimensional channels.

The HP behavior of the natural zeolite yugawaralite and of the synthetic zeolite Na-A was studied by in situ synchrotron X-ray powder diffraction, using a non-penetrating P-transmitting medium. The unit-cell parameters of yugawaralite were refined up to the pressure of 10 GPa, at which reductions were found of about 7, 2.4, 7, 1.3. and 15% for a, b, c, [beta], and V, respectively. Contractions of 6.5 and 18.4% were found for a and V, respectively, for zeolite Na-A in the range [10.sup.-4] to 6.8 GPa. Diffraction patterns collected during decompression show that the effects induced by high pressure on both samples are almost completely reversible. These results are compared with those obtained under similar experimental conditions for other natural zeolites, with the aim of rationalizing the deformation mechanisms of these porous materials and comparing their flexibility under high-pressure and high-temperature conditions.

Pressure-induced structural modifications in the zeolite bikitaite are studied by means of in situ synchrotron X-ray powder diffraction and ab initio molecular dynamics. The experimental cell parameters were refined up to 9 GPa, at which pressure we found reductions of 4.5, 4.5, 6.3, and 15% in a, b, c, and V, respectively. Minor variations were observed for the cell angles. Complete X-ray amorphization is not achieved in the investigated P range, moreover the P-induced effects on the bikitaite structure are completely reversible. Because it was possible to extract only the cell parameters from the powder patterns, the atomic coordinates at 5.7 and 9.0 GPa were obtained by means of Car-Parrinello simulations using the unit-cell parameters experimentally determined at these pressures. Analysis of the computational results for increasing pressures showed that the volume contraction is essentially due to rotations of the tetrahedra; the 8-ring channels become more circular; the pyroxene chain becomes more corrugated in the b-c plane; and the mean Li-O bond distances and coordination polyhedral volumes decrease with increasing pressure without significant distortion of the internal angles. The peculiar aspect of the bikitaite structure, i.e., the presence in the channels of a 'floating' one-dimensional water chain, is only partially maintained at high pressure; the compression brings framework O atoms close enough to water hydrogen atoms to allow the formation of host-guest hydrogen bonds, without, however, destroying the one-dimensional chain.

Pressure-induced structural modifications in scolecite were studied by means of in situ synchrotron X-ray powder diffraction and density functional computations. The experimental cell parameters were refined up to 8.5 GPa. Discontinuities in the slope of the unit-cell parameters vs. pressure dependence were observed; as a consequence, an increase in the slope of the linear pressure-volume dependence is observed at about 6 GPa, suggesting an enhanced compressibility at higher pressures. Weakening and broadening of the diffraction peaks reveals increasing structural disorder with pressure, preventing refinement of the lattice parameters above 8.5 GPa. Diffraction patterns collected during decompression show that the disorder is irreversible. Atomic coordinates within unit cells of different dimensions were determined by means of Car-Parrinello simulations. The discontinuous rise in compressibility at about 6 GPa is reproduced by the computation, allowing us to attribute it to re-organization of the hydrogen bonding network, with the formation of water dimers. Moreover we found that, with increasing pressure, the tetrahedral chains parallel to c rotate along their elongation axis and display an increasing twisting along a direction perpendicular to c. At the same time, we observed the compression of the channels. We discuss the modification of the Ca polyhedra under pressure, and the increase in coordination number (from 4 to 5) of one of the two Al atoms, resulting from the approach of a water molecule. We speculate that this last transformation triggers the irreversible disordering of the system.

Scandiobabingtonite, ideally [Ca.sub.2]([Fe.sup.2+], Mn)Sc[Si.sub.5][O.sub.14](OH) is the scandium analogue of babingtonite; it was found in a pegmatitic cavity of the Baveno granite associated with orthoclase, albite, muscovite, stilbite, and fluorite. Its optics are biaxial (+) with 2V = 64(2) [degrees], [Alpha] = 1.686(2), [Beta] = 1.694(3), [Gamma] = 1.709(2). [D.sub.meas] = 3.24(5) g/[cm.sup.3], [D.sub.calc] = 3.24 g/[cm.sup.3], and Z = 2. Scandiobabingtonite is colorless or pale gray-green, transparent, with vitreous luster. It occurs as submillimeter sized, short, tabular crystals, slightly elongated on [001], and characterized by the association of forms {010}, {001}, {110}, [Mathematical Expression Omitted], and {101}. It occurs also as a thin rim encrusting small crystals of babingtonite. The strongest lines in the X-ray powder pattern are at 2.969 (S), 2.895 (S), 3.14 (mS), and 2.755 (mS) A. The mineral is triclinic, space group PT, with a = 7.536(2), b = 11.734(2), c = 6.748(2) A, [Alpha] = 91.70(2), [Beta] = 93.86(2), [Gamma] = 104.53(2) [degrees] . Scandiobabingtonite is isostructural with babingtonite, with Sc replacing [Fe.sup.3-] in six-fold coordination, but no substitution of [Fe.sup.2+] by Sc takes place. Due to the lack of a suitably large crystal of the new species, such a replacement has been confirmed by refining the crystal structure of a Sc-rich babingtonite (final R = 0.047) using single-crystal X-ray diffraction (XRD) data.

Scandiobabingtonite, ideally [Ca.sub.2]([Fe.sup.2+], Mn)Sc[Si.sub.5][O.sub.14](OH) is the scandium analogue of babingtonite; it was found in a pegmatitic cavity of the Baveno granite associated with orthoclase, albite, muscovite, stilbite, and fluorite. Its optics are biaxial (+) with 2V = 64(2) [degrees], [Alpha] = 1.686(2), [Beta] = 1.694(3), [Gamma] = 1.709(2). [D.sub.meas] = 3.24(5) g/[cm.sup.3], [D.sub.calc] = 3.24 g/[cm.sup.3], and Z = 2. Scandiobabingtonite is colorless or pale gray-green, transparent, with vitreous luster. It occurs as submillimeter sized, short, tabular crystals, slightly elongated on [001], and characterized by the association of forms {010}, {001}, {110}, [Mathematical Expression Omitted], and {101}. It occurs also as a thin rim encrusting small crystals of babingtonite. The strongest lines in the X-ray powder pattern are at 2.969 (S), 2.895 (S), 3.14 (mS), and 2.755 (mS) A. The mineral is triclinic, space group PT, with a = 7.536(2), b = 11.734(2), c = 6.748(2) A, [Alpha] = 91.70(2), [Beta] = 93.86(2), [Gamma] = 104.53(2) [degrees] . Scandiobabingtonite is isostructural with babingtonite, with Sc replacing [Fe.sup.3-] in six-fold coordination, but no substitution of [Fe.sup.2+] by Sc takes place. Due to the lack of a suitably large crystal of the new species, such a replacement has been confirmed by refining the crystal structure of a Sc-rich babingtonite (final R = 0.047) using single-crystal X-ray diffraction (XRD) data.

The water chains in Li-ABW are less stable than those in bikitaite. The stability of such peculiar one-dimensional arrangement is strongly influenced by the zeolite's framework structure.