Giuseppe Cruciani

Rietveld refinement of the zorite structure of ETS-4 molecular sieves

Abstract The crystal structure of ETS-4 titanosilicate [Na9Si12Ti5O38(OH)·12H2O] was refined using synchrotron X-ray powder diffraction and the Rietveld method (Rp=0.050; Rwp=0.063; RB=0.169). Our diffraction data provide direct evidence that ETS-4 has the same structure as the zorite mineral. The framework of zorite can be described as nenadkevichite-like chains, laterally connected by 4[SiO4]/[TiO6] units, corresponding to two different chemical environments for Si. Similar to previous observations, a discrepancy was found between the population ratio of Si positions as calculated by the occupancy refinement of atomic sites related to these two environments, and that inferred from 29 Si MAS NMR. This is tentatively explained here on the basis of the large amounts of protons in the proximity of the Si2 site as indicated by 29 Si CP/MAS NMR. The low thermal stability of the zorite/ETS-4 structure is explained in terms of relevance of the hydrogen bonding between extra framework water molecules and framework oxygens.

Hydrogen deficiency in Ti-rich biotite from anatectic metapelites (El Joyazo, SE Spain): Crystal-chemical aspects and implications for high-temperature petrogenesis

Abstract Titanium-rich biotites from graphite-bearing metapelitic xenoliths, equilibrated at ca. 850 °C and 7 kbar in the presence of a granitic melt, have been studied through complete chemical analysis and single-crystal XRD refinements. The chemical study combines EMP analyses, hydrogen determination by both SIMS and C-H-N elemental analysis, and Mössbauer spectroscopy. Biotites in the analyzed xenoliths have TiO2 contents ranging from 4.5 to 4.9 wt% and an XFe of 0.67. Their F and Cl contents are negligible, and Fe3+/Fetot ranges from 10 to 16%. The H2O content of the biotites ranges from 2.4 to 2.8 wt%, and a small fraction of H is accommodated in the lattice as NH4. Based on these full chemical analyses, the calculated OH occupancy is 1.26 to 1.30 groups per formula unit, more than one third less than the stoichiometric value. The entrance of Ti in the octahedral site of biotite is consistent with the Ti-oxy exchange, whereas Ti-Tschermak or Ti-vacancy substitutions play a very minor role. The Fe3+-oxy exchange cannot account for the observed OH deficiency. From single-crystal XRD, biotites belong to the 1M polytype and contain variable amounts of stacking faults. The c cell parameter, K-O4 and outer distances provide an independent estimate of the OH content, which agrees with SIMS determinations. The linear relationship between VITi4+ and the bond-length distortion of the cis-M2 octahedron reveals the partitioning of Ti4+ in M2, and the Ti4+ off-center shift toward O4 supports the occurrence of the Ti-oxy exchange. The ordering of Ti4+ over two non-equivalent M2 sites, which would be favored energetically, is in agreement with the evidence for a third octahedral site suggested by Mössbauer spectroscopy. The biotite dehydrogenation combined with the partitioning of Ti4+ in M2 and the low thermal expansion of Ti4+- containing octahedra, are the keys to understanding the thermal stabilization of Ti substitution in biotites.

Dehydration dynamics of analcime by in situ synchrotron powder diffraction

Abstract The continuous structural transformation of tetragonal analcime (Na15.87Al15.20Si32.64O96·16.3H2O) upon dehydration was studied, using Rietveld structure analysis of temperature- resolved powder diffraction data collected using synchrotron radiation. The variation of the a-c axis length difference and normalized intensity of the (200) reflection as a function of temperature suggest that tetragonal analcime evolves toward a cubic structure at high temperature. The removal of water was accompanied by a spreading of the initial Na sites into many positions bonded to the framework O atoms. The migration of H2O molecules through the [111] channels during dehydration caused the six-member ring apertures to open as widely as possible: this was accompanied by a twisting of the tetragonal prism, constituting the analcime framework, which led to an opposite tilting of tetrahedra connecting the prisms. These modifications induced by water diffusion are not energetically favored because they would increase the elastic energy of the system, and require a substantial thermal activation energy. The analcime framework reached a maximum distortion at about 650 K, the temperature of complete water loss, then underwent a relaxation process during which the T-O-T angles were restored to the starting value. The relative variation of cell volume associated with the opening of wide six-member ring channels during water migration, and then due to the framework relaxation process after complete dehydration, provides an explanation of the ‘‘negative thermal expansion’’ (i.e., volume contraction) effect in dehydrated analcime, which is complementary to that based on the Rigid Unit Modes theory.

Dehydration dynamics of stilbite using synchrotron X-ray powder diffraction

Abstract The continuous structural transformation of the natural zeolite stilbite (Na3.62K0.44Ba0.03Ca6.32Sr0.28Mg0.04[Fe3+001Al17.33Si54.64O144] · 60H2O) upon dehydration has been studied using Rietveld structure analysis of temperature-resolved powder diffraction data collected with synchrotron radiation. In the initial stage of heating, the monoclinic F2/m stilbite structure (the so-called A phase) behaves as a noncollapsible framework, featuring only a slight framework distortion and a slight cell-volume contraction. At about 420 K, a first-order phase transition occurs changing the symmetry to an orthorhombic Amma phase, whose framework is collapsible and shows a large cell-volume contraction with temperature. The cell contraction is related to the process of T-O-T bond breaking and leads to a high-temperature stilbite phase with the same Amma space group and a collapsed structure similar to the previously described B phase in stellerite and barrerite. The structural refinement indicates that the dynamics of bond breaking is related to the shift of the Ca cations in the channels to achieve optimal coordination after the release of the H2O molecules. Refined statistical occupancies of the tetrahedral atoms involved in the bondbreaking process (T1 and T1P) are consistent with a random rupture and re-formation of the T-O-T bonds. This is the first experimental study of the dynamic bond breaking of T-O-T bonds in a framework structure.

Crystal structure of the zeolite mutinaite, the natural analog of ZSM-5

We describe the crystal structure of the high-silica zeolite mutinaite, recently found at Mt. Adamson (Northern Victoria Land, Antarctica). Mutinaite is the natural counterpart of the synthetic zeolite ZSM-5. The new mineral, (Na 2.76 K 0.11 Mg 0.21 Ca 3.78 ) (Al 11.20 Si 84.91 ) · 6O H 2 O H 2 O, is orthorhombic, space group Pnma, with a = 20.201(2), b = 19.991(2), and c = 13.469(2) A. A single-crystal X-ray diffraction experiment was performed at the synchrotron radiation source ESRF (Grenoble). No Si-Al order in the framework has been detected. Large distances between ions in the channels and framework oxygens suggest weak interactions between the framework and extraframework species.

Siting and coordination of cobalt in ferrierite: XRD and EXAFS studies at different Co loadings

Abstract The crystal structure of three synthetic ferrierite samples loaded with different amounts of Co cations (2.5 wt.%, 4.3 wt.% and 13.7 wt.% respectively) were refined by means of synchrotron X-ray powder diffraction data. The unit cell content of cobalt exchanged ferrierites resulted: ∣Na 0.18 K 0.03 Co 1.12 H 1.36 (H 2 O) 17.2 ∣[Al 3.81 Si 32.19 O 72 ]–– FER , ∣Na 0.18 K 0.03 Co 1.89 (H 2 O) 17.2 ∣[Al 3.81 Si 32.19 O 72 ]–– FER , and ∣Na 0.18 K 0.03 Co 1.89 (H 2 O) 17.2 ∣[Al 3.81 Si 32.19 O 72 ]–– FER respectively. In the overloaded sample the excess of Co (9.4 wt.%) is located outside the zeolite structure as cobalt oxide phases. After cation exchange the space group, which was P 2 1 / n in the as-synthesised form, becomes Immm . The crystal structure of the three synthetic samples is characterised by the presence of a Co(H 2 O) 6 2+ polyhedron inside the ferrierite cage. The occupancy of this polyhedron varies as a function of the Co amount in the extra-framework sites of the ferrierite structure: up to 50% for a complete ion-exchange. The residual cobalt occupies two further cation sites.

Ni2+ ion sites in hydrated and dehydrated forms of Ni-exchanged zeolite ferrierite

Abstract The location of extra-framework Ni 2+ cations in the hydrated and dehydrated nickel ion-exchanged zeolite ferrierite was determined from the analysis of synchrotron X-ray powder diffraction data. Rietveld refinements were performed in the Immm space group. One Ni 2+ was found at the centre of the ferrierite cage coordinated to six water molecules in a quite regular octahedral coordination. The Ni(H 2 O) 6 2+ octahedron assumes two equally probable statistical orientations, which differ by a rotation of about 45° around the z axis. These two configurations of the Ni(H 2 O) 6 2+ polyhedron show strong similarities with the two configurations of the Mg(H 2 O) 6 2+ polyhedron found in natural Mg-rich ferrierite. However, whereas the cation site in natural Mg-ferrierite is fully occupied, in the Ni-exchanged form, this site is only half occupied. The unit cell volume of dehydrated Ni-ferrierite was 1.4% smaller than the unit cell volume of the hydrated phase. Two other Ni 2+ extra-framework sites were found, one near the centre of the 10-ring channel, the other not far from the centre of the 8-ring window of the ferrierite cage. Four Ni 2+ extra-framework sites were localised in dehydrated Ni-ferrierite. The first is near the centre of the 6-ring window of the ferrierite cage, the second is located on the walls of the cage, the third is in the 10-ring channel, and the last is at the intersection of the 10- and 8-ring channels. The last Ni 2+ site is very close to the Cu(1) site found by Attfield et al. [M.P. Attfield, S.J. Weigel, A.K. Cheetham, J. Catal. 172 (1997) 274–280] in dehydrated Cu-ferrierite, whereas the other three Ni 2+ sites resemble the α-, β- and γ-type sites of Kaucký et al. [D. Kaucký, J. Dědecek, B. Wichterlova, Micropor. Mesopor. Mater. 31 (1999) 75–87] in dehydrated Co-ferrierite.

Structural relaxation in tetrahedrally coordinated Co2+ along the gahnite-Co-aluminate spinel solid solution

Abstract The structural relaxation around the Co2+ ion along the gahnite (ZnAl2O4)-Co-aluminate (CoAl2O4) join was investigated by a combined X-ray diffraction (XRD) and electronic absorption spectroscopy (EAS) approach. Monophasic spinel samples (Zn1-yCoyAl2O4 with y = 0, 0.25, 0.5, 0.75, and 1 apfu) were obtained through solid-state reaction (1300 °C with slow cooling). The cobalt incorporation induces a linear increase of the unit-cell parameter (a) accompanied by an increasing inversion parameter (up to 0.07) so that the Co2+ for Al3+ substitution in the octahedral site is, at a first approximation, the cause of the lattice expansion. However, a careful consideration of T-O distances highlights the role played by an enhanced covalence degree of Zn-O bonds. The optical spectra are characterized by the occurrence of electronic transitions of Co2+ in tetrahedral coordination affected by a strong spin-orbit coupling, causing a threefold splitting of spin-allowed bands. Further complications stem from mixing of quadruplet and doublet states (leading to a consistent intensity gain of spin-forbidden bands) and vibronic effects (producing intense sidebands). Crystal field strength goes from 4187 to 4131 cm-1 with increasing cobalt amount, while the Racah B parameter is in the 744-751 cm-1 range (C ∼3375 cm-1). To achieve a reliable estimation of the local Co-O distance, the tetrahedral distance evolution was recast to eliminate the effects of the inversion degree. By this way, a relaxation coefficient as low as ε = 0.47 was obtained, i.e., significantly smaller than literature data for other spinel systems. The gahnite-Co-aluminate join seems to be constrained by the strong preference of Zn2+ for the tetrahedral site in which its enhanced covalency can be exerted, limiting the cation exchange between tetrahedral and octahedral sites as well as the lattice flexibility.

Colour development of red perovskite pigment Y(Al, Cr)O3 in various ceramic applications

Abstract Ceramic pigments based on the perovskite structure develop a promising red hue and furthermore present no environmental or health problems. Pigment colouring efficiency was tested in different ceramic applications, both in the body and in glazes. Final products, with variations in both maximum firing temperature and soaking time, were characterised by colorimetry. The investigation focused on the factors and mechanism influencing perovskite dissolution by calculating phase compositions of different ceramic matrices and observing pigment microstructure. Pigment powders were also characterised by structural analysis and spectroscopy. Pigment dissolution was only slightly affected by the firing cycle, mostly being governed by the amount and chemical composition of the liquid phase. In particular, the larger the content of chemically aggressive components such as calcium, magnesium, zinc and lead oxides, the stronger and faster the pigment dissolution.

Mineral chemistry of Ti-rich biotite from pegmatite and metapelitic granulites of the Kerala Khondalite Belt (southeast India): Petrology and further insight into titanium substitutions

Abstract Precise chemical composition, including Fe3+ and H, of biotite from a pegmatite dike and its host granulite from the Kerala Khondalite Belt of SE India has been determined using a multi-technique approach involving EMP, SIMS, Mössbauer, and C-H-N elemental analysis. Biotite in these rocks formed at T > 800-850 °C and P = 5 ± 1 kbar. The full analyses were normalized on the basis of [O12-(x+y+z)(OH)xClyFz]. Biotite in the pegmatite is Ti-, F-, and Cl-rich (0.33, 0.46, and 0.16 apfu, respectively), H2O-poor (OH = 0.86 pfu), has XMg = 0.49 and Fe3+/Fetot ≤ 3%. The low octahedral vacancies (0.06 pfu) and the high oxygen content in the hydroxyl site (OH + F + Cl = 1.49 pfu) confirm the role of the Ti-oxy substitution as a major exchange vector in these high-T biotites. In the host granulite, fine-grained biotite is Fe3+-free, has low Cl (0.03 apfu), and more variable composition, with Ti, F, and XMg in the ranges 0.26-0.36, 0.52-0.67, and 0.67-0.77, respectively. The number of octahedral vacancies is relatively large (0.10-0.18 pfu) and the sum of volatiles (OH + F + Cl) varies from 1.71 to 2.06 pfu. Systematic variations of XMg are a function of the microstructural position and are in agreement with retrograde exchange reactions: biotite included in or in contact with garnet has the maximum values, whereas crystals in the matrix have the minima. Titanium has systematic negative correlations with F, XMg, and (OH + F + Cl), whereas Al and octahedral vacancies are virtually constant. These trends indicate that the Ti-vacancy, along with substitutions involving Al, cannot explain the observed short-scale variations. Conversely, the Ti-oxy exchange appears to be active, resulting from combination of two vectors: the more conventional hydroxylation Ti4+ + 2O2- = (Fe,Mg)2+ + 2OH- and the “fluorination” Ti4+ + 2O2- = (Fe,Mg)2+ + 2F-. The systematic retrograde redistribution involves not only Fe and Mg as commonly observed, but also Ti, F, and H, in a way such to eliminate the primary Ti-oxy component of biotite.