Maria Chiara Dalconi

Rietveld Refinement on X-Ray Diffraction Patterns of Bioapatite in Human Fetal Bones

Bioapatite, the main constituent of mineralized tissue in mammalian bones, is a calcium-phosphate-based mineral that is similar in structure and composition to hydroxyapatite. In this work, the crystallographic structure of bioapatite in human fetuses was investigated by synchrotron radiation x-ray diffraction (XRD) and microdiffraction ( micro -XRD) techniques. Rietveld refinement analyses of XRD and micro -XRD data allow for quantitative probing of the structural modifications of bioapatite as functions of the mineralization process and gestational age.

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.

Site preference and local geometry of Sc in garnets: Part II. The crystal-chemistry of octahedral Sc in the andradite-Ca3Sc2Si3O12 join

Abstract Investigation of scandium incorporation in garnets along the synthetic Ca3Fe23+Si3O12-Ca3Sc2Si3O12 (adr.CaSc) join, based on the same multi-technique approach used in the companion paper (Oberti et al. 2006a), shows that (1) Sc is incorporated exclusively at the Y octahedron; (2) the local coordination of Sc is slightly different in Sc-poor than in Sc-rich compositions (Sc-O = 2.06 Å in CaSc10 vs. 2.10 Å in CaSc30-90); (3) the local coordination of Ca is also slightly different in Sc-poor than in Sc-rich compositions [Ca1,2-O are 2.34(2) and 2.48(2) Å in CaSc10 and 2.36(2) and 2.50(2) Å in CaSc90, with Δ fixed at 0.14 Å in all the samples]; (4) the linear increase of the unit-cell edge along the join derives from multiple changes in the geometry of the different polyhedra and from the rotation of the tetrahedron around the 4̄ axis (α rotation), and cannot be modeled from extrapolation of the behavior observed along the Ca3Al2Si3O12-Ca3Fe23+Si3O12 (grs.adr) join. CaSc-rich garnets, where a large X dodecahedron coexists with a large Y octahedron and a Z tetrahedron occupied by Si, similar to pyrope-grossular garnets, have the highest α values observed to date in calcium silicate garnets. Slightly lower α values are observed in pyrope and almandine, but correspond to a different structural arrangement, where a small X dodecahedron coexists with a small Y octahedron. These results further confirm the efficiency of a combined short- and long-range approach for understanding the properties of garnet solid solutions.

Structure of bioapatite in human foetal bones: An X-ray diffraction study

Bioapatite is a calcium phosphate closely resembling the hydroxyapatite (Ca10(PO4)6(OH)) and is considered to be the major component of the mineralised part in mammalian bones. Even if bones are widely investigated, several aspects concerning the initial bone formation stages are unclear and need to be clarified. In this work advanced X-ray powder diffraction techniques have been exploited to get insights on the structural evolution of human bioapatite during the early stages of ossification processes.

Site preference and local geometry of Sc in garnets: Part I. Multifarious mechanisms in the pyrope-grossular join

Abstract We applied different independent techniques (electron microprobe analysis, structure refinement, and X-ray absorption spectroscopy) to unravel the possible mechanisms of Sc incorporation in the pyrope-grossular join. Samples were synthesized at elevated pressure and temperature by adding 5 wt% of Sc2O3 to selected nominal compositions (pyrope, pyrope60grossular40, pyrope20grossular80, and grossular). In this way, the site of incorporation was not pre-determined, and only depends on the availability of a mechanism for local charge-balance. The EXAFS spectra of the two end-members could be analyzed by a multi-shell fit procedure, whereas the chemical heterogeneity of the Sc-doped solid-solution terms prevented this approach. However, the available information allows detection of different mechanisms of incorporation, which are active as a function of the bulk composition. In pyrope, Sc mainly enters the dodecahedral X site, and the local charge balance is achieved by incorporation of Mg at the adjacent tetrahedral Z site. Local charge-balance requirements suggest that a Z site occupied by Mg bridges two X sites occupied by Sc. When the entrance of Ca provides relaxation of the averaged structure, Sc may enter all the three available cation sites via the coupled heterovalent exchange XSc1ZSc1XMg-1ZSi-1 and the homovalent exchange YSc1YAl-1. In the samples of this work, there is an apparent limit in the Sc incorporation at the Y site, which is in contrast to the favored mechanism of incorporation in Sc-doped andradites. This limit can be explained in terms of relative dimensions of the structural sites when Al is the dominant Y cation. These results must be taken into account when evaluating trace-element behavior in garnets for geochemical purposes. In particular, they explain why DSc can be treated together with DREE in models based on the elastic strain theory in garnets close to the pyrope composition, but deviate from the parabolic fit in grossular-rich garnets.

Towards three-dimensional quantitative reconstruction of cement microstructure by X-ray diffraction microtomography

Quantitative characterization of the microstructure of cement-based materials is of fundamental importance for assessing the performance and durability of the final products. However, accessing the three-dimensional microstructural information of hydrating cement pastes without introducing any perturbation is not trivial. Recently, a novel non-invasive method based on X-ray diffraction computed microtomography (XRD-CT) has been applied to cement-based materials, with the aim of describing the three-dimensional spatial distribution of selected phases during the hydration of the cement paste. This paper illustrates a method based on XRD-CT, combined with Rietveld-based quantitative phase analysis and image processing, which provides quantitative information relative to the distribution of the various phases present in the studied samples. In particular, it is shown how this method allows the estimation of the local volume fraction of the phase ettringite within a hydrating cement paste, and construction of a three-dimensional distribution map. Application of this method to the various constituents of a cementitious material, or, more generally, of a composite polycrystalline material, may provide a non-invasive tool for three-dimensional microstructural quantitative characterization.

Dehydration and rehydration processes in gmelinite: An in situ X-ray single-crystal study

Abstract The dehydration-rehydration process in gmelinite-Na |Na7.27K0.28Ca0.15(H2O)21.85|[Al7.71Si17.24O48]- GME, a natural zeolite that can be described as a parallel stacking of double six rings of tetrahedra in the AABB sequence, was studied by X-ray diffraction data. Its space group is P63/mmc with a = 13.764(1) and c = 10.078(1) Å cell parameters. Single-crystal data collections were performed at room conditions and at increasing temperatures, in a hot nitrogen stream, up to the fragmentation of the crystals, which occurs at a temperature as low as 100 °C, and afterward the crystal was cooled down to room conditions. X-ray powder diffraction data showed that gmelinite-Na transforms into a new structure with an AFI-type topology at about 300 °C. At room conditions, extraframework cations are located in two symmetrically independent positions, both of which are coordinated to either framework O atoms or water molecules. When the mineral is heated to 90 °C, about 40% of H2O is lost, and one cation site splits over two positions, which are three-coordinated to the framework O atoms. The dehydration process is completely reversible over a period of hours. X-ray single-crystal data has highlighted that gmelinite-Na when quenched at 100 K displays remarkable modifications in its extraframework content, resulting in a strong disorder in its extraframework ions. As in the case of heating, the mineral restores its structural features when brought back to room temperature.

X-ray powder diffraction clustering and quantitative phase analysis on historic mortars

This paper describes the application of cluster analysis to X-ray powder diffraction patterns (XRPD) to define homogeneous groups of mortar-based materials according to their mineralogical composition. For this purpose, the diffraction patterns of 110 samples of mortars from the Temple of Venus (Pompeii, southern Italy) were used to test the method. Rietveld refinement, for quantitative mineralogical phase analysis, was performed on the most representative sample of each cluster. The mineralogical grouping yielded by cluster analysis of XRPD data turned out to be consistent with the petrographic groups.

3D imaging of complex materials: the case of cement

Abstract Absorption-based X-ray micro-tomography (X-μCT) provides fundamental in-situ information on the 3D microstructure of complex multiphase materials such as cements. However, since the phases present in a hydrating cement paste may be characterized by similar values of the attenuation coefficient, leading to low absorption contrast between different crystalline or amorphous phases, micro-structural interpretation can be equivocal. 3D phase mapping by X-ray diffraction micro-tomography proved to be a successful technique for investigating the spatial distribution of the products in the paste during the hydration process, in a totally non-invasive mode and with enhanced phase selectivity compared to absorption tomography. Phase-selective maps, in the case of crystalline phases, can be extracted from single Bragg peaks or from the Rietveld-refined scale factor. However, even poorly crystalline and/or amorphous phases present in the cement paste, such as calcium silicate hydrates, can be successfully mapped by the use of selected portions of the measured powder data containing the relevant scattering of the phase. The reconstructed maps can be directly modeled by multifractal analysis and compared with computer-generated distributions.