1,721,053 research outputs found
Elastic behavior, phase transition, and pressure induced structural evolution of analcime
No full textElastic and structural behavior of a natural cubic analcime (space group:Ia3d)from Sardinia (Italy) was investigated at high pressure by in situ single-crystal X-ray diffraction. A first-order phase transition is observed in the pressure range between 0.91(5) and 1.08(5) GPa. Unit-cell constants and reflection conditions confirm that the space group of the HP-polymorph is P1. No further phase-transition has been observed at least up to 7.14 GPa. Fitting the volume data of the cubic polymorph with a second-order BM-EoS we obtain: V0 = 2571.2(4) Å3, KT0 = 56(3) GPa, and K′ = 4 (fixed). For the triclinic polymorph, a third-order EoS gives: V0 = 2607(9) Å3 KT0 = 19(2) GPa, and K′ = 6.8(7). Axial bulk moduli of the triclinic polymorph, calculated with a linearized BM-EoS, are: KT0(a) = 29(2) GPa, with K′(a) = 4.9(6) and a0 = 13.727(10) Å; KT0(b) = 20(1) GPa, with K′(b) = 5.2(5), and b0 = 13.751(15) Å; KT0(c) = 11 (1) GPa, with K′(c) = 12.6(6) and c0 = 13.822(31) Å. The elastic behavior of the HP-polymorph appears to be strongly anisotropic, being KT0(a):KT0(b):KT0(c) = 2.64:1.82:1.00. The relevant structural variations in response to the cubic → triclinic phase transition are due to tetrahedral tilting. The tetrahedral framework distortion gives rise to a change of the eight- and six-ring channels ellipticity and of the extra-framework topological configuration: it appears in fact that for the high-pressure triclinic polymorph the coordination number of some of the Na atoms is seven (2H2O + five framework O atoms) instead of six (2H2O + four framework O atoms)
Ab initio calculation at DFT level of NaGaSi2O6 and LiAlSi2O6 pyroxenes: a comparison from the Bader's topology viewpoint
No full textThe crystal structure of intermediate clinopyroxenes along the join diopside-enstatite (CaMgSi_2O_6-Mg_2Si_2O_6)
No full textThe structures of five clinopyroxenes on the diopside–enstatite join have been refined by Rietveld analysis of powder-diffraction patterns obtained with synchrotron radiation. The reliability of the present Rietveld refinements was checked against previous single-crystal data on the sample with composition Di80En20, and it showed reasonable agreement. A difference in the y/b M2 coordinate is related to the lower resolution of the powder-diffraction data of this work, not sufficient to detect the splitting at the M2 site. The Rietveld analysis allowed us to detect the strain modulations on (010) prior to exsolution. The small size of primitive antiphase domains, less than the length-scale of X-ray diffraction, prevents P21/c refinement in samples with composition Di59En41 and Di52En42CaTs2. The refined clinopyroxenes encompass the transition from P21/c to C2/c structures. In the P21/c structure, the A and B chains of tetrahedra undergo a slight straightening with increasing Ca content, and changes similar to those observed at high temperature. The results are in general agreement with large changes in the configuration of the chains at the transition, as observed during the P21/c – C2/c transition at high pressure and high temperature
Arsenic-rich fergusonite-beta-(Y) from Mount Cervandone (Western Alps, Italy) : crystal structure and genetic implications
No full textAn As-rich variety of fergusonite-beta-(Y) occurs as greenish yellow pseudo-bipyramidal crystals up to 1 mm in length in centimeter-sized secondary cavities within sub-horizontal pegmatite dikes at Mount Cervandone (Western Alps, Italy). The mineral is associated with quartz, biotite, potassium feldspar, and orange-yellow barrel-shaped hexagonal crystals of synchysite-(Ce) up to 2 mm in length. Fergusonite-beta-(Y) crystallized during the Alpine metamorphism under amphibolite-facies conditions, as a result of interaction between As-enriched hydrothermal fluids, circulating through the pegmatite dikes, and precursor accessory minerals in the pegmatites enriched in high-field-strength elements. These pegmatites are of NYF (niobium-yttrium- fluorine) geochemical type and served as the principal source of Be, Y, Nb, Ta, and rare-earth elements (REE) that were liberated and redeposited as rare Be-As-Y-REE minerals, including the As-rich fergusonite-beta-(Y). The latter mineral crystallizes with monoclinic symmetry [a = 5.1794(14), b = 11.089(3), c = 5.1176(14) Å, β = 91.282(8)°, V = 293.87(14) Å3, space group I2/a] and has the empirical formula (Y 0.70Dy0.07Er0.05Ca0.05Gd 0.02U0.02Yb0.01 Tb0.01Th 0.01Nd0.01)Σ0.95 (Nb 0.68As0.275+W0.06Ta 0.01Si0.01)Σ1.03O4. The crystal structure of fergusonite-beta-(Y) has been refined using a thermally untreated single crystal to R1 = 6.6% for 441 observed reflections with Fo/σFo > 4. The incorporation of As in the structure of monoclinic fergusonite-type phases is discussed in the context of the data available for synthetic analogs
New insight into the crystal chemistry of alkali-poor beryl: the real topological configuration of the extra-framework content
No full textThe crystal chemistry of natural alkali/water-poor beryl from Gilgit, Pakistan (H2O+Na2O+Cs2O<1.2wt%) is reinvestigated by means of laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, neutron diffraction and polarised infrared spectroscopy in order to determine the real topological configuration of the extra-framework content in the 6-membered ring channels. Fine chemical analysis was performed by means of laser ablation inductively coupled plasma mass spectroscopy with a laser ablation system made by Wave UP213 (Nd:YAG laser source) coupled with ICP-MS Thermo Electron X7. Calibration was performed using NIST SRM 612 as external calibration sample in conjunction with internal standardization using 29Si, previously measured by EMPA-WDS. The following masses (isotopes) have been analysed: 7Li, 9Be, 23Na, 24Mg, 27Al, 39K, 44Ca, 55Mn, 56Fe, 85Rb and 133Cs.
Thermal analysis were performed with a NETZSCH STA 449C instrument using 7.442 mg of powdered sample, previously dried at 130oC for 4h. The thermal curve (weight loss vs T, 1oC/min), showed a monotonic trend that became flat at about 1100 oC. The experiment was conducted up to 1200 oC. The calculated amount of H2O, on the basis of the weight loss, is 0.86 wt%. The chemical composition of sample is: Al1.93Fe0.03Be2.94Si5.93O18(Na 0.03 0.26H2O).
Single-crystal polarised IR-spectra were obtained with a Bruker IFS-120-HR spectrometer coupled with a Bruker IR-microscope. In order to avoid absorption bands due to external contaminants (H2O vapour or CO2), the optics of the spectrometer were kept under vacuum and the optics of the microscope were continuously purged with purified air (free of H2O and CO2) during the measurements. The resolution of the spectra was 1 cm-1. IR-spectra were obtained using radiation polarised parallel to the a and c axes of the beryl samples. To do this, two small single-crystals were oriented using a Huber four-circle X-ray diffractometer, and then cut and polished in thin platelets (50 μm thick) with the polished surfaces parallel to (001) and (100) respectively.
The single-crystal neutron diffraction experiment was performed at room T with a Huber four-circle diffractometer (SV28 beam-line) installed at the DIDO reactor - Forschungszentrum Juelich, Germany. The incident radiation (CW, λ=1.24126Å) was obtained using a Cu(200) monochromator. The unit-cell parameters of the neutron measurement are: a=b=9.2099(35)Å, c=9.1894(18)Å (c/a=0.9977). A total of 1534 reflections were recorded (2θmax = 100.78 ̊), of which 191 were unique. The systematic extinction rules agreed with the space group P6/mcc. Diffraction data were then corrected for Lorentz effect. No absorption correction was applied. Analysis of the nuclear density Fourier map suggests that the (water) oxygen is located along the 6-fold axis at the 2a-position site (0,0,1/4), whereas the (water) protons are at -0.028(7), -0.071(3), 0.332(1). The hydrogens are distributed in 6 x 2 equivalent positions, above and below the oxygen site. Geometrical configuration of the water molecule is well defined: the O-H bond distance is 0.949(18) Å and the H-O-H bond angle is 106.9(2.2) ̊. The H•••H vector is oriented at ~4 ̊ from [001]. This configuration is completely different from that found in alkali-rich beryl, where the H•••H vector is perpendicular to [001]. Na is also located, with the H2O-oxygen, at the 2a-position site. The final agreement index (R1) of the structural refinement was 0.037 for 34 refined parameters and 160 unique reflections with Fo > 4σ(Fo). The topological configuration of the H2O-molecule into the channel is confirmed by the spectroscopic investigation. Polarised single-crystal IR spectra show that the H2O-molecule is oriented with the molecular symmetry axis perpendicular to the hexagonal axis and H•••H vector parallel (or quasi-parallel) to [001]
The effect of Ca substitution on the compressional behavior of enstatite (Mg2Si2O6) up to 10 GPa
No full textElastic and structural behaviour of analcite at high pressure
No full textAnalcite (ANL), NaAlSi2O6H2O, is a natural zeolite (or felspathoid). The crystal structure is based on a Secondary Building Unit (SBU) consisting of four-member tetrahedral rings, which are joined to form a complex tetrahedral framework with two different systems of channels: highly distorted 8-rings and regular 6-ring channels. Na and H2O lie into the structural voids. Two H2O and four framework oxygens coordinate the Na-atoms. Elastic and structural behaviour of a natural cubic ANL from Sardinia (Italy) was investigated at HP by in situ single-crystal X-ray diffraction. A first-order phase transition was observed at P = 0.98(7)GPa. Lattice parameters and reflection conditions confirm that the HP-polymorph has a P-1 space group. The experiment is in progress but no further phase-transition has been observed at least up to 6.3 GPa. Fitting the volume data of the cubic polymorph with a BM-EoS we obtain: V0= 2571.2(4)Å3, KT0= 56(3) GPa and K’= 4 (fixed). For the triclinic polymorph: V0= 2618(13)Å3, KT0= 17(1) GPa and K’= 7(1). Tetrahedral tilting produces the relevant structural variations in response of the cubictriclinic phase transition. The SBU distortion gives rise to a change of the 8- and 6-ring channels ellipticity and of the extra-framework content topological configuration: it appear in fact that the coordination number of half of the Na atoms is 7 (2H2O + 5 framework oxygens) instead of 6
High-pressure behaviour of feldspathoids: the case of analcite
No full textFeldspathoids are low silica minerals and, similar to zeolites, have large openings in the crystal structure. Elastic and structural behaviour of a natural cubic feldspatoid analcite (NaAlSi2O6•H2O) was investigated up to 8.5 GPa by in situ single-crystal X-ray diffraction. A first-order phase transition was observed at P = 0.98±0.07GPa. Lattice parameters and reflection conditions show that the HP-polymorph has a P -1 Sp. Gr.. Volume data of the low-P (cubic) and high-P (triclinic) polymorphs were fitted with a second- and third-order Birch-Murnaghan Equation of State [1], respectively. The refined elastic parameters are: V0= 2571.2(4)Å3, KT0= 56(3) GPa and K’= 4 (fixed), for the cubic polymorph, V0= 2613(10)Å3, KT0= 18(1) GPa and K’= 7.2(7), for the triclinic polymorph. The elastic behaviour of the HP-polymorph, calculated on the basis of the linearised bulk moduli, appears to be strongly anisotropic (K(a):K(b):K(c) = 2.07:1.36:1.00). Tetrahedral tilting produces the main deformation mechanism in response of the cubic-triclinic phase transition. The distortion of the secondary building units gives rise to a change of the 8- and 6-ring channels ellipticity. As a consequence, the extra-framework topological configuration changes: it appears in fact that the coordination number of part of the Na atoms becomes 7 (2H2O + 5 framework oxygens) instead of 6 (2H2O + 4 framework oxygens).
[1]Birch,F., Phys. Rev., 1947, 71, 809
High-brilliance X-ray system for high-pressure in-house research: applications for studies of superhard materials
No full textNewly installed, at Bayerisches Geoinstitut, X-ray system consists of three major components:
RIGAKU FR-D high-brilliance source, Osmic’s Confocal Max-Flux optics, and SMART APEX 4K
CCD area detector. We tested the system for single-crystal structural studies on example of garnet,
Co3Al2Si3O12, for powder diffraction analysis on example of a small sample of boron carbide, B4C,
recovered after treatment in a multi-anvil apparatus, and for in situ experiments at high pressure
on example of FeO compressed in a diamond anvil cell to >100 GPa. In all the cases, the system
demonstrated its reliability and provided results comparable with those obtained using the best currently
available instruments. Using a new X-ray system, we measured the compressibility of superhard
nanocrystalline diamond at pressures up to 26 GPa and found it to be extremely incompressible with
KT = 488(4) GPa, K' = 3.1(2), and V0 = 3.401(6) cm3/mol
Temperature dependence of yttrium partitioning between garnet and xenotime : an experimental study
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