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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
Elastic 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
The effect of Ca substitution on the compressional behavior of enstatite (Mg2Si2O6) up to 10 GPa
No full textLeucite at high-pressure : elastic behaviour, phase stability and petrological implications
No full textElastic and structural behavior of a natural tetragonal leucite from the volcanic Lazium district (Italy) were investigated at high pressure by in-situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. A first-order phase transition, never reported in the literature, was observed at P=2.40.2 GPa from tetragonal (I41/a) to triclinic symmetry (diffraction intensities analysis suggests the space group P ), accompanied by a drastic increase of density of about 4.7%. The transition pressure was bracketed by several measurements in compression and decompression. No further phase-transition has been observed up to 7 GPa. Fitting a second-order Birch-Murnaghan Equation-of-State (BM-EoS) to the pressure-volume data of the tetragonal polymorph, we obtain: K0=41.9(6) GPa and K’= 4 (fixed). In the case of the triclinic polymorph, a second-order BM-EoS gives: K0= 33.2(5) GPa. The eulerian finite strain (fe) vs normalised stress (Fe) curves were calculated for the low- and high-P polymorphs, providing Fe(0)=42(1) and Fe(0)=33.2(4) GPa, respectively. The axial bulk modulus values of the tetragonal polymorph, calculated with a linearised BM-EoS, are: K0(a)=34.5(5) GPa and K0(c) = 78(1) GPa. For the triclinic polymorph, we obtain: K0(a)= 35.9(5), K0(b)= 34.9(7) and K0(c) = 35.5(7) GPa. The elastic behavior of the low-P polymorph appears to be more anisotropic than that of the high-P polymorph. The HP-crystal structure evolution of the tetragonal polymorph of leucite was studied on the basis of six structural refinements at different pressures between 0.0001-1.8 GPa. The main deformation mechanisms at high-pressure are due to tetrahedral tilting, giving rise to an increase of the ellipticity of the 4- and 6-membered rings of the tetrahedral framework. The tetrahedral T-O bond distances are practically invariant within the stability field of the tetragonal polymorph. The complex P-induced twinning, due to the tetragonal-to-triclinic phase-transition, and the low quality of the diffraction data at pressure above the phase-transition, did not allow the refinement of the crystal structure of the triclinic polymorph
Comparative compressibility and equation of state of orthorhombic and tetragonal edingtonite
No full textThe high-pressure (HP) behaviour of a natural orthorhombic and tetragonal edingtonite from Ice River, Canada, has been investigated using in situ single-crystal X-ray diffraction. The two isothermal equations of state up to 6.74(5) GPa were determined. V0, KT0 and K′ refined with a third-order Birch-Murnaghan equation of state (BM-EoS) are: V0 = 598.70(7) Å3, KT0 = 59(1) GPa and K′= 3.9(4) for orthorhombic edingtonite and V0 = 600.9(2) Å3, KT0 = 59(1) GPa and K′= 4.2(5) for tetragonal edingtonite. The experiments were conducted with nominally hydrous pressure penetrating transmitting medium. No overhydration effect was observed within the pressure range investigated. At high-pressures the main deformation mechanism is represented by cooperative rotation of the secondary building unit (SBU). Si/A1 distribution slightly influences the elastic behaviour of the tetrahedral framework: the SBU bulk moduli are 125(8) GPa and 111(4) GPa for orthorhombic and tetragonal edingtonite, respectively. Extra-framework contents of both zeolites show an interesting behaviour under HP conditions: the split Ba2 site at P > 2.85 GPa is completely empty; only the position Ba 1 is occupied
The mechanism of coupling in the modulated structure of nephline
No full textThe crystal structures of a nepheline, K0.54Na3.24Ca0.03□0.19Al3.84Si4.16O16, and that of the same sample annealed at high
temperature to induce K–vacancy disorder, have been determined at several temperatures down to 15 K by single-crystal
X-ray diffraction. The largest structural change in both crystals with decreasing temperature is the decrease of the T1–O1–T2
angle, corresponding to an increase in the tilt of the T1 and T2 tetrahedra within the framework. The tetrahedra in the annealed
sample have a smaller tilt than in the natural sample at any given temperature. The correlation of the tilts of the tetrahedra with
changes in the intensities of satellite reflections confirms that the satellites arise from a displacive modulation of the framework
of tetrahedra. Distance-least-squares simulations suggest that the modulation creates larger and smaller cavities within the
extra-framework channels that contain the K atoms. Analysis of the K–O bond lengths with both the state of K–□ order and temperature indicate that the coupling between K–□ order and the framework modulation occurs through the K–O2 bond. An increase in the average K–O2 bond length with decreasing temperature or increasing K–□ order supports the modulation of the framework. Shortening of the K–O2 bond leads to rotations of the tetrahedra that are opposite to those associated with the modulation, and thus suppresses it
Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order
No full textThe equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions,
were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K′, were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) Å3, KT0 = 193(1) GPa, K′ = 5.6(3) for the ordered sample and V0 = 528.39(2) Å3, KT0 = 192(1) GPa, K′ = 5.4(3) for the disordered one. Complete
intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence
on the degree of order in the MgAl2O4 spinel
Effect of non-hydrostatic conditions on the elastic behaviour of magnetite: An in-situ single-crystal X-ray diffraction study
No full textThe high-pressure elastic behaviour and the
pressure-induced structural evolution of synthetic magnetite
were investigated up to 11.11(5) GPa by means of
in situ single-crystal X-ray diffraction with a diamond anvil
cell, using the mix methanol:ethanol:water = 16:3:1 as
pressure-transmitting medium and the ruby-fluorescence
method for pressure-calibration. The evolution of the ruby
R1-fluorescence line with P, with a drastic increase of the
full-width-at-half-maximum (FWHM) of the Lorentzian
profile at P > 9 GPa, shows that the P-medium is not
hydrostatic above 9 GPa. Such a condition is well reflected
by the drastic increase (by 20–22%) of the FWHM of the
diffraction peak profiles of magnetite, by the behaviour of
the Eulerian finite strain versus normalized pressures plot
(fe–Fe plot) and by the slight EoS-misfit. However, the
diffraction data collected during decompression showed a
reversible/complete restoration of the diffraction profiles
and of the elastic behaviour in the fe–Fe plot. The reflection
conditions dictated by the Fd 3m space group confirm that
symmetry of magnetite is maintained within the P-range
investigated. The structural refinements performed at
0.0001, 4.99(3) and 9.21(8) GPa show that the evolution of
the oxygen u-parameter is almost constant within the
P-range investigated. A weighted linear regression through
the data points gives only a slight negative slope and no
discontinuity is observed within the P-range investigated.
A similar continuous behaviour is also observed in the
evolution of the T- and M-polyhedral volumes, bond distances
and angles with P. On the basis of data reported in
this study, it appears that the elastic behaviour and the
structural evolution of magnetite is drastically influenced
by the experimental conditions (i.e., hydrostatic or nonhydrostatic),
and diffraction data of magnetite collected
under non-hydrostatic conditions are unusable for a reliable
description of the elastic behaviour and for the P-induced
structural rearrangements. Comparisons are carried out
between the experimental findings of this study and those
reported in the previous ones, in which an inverse-to-direct
spinel phase transition at P > 8 GPa is suggested
Elastic behavior and phase-stability of pollucite, a potential host for nuclear waste
No full textThe elastic behavior and the phase stability of natural pollucite, (Cs,Na)16Al16Si32O96·nH 2O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P-induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic (Ia3̄d) to triclinic symmetry (P1̄). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch- Murnaghan Equation-of-State (BM-EoS), we obtain V0 = 2558.3(4) Å3, KT0 = 41(2) GPa, and K'T = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V0 = 2577.5(40) Å3, KT0 = 25.1(9) GPa, and K'T = 6.5(4). The axial bulk moduli of the high-pressure triclinic polymorph were calculated with a third-order "linearized" BM-EoS. The EoS parameters are a0 = 13.699(12) Å, KT0(a) = 25.5(17) GPa, and K'T(a) = 6.8(6) for the a axis; b0 = 13.728(12) Å, KT0(b) = 23.2(15)GPa, and K'T(b) = 7.7(7) for the b axis; c0 = 13.710(7) Å, KT0(c) = 25.2(10) GPa, and K'T(c) = 6.8(4) for the c axis [KT0(a):K T0(b):KT0(c) = 1.10:1:1.09]. Brillouin light-scattering was used to investigate the single-crystal elastic properties of pollucite at ambient conditions. The aggregate adiabatic bulk modulus (Ks) and shear modulus (G), calculated using the Voigt-Reuss-Hill averaging procedures, are K s = 52.1(10) GPa and G = 31.5(6) GPa. The elastic response of pollucite and other isotypic materials (e.g., analcime, leucite, and wairakite) is compared. The high thermo-elastic stability of pollucite, reflected by the preservation of crystallinity at least up to 9 GPa (at room T) and 1470 K (at room P) in elastic regime, the large amount of Cs hosted in this material (Cs2O ∼ 30 wt%), the immobility of Cs at high-temperature and high-pressure conditions, and the extremely low leaching rate of Cs, make of this open-framework silicate a functional material with potential use for fixation and deposition of Cs radioisotopes in high-level nuclear waste
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