1,720,968 research outputs found
M1-site in dioctahedral micas: a novel approach from information theory to fix its content
No full textThe reliability of the Maximum Entropy Method MEM)
to reconstruct finite temperature electron density
(ED) is here discussed, investigating the case of
periclase (MgO). A theoretical electron density has been generated by quantum mechanic calculations and folded with a function simulating
atomic thermal motion, in order to produce a reference errorless ED [q(r)REF]. The Fourier coefficients of q(r)REF have been calculated, and used as “observed” diffraction intensities to reconstruct via MEM the original ED. The electron density attained by MEM [q(r)MEM] and q(r)REF have
been compared with each other (pixel-by-pixel and
critical points) to assess the ability of MEM to retrieve EDs, on the basis of a set of observed structure factors. We have carried out our study varying the number of observed structure factors [i.e. sin (q)/l cut-off], the nature of the prior-density [uniform density and procrystal-like model] and the way in which the prior-density
is treated during MEM maximization [fixed or free
to change]. We observe that (i) it is recommendable to use the prior-density as a start point only, and allow it to change during maximization; (ii) the closer is the prior-density
o q(r)REF, the easier one attains by MEM a correct
D; (iii) if the prior-density is varied and a sufficient large number of observed structure factors used, then MEM tends to yield converging EDs, regardless of the prior-density chosen as a start point
Structural evolution of a 3T phengite mica up to 10 GPa : an in-situ single-crystal X-ray diffraction study
No full textThe high-pressure structural evolution of a natural 3T- phengite [(K0.90Na0.05)Σ=0.95(Al1.51Mg0.32Fe0.18Ti0.03)Σ=2.04 (Si3.40Al0.60)O10(OH)2, a = b =5.2279(11) and c = 29.752(7)Å, space group: P3112] from Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to ~10 GPa. Nine structural refinements were performed at selected pressures within the P-range investi-gated. The compressional behavior of the same phengite sample was previously studied up to ~27 GPa by synchro-tron X-ray powder diffraction, and the corresponding P-V curve was modeled by a third-order Birch–Murnaghan Equation of State (BM-EoS). The significant elastic anisot-ropy of the 3T-phengite (i.e. (c)>> (a)) is mainly con-trolled by the compression of the K-polyhedra. The evolu-tion of the volume of the inter-layer K-polyhedron as a function of P is monotonic, without any evidence of discon-tinuity. Fitting the P-V data with a truncated second-order BM-EoS, we obtain a bulk modulus value of K0(K-polyhedron) = 35(3) GPa. The tetrahedra and octahedra in the 3T-phengite structure are significantly less compressible than the K-polyhedron, and behave similarly to rigid units within the P-range investigated. The main P-induced effect on the tetrahedral sheet consists in a cooperative rotation of the tetrahedra, describable by the evolution of the “tetrahedral rotation angle” (or “ditrigonal rotation angle”, ) as a func-tion of P. The value of the ditrigonal rotation angle in-creases significantly with P: (°) = P0 + 0.57(2)P (GPa) [R~99%]. The volume of the K-polyhedron and the value of ditrigonal rotation parameter () are not independent of one another, showing a correlation of about 99%
Structural evolution of coexisting 3T and 2M1 phengite micas up to 11 GPa: An in-situ single-crystal X-ray diffraction study
No full textThermal expansion and de-hydroxylation of phengite micas
No full textPhengite samples (2M 1 and 3T politypes) and a synthetic end-member muscovite specimen were studied by in situ high-temperature synchrotron radiation X-ray diffraction. The measured volume thermal expansion of 2M 1 phengite ( ≈ 36.6 × 10−6 K−1) was systematically greater than of the 3T polytype (≈33.3 × 10−6 K−1). A positive linear correlation between the average thermal expansion on (001) plane and the mean tetrahedral rotation angle at ambient condition is proposed on the ground of new measurements and literature data. Dehydroxylation processes were observed in 2M 1, starting at 1,000 K in 3T at 800 and 945 K in synthetic muscovite. Rietveld refinements allowed a determination of structural variations upon heating of phengite samples and their dehydroxylate phases. The phengite structure expands by regularizing the tetrahedral sheet and by reducing the bond length differences between the outer and inner coordination shell of the interlayer site. The dehydroxylate phase derived from 2M 1 is characterized by fivefold polyhedra in the low temperature form as a consequence of two OH groups reacting to form H2O + O (residual). The dehydroxylate exhibits an increase of the cation–cation distances along the M–Or–M bonds with respect to low-temperature phengite structures. For the 3T phase, we were unable to achieve completion of dehydroxylation. The refined structural model of the dehydroxylate phase shows two hydroxyl sites, but at a short distance from one another. This result suggests that the dehydroxylation reaction did not proceed to completion
Elastic properties and stability of coexisting 3T and 2M(1) phengite polytypes
No full textThe elastic properties of coexisting natural 3T and 2M (1) phengite samples (Cima Pal, Sesia Zone; Val Savenca; Western Alps, Italy) with similar chemical compositions have been studied by room temperature-high pressure powder diffraction, using synchrotron radiation on the ID9A beam-line at ESRF (Grenoble, France). The P-V curves have been modelled by the Birch-Murnaghan model; a third-order expansion fitted to the experimental data yields for 3T and 2M (1) K (0)=60.4(+/- 0.7) GPa, K'=5.79(+/- 0.11) at V (0)=703.8851 angstrom(3), and K (0)=57.3(+/- 1.0) GPa, K'=6.97(+/- 0.24) at V (0)=938.8815 angstrom(3), respectively. The relative stability of 3T vs. 2M (1) has been explored as a function of pressure and temperature in terms of configuration and deformation contributions to the Gibbs energy, using the elastic properties determined here and other thermodynamic parameters from earlier investigations. The results presented agree with the hypothesis of stability of the 3T polytype in the high pressure regime
Structural evolution of a 2M1 phengite mica up to 11 GPa : an in-situ single-crystal X-ray diffraction study
No full textThe structural evolution at high pressure of a natural 2M 1-phengite [(K0.98Na0.02)Σ=1.00(Al1.55Mg0.24Fe0.21Ti0.02)Σ=2.01(Si3.38Al0.62)O10(OH)2; a = 5.228(2), b = 9.057(3), c = 19.971(6)Å, β = 95.76(2)°; space group: C2/c] from the metamorphic complex of Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to ~11 GPa. A series of 12 structure refinements were performed at selected pressures within the P range investigated. The compressional behaviour of the same phengite sample was previously studied up to ~25 GPa by synchrotron X-ray powder diffraction, showing an irreversible transformation with a drastic decrease of the crystallinity at P > 15–17 GPa. The elastic behaviour between 0.0001 and 17 GPa was modelled by a third-order Birch–Murnaghan Equation of State (BM-EoS), yielding to K T0 = 57.3(10) GPa and K′ = ∂K T0/∂P = 6.97(24). The single-crystal structure refinements showed that the significant elastic anisotropy of the 2M 1-phengite (with β(a):β(b):β(c) = 1:1.17:4.60) is mainly controlled by the anisotropic compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of P shows a negative slope, Fitting the P–V(K-polyhedron) data with a truncated second-order BM-EoS we obtain a bulk modulus value of K T0(K-polyhedron) = 26(1) GPa. Tetrahedra and octahedra are significantly stiffer than the K-polyhedron. Tetrahedra behave as quasi-rigid units within the P range investigated. In contrast, a monotonic decrease is observed for the octahedron volume, with K T0 = 120(10) GPa derived by a BM-EoS. The anisotropic response to pressure of the K-polyhedron affects the P-induced deformation mechanism on the tetrahedral sheet, consisting in a cooperative rotation of the tetrahedra and producing a significant ditrigonalization of the six-membered rings. The volume of the K-polyhedron and the value of the ditrigonal rotation parameter (α) show a high negative correlation (about 93%), though a slight discontinuity is observed at P >8 GPa. α increases linearly with P up to 7–8 GPa (with ∂α/∂P ≈ 0.7°/GPa), whereas at higher Ps a “saturation plateau” is visible. A comparison between the main deformation mechanisms as a function of pressure observed in 2M 1- and 3T-phengite is discussed
Maximum entropy method: an unconventional approach to explore observables related to the electron density in phengites
No full textThe maximum entropy method (MEM) is used here to get an insight into the electron density [ρ(r)] of phengites 2M 1 and 3T, paying special attention to the M1-formally empty site and charge distribution. Room temperature single crystal X-ray diffraction data have been used as experimental input for MEM. The results obtained by MEM have been compared with those from conventional structure refinement which, in turn, has provided the prior-electron density to start the entropy maximization process. MEM reveals a comparatively non-committal approach, able to produce information related to the M1-site fractional occupancy, and yields results consistent with those from the difference Fourier synthesis, but free of the uncertainties due to the abrupt truncation of the series. The charge distribution is investigated by means of the notion of ‘‘site basin’’, i.e., those site-centered volumes delimited by a surface such as ∇ρ·n = 0. In particular, we observe: (1) the overall partitioning of the basin total charge between cation and anion sites, and the interlayer site charge seems to depend on sample composition, and (2) the apical-oxygen plane total basin charge and hydroxyl basin charge are presumably related to the polytype. The MEM-determined electron density does not allow full exploration of the critical points for very complex structures as micas, insofar as conventional room temperature experimental diffraction data are used
Equation of state and compressibility of pholgopite by in situ high pressure X-ray powder diffraction experiments
No full textThe elastic properties of a natural phlogopite have been studied by in-situ high-pressure X-ray powder diffraction experiments on the ID9 beamline at the ESRF, at room temperature. Several EoS models (Birch-Murnaghan, Vinet, Poirier-Tarantola) have been fitted to the experimental P-V data and the results obtained are presented and discussed. The third-order Birch-Murnaghan EoS, assuming Vo fixed at its experimental value, yields Ko=49.7(±0.5) GPa, Ko'=8.59(±0.19), Ko"=-0.5953 GPa-1 (implied value). The axial compressibilities at room conditions, determined by the third-order Birch-Murnaghan EoS, result in βEoSa0=3.48(5), βEoSb0=3.2(1), βEoSc0=13.2(1) 10-3 GPa-1. Equilibrium thermodynamic calculations have been carried out to show how the new elastic parameters here reported affect the stability field of phlogopite
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