Vittorio Tazzoli

Structural mechanisms of solid solution and cation ordering in augite-jadeite pyroxenes: I. A macroscopic perspective

Abstract Infrared spectra of ordered (P2/n) and disordered (C2/c) pyroxenes belonging to the join augite-jadeite were investigated at room temperature in the range 70-1400 cm-1. The spectra change as a function of composition and the phase transition P2/n → C2/c produces an increase in band widths. The autocorrelation function quantified changes in line widths due to these effects. Phonons at low frequencies are very sensitive to both changes in composition and degree of order and are indicative of a non-ideal mixing behavior for the C2/c solid solution. Phonons at high frequencies depend on the average composition of the samples and not on local configurational changes related to the order-disorder transition. High degrees of local heterogeneity were suggested by the Δcorr values of the C2/c omphacites at intermediate compositions. This effect is most evident in the low frequency region of the spectra, corresponding to modes involving, primarily, the M cations. The changes in frequency due to ordering, Δω, and the change in line widths δ(Δcorr)100-200 for the spectral region 100-200 cm-1 and δ(Δcorr)210-800 for the range 210-800 cm-1 have been used to characterize the state of local order at different compositions. The spectra yield information related to the local structural states of augite-jadeite pyroxenes, down to a unit-cell length scale, which may then be compared with the average structure determined by X-ray diffraction studies of the same samples.

Sheet superconductivity in : crystal structure of the tetragonal matrix

Sheet conductivity was found in oxygen reduced . The sheets are aligned along the twin boundaries of the unreduced starting material . The bulk transforms during the oxygen loss to a phase with tetragonal crystal structure. The space group is with lattice parameters a = 0.739 nm and c = 0.388 nm. The perovskite-like structure contains distorted octahedra with W - O distances between 0.17 nm and 0.218 nm.

Structural properties of ferromagnesian cordierites

Abstract A set of natural and heat-treated cordierite crystals has been analyzed using single-crystal X-ray diffraction, EMPA, SIMS, and 57Fe Mössbauer-spectroscopy. Structure determination of natural cordierite shows that the average size of the tetrahedrally coordinated T11 cation increases systematically by about 0.01 Å as the Fe content of the solid solution decreases toward the Mg end-member. For Fe-rich compositions, the mean tetrahedral bond length T11-O is close to 1.749 Å, a value expected for an AlO4 tetrahedron in an alumosilicate framework structure. It is suggested that the structural dilation of the tetrahedral sites, which is driven by the decreasing average size of the octahedral cation, is sustained by substitution of the large cations Mg and Fe2+ for Al. Mössbauer spectroscopy shows that up to 11% of Fe2+ can be attributed to tetrahedral coordination in Mg-rich cordierite. Charge balance for the substitutions is either provided by introduction of Na+ into vacant Ch2(0,0,0) channel sites or by substitution of an additional Al3+ by Si4+. Tetrahedral site occupancies and corresponding Al,Si order parameters are calculated on the basis of a simple hard sphere model, based on refined cation-oxygen mean bond length, chemical composition, and Fe site-occupancy refinements. Almost complete Al,Si ordering among ring sites T2 is encountered for most natural cordierite samples. Al,Si ordering among the T1 tetrahedra is less pronounced for Fe-rich compositions. Partial disorder is attributable to Al/Si ratios in excess of 4/5. Orthorhombic shear strain is shown to correlate with the derived average order parameter, if corrections for strain contributions from size effects of the octahedral cation and from channel constituents are employed. Crystal-chemical similarities to other compounds and the implications of the minor cation substitutions for the properties of structural phase transitions in cordierite are discussed.

Kinetics of Fe2+-Mg distribution in aluminous orthopyroxenes

Kinetic studies of isothermal heating experiments (600–800° C) on aluminous pyroxenes (Mg0.942Fe0.8802+Fe0.0683+Mn0.016Ca0.010Al0.084) (Si1.848Al0.152) permit the determination of rate constant of isothermal disordering as 2.5457 E13(±1.4 E13) min−1. The activation energy is determined as 278 (±23) kJ/mol. Data on two other aluminous pyroxenes at 700° C indicate that the rate constant decreases significantly with increasing amount of trivalent cations. There is a similar but reverse correlation between the concentration of trivalent cations and the Fe2+-Mg equilibrium distribution between sites. The site distribution coefficient increases with increasing concentration of trivalent cations at constant temperature.

X-ray diffraction study of Fe2+-Mg order-disorder in orthopyroxene. Some kinetic results

AbstractKinetic rates of Fe2+-Mg disordering in three orthopyroxenes (mean value of XFe = Fe2+/(Fe2++Mg) = 0.175,0.482,0.770 respectively) have been determined employing heating experiments and single crystal X-ray structural refinements. Disordering rate constants

Kinetics of Fe2+-Mg order-disorder in P21/c pigeonite

(\vec K)

Order-disorder kinetics in orthopyroxene with exsolution products

(550800° C) for two pyroxenes are given by: ln