Thomas Malcherek

Autocorrelation analysis of infrared spectra from minerals

The autocorrelation function, Corr(α, ω′) = \({\int}_{{-}{\infty}}^{{\infty}}\) α(ω + ω′)α(ω)dω, may be used to parameterise effective line widths of absorption bands in IR spectra. It has the advantage of not requiring any peak fitting to the primary spectra and can be applied to individual bands or groups of bands in a spectrum. A new procedure for analysing autocorrelation spectra which result from the application of the autocorrelation function to primary spectra is presented. The approach is well suited for quantifying line width variations in powder absorption spectra from sequences of samples with varying composition, degree of cation order or structural state. Worked examples are set out to illustrate different applications of the approach, including the characterisation of structural phase transitions in tridymite, Al/Si ordering under non-equilibrium conditions in Mg-cordierite, short range Al/Ge order in BaAl 2 Ge 2 O 8 feldspar and mixing behaviour in the jadeite-augite solid solution.

Localized defects in radiation-damaged zircon

The crystal structure of a radiation-damaged natural zircon, ZrSiO(4) (alpha-decay radiation dose is ca 1.8 x 10(18) alpha-decay events g(-1)), has been determined. The anisotropic unit-cell swelling observed in the early stages of the amorphization process (0.17% along the a axis and 0.62% along the c axis compared with the undamaged material) is a consequence of the anisotropy of the expansion of ZrO(8) polyhedra. Larger anisotropic displacement parameters were found for Zr and O atoms, indicating that the distortion produced by alpha particle-induced localized defects mainly affects the ZrO(8) unit. The overall shape of SiO(4) tetrahedra remains essentially undistorted, while Si-O bonds are found to lengthen by 0.43%.

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.

Structure and stability of Cd 2 Nb 2 O 7 and Cd 2 Ta 2 O 7 explored by ab initio calculations

Structural instabilities of

Micro-Raman study of copper hydroxychlorides and other corrosion products of bronze samples mimicking archaeological coins

{\text{Cd}}_{2}{\text{Nb}}_{2}{\text{O}}_{7}

Structures of the pseudo-trigonal polymorphs of Cu2(OH)3Cl

have been explored by all-electron ab initio calculations in the framework of density-functional theory. The calculated phonon-dispersion curve of the cubic high-temperature phase displays several modes with imaginary frequencies. A distortion according to a

Exploring the potential of Raman spectroscopy for crystallochemical analyses of complex hydrous silicates: II. Tourmalines

{T}_{1u}

Diffuse scattering anisotropy and the P21/a

mode at the

A2/a phase transition in titanite, CaTiOSiO4

\ensuremath{\Gamma}

High Structural Complexity of Potassium Uranyl Borates Derived from High-Temperature/High-Pressure Reactions

point leads to the energetically most favorable configuration. The resulting structure has been optimized in space-group symmetries