EXAMINATION OF Ti XANES SPECTRA OF MINERALS AND SOLIDS : EFFECTS OF SITE GEOMETRY ON SPECTRAL FEATURES

The X-ray absorption near edge structure of Ti in 16 oxides and silicates has been collected and analyzed by computer fitting Gaussian lines to the spectral envelopes. Comparison of feature intensities and energies with geometric site parameters reveals that the near edge changes systematically with site symmetry variation and mean Ti-O bond length

DETERMINATION OF SHORT RANGE ORDER (SRO) PARAMETERS FROM EXAFS PAIR DISTRIBUTION FUNCTIONS OF OXIDE AND SILICATE SOLID SOLUTIONS

Short Range Order (SRO) in certain solid solutions can be measured directly from observed EXAFS structure function peak areas. Results for MgO-FeO, MgO-LiFeO2, and CaMgSi2O6-CaFe2+Si2O6 solutions indicate that these are almost completely disordered at their synthesis temperatures. The necessity of structural modeling to obtain interatomic distances for varying compositions and degree of ordering is emphasized

Structure, chemistry, and properties of mineral nanoparticles

Nanoparticle properties can depart markedly from their bulk analog materials, including large differences in chemical reactivity, molecular and electronic structure, and mechanical behavior. The greatest changes are expected at the smallest sizes, e.g. 10 nm and below, where surface effects are expected to dominate bonding, shape and energy considerations. The precise chemistry at nanoparticle interfaces can have a profound effect on structure, phase transformations, strain, and reactivity. Certain phases may exist only as nanoparticles, requiring transformations in chemistry, stoichiometry and structure with evolution to larger sizes. In general, mineralogical nanoparticles have been little studied

Structures and Charging of Alpha-Alumina (0001)/ Water Interfaces Studies by Sum-Frequency Vibrational Spectroscopy

Sum-frequency vibrational spectroscopy in the OH stretch region was employed to study structures of water/{alpha}-Al{sub 2}O{sub 3} (0001) interfaces at different pH values. Observed spectra indicate that protonation and deprotonation of the alumina surface dominate at low and high pH, respectively, with the interface positively and negatively charged accordingly. The point of zero charge (p.z.c.) appears at pH {approx}6.3, which is close to the values obtained from streaming potential and second harmonic generation studies. It is significantly lower than the p.z.c. of alumina powder. The result can be understood from the pK values of protonation and deprotonation at the water/{alpha}-Al{sub 2}O{sub 3} (0001) interface. The p.z.c. of amorphous alumina was found to be similar to that of powder alumina

NEAR-EDGE STRUCTURE OF OXYGEN IN INORGANIC OXIDES : EFFECT OF LOCAL GEOMETRY AND CATION TYPE

NEXAFS measurements at the oxygen K-edge have been carried out on a variety of crystalline oxide samples to examine the effects of differences in local coordination environment of oxygen on near-edge structure. All spectra display an intense white line at about 543 eV and a strong shape resonance about 20 eV above the edge. The white line shifts to higher energies with increasing oxygen coordination number, although differences in nearest-neighbor type can affect this correlation. In addition, the Ca and transition-metal containing oxides display features 6-10 eV below and 6-9 eV above the white line. The intensities of these features vary inversely with the number of 3d-electrons but their energies are independent of cation oxidation state. Large changes in coordination number and geometry affect edge structure less than variations in types of nearestneighbor cations. The inverse distance-energy relationship is poorly obeyed in these compounds when oxygencation bond lengths are compared with the position of the strongest shape resonance for a given structure type. The near-edge structure is qualitatively interpreted using the results of recent Xα multiple scattered-wave calculations

EXAFS AND NEXAFS STUDIES OF CATION ENVIRONMENTS IN OXIDE GLASSES

X-ray absorption spectroscopic studies of cation environments in oxide glasses are selectively reviewed. New results are presented on K and Yb environments in silicate glasses and on Fe in silicate melts at temperatures up to 1173° K

Reanalysis of the Schwertmannite structure and the incorporation of SO42- groups: An IR, XAS, WAXS and simulation study

Schwertmannite is a poorly crystallized iron oxyhydroxide with essential structural sulfate that can be a major component in acid mine drainage environments. Original characterization work concluded that the sulfate was largely contained within tunnels of an ordered akaganeite-like structure based on powder XRD, analysis of IR spectra, and sulfate extraction procedures [1]. Since the original description, problems have emerged with the nature of the tunnel sulfate, and with the interpretation of the IR spectra. Other related work has shown that it is now possible to determine sulfate-iron oxide inner sphere binding unambiguously from the S K-edge XANES spectrum. Hence a reassessment of the evidence for the original schwertmannite structure was deemed necessary and timely