CHEMICAL BONDING AND ELECTRONIC STRUCTURES OF THE Al_2Si_2O_5(OH)_4 POLYMORPHS KAOLINITE, DICKITE, NACRITE, AND HALLOYSITE BY X-RAY PHOTOELECTRON SPECTROSCOPY

抄録

A detailed analysis was undertaken of the X-ray photoelectron spectra of the three polymorphs of Al_2Si_2O_5(OH)_4; kaolinite, dickite, nacrite plus the related mineral halloysite. Comparison of the spectra was made based on the chemical bonding and structural differences in the Al- and Si-coordination within each polymorph. The spectra for Si(2p) for all four polymorphs are nearly identical, consistent with the fact that all the Si atoms are in 4-fold (tetrahedral) coordination, whereas the binding energies for Al(2p) vary slightly depending on the type of polymorph and the corresponding change in the stacking order of the layers. The overall shapes of the O 1s peaks observed in the four polymorphs are similar. Theoretically the ratio of oxygen atoms versus oxygen in hydroxyl groups is 5:4 (55% vs 45%). For all four polymorphs the observed values are between 53 and 58% for the oxygen atoms and 38 to 45% for the oxygen atom in the hydroxyl groups. The lower-VB spectra for the kaolin polymorphs are similar to those of α-SiO_2 in terms of binding energies, but appear 〜2 eV higher than that for α-Al_2O_3. Compared with O 1s peaks, the lower VB peaks are considerably broader (〜3-4 eV FWHM), and therefore, detailed structures cannot be resolved. Nevertheless, this difference implies that the bonding character for the kaolin polymorphs is more covalent than that of α-Al_2O_3, but similar to that of α-SiO_2.