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Dissertation zugänglich unter
Annealing induced recrystallization of radiation damaged titanite and allanite
Thermisch induziertes Rekristallisationsverhalten von strahlengeschädigtem Titanit und Allanit
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Titanit , Allanit , Röntgenbeugung , Raman-Spektroskopie , Rekristallisation
Freie Schlagwörter (Deutsch):
Metamikt , Titanit , Allanit , Rekristallisation , Raman-Spektroskopie
Freie Schlagwörter (Englisch):
recrystallization , metamict , titanite , allanite , radiation damage
38.30 , 38.31
Bismayer, Ulrich (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
Minerals can become metamict over geological time periods as a result of structural damage induced by α-decay events resulting from incorporated radiogenic elements. The structural state and the thermally induced recrystallization behavior of metamict titanite were studied by Raman spectroscopy, synchrotron single-crystal X-ray diffraction, nanoindentation and complementary high-resolution transmission electron microscopy.
The results of the Raman spectroscopic measurements show that Raman scattering collected from metamict titanite is still anisotropic, which is typical of single crystals and indicates oriented, basically coherent structural elements. The observed Raman scattering dependence on the sample orientation is on the other hand much more pronounced for heavily metamict than for weakly metamict titanite samples. Radiation-induced anisotropic effects are related to the specific atomic arrangements in the structure of metamict titanite. This leads to the opportunity to study separately the structural transformations of the crystalline and amorphous fractions in metamict titanite by using Raman spectroscopy. That is possible because the Raman modes in the spectra collected from a plane nearly perpendicular to the chains of corner-sharing TiO6 octahedra arise predominantly from phonon modes in crystalline nanoregions with radiation-induced defects. In contrast to the contribution of atomic vibrations in radiation-induced amorphous nanoregions, which is better pronounced in spectra collected from a plane containing TiO6 chains. The results show that radiation-induced periodic faults in the crystalline matrix are related to the disturbance of SiO4-TiO6-SiO4-TiO6 rings comprising TiO6 octahedra from different chains. The radiation-induced amorphization by contrast is related to the partial change of Ti coordination from octahedral to pyramidal and/or tetrahedral, which leads to a rising Ti-O bond strength. This in turn violates the Ti-O-Ti intrachain linkages in the titanite structure. Hence, the planes containing Si-O-Ti-O bond rings are less susceptible to a self-accumulation of radiation-induced defects resulting in the development of amorphous regions as compared to the perpendicular plane containing Ti-O bond chains. Multistep annealing gradually suppresses the structural defects in the crystalline fraction of the titanite as the improvement of the SiO4-TiO6 connectivity within planes near perpendicular to the TiO6 chains reaches saturation near 900 K. Annealing-induced recrystallization of the radiation-induced amorphous nanoregions takes place in the temperature range between approximately 650 and 950 K, with a maximum near 750 K.
To determine the influence of radiation damage on the behavior of the elastic material properties of titanite, nanoindentation measurements were performed on partially metamict titanite (sample E2312) and for comparison additionally on nearly undamaged crystalline titanite (Rauris sample) and titanite glass. Metamict titanite E2312 shows hardness (H) and elastic modulus (E) values close to those of titanite glass. Rauris titanite shows strong anisotropy and the H and E values are clearly larger than those of E2312. Thermally induced stepwise recrystallization of metamict titanite E2312 leads to a decrease in the hardness until approximately 950 K and afterwards to an increase at higher temperatures, while the elastic modulus increases continuously (H and E values measured always at room temperature). Changes of the hardness and elastic modulus are related to increasing long-range order and vanishing amorphous interface areas in the titanite structure.
In further studies the structural recovery of the metamict epidot group mineral allanite (sample number R1) produced by thermal annealing was followed by powder X-ray diffraction, single-crystal synchrotron X-ray diffraction and infrared spectroscopy.
Allanite contains in contrast to titanite structural OH groups. But no evidences for a marked influence of this OH groups on the recrystallization behavior were found. Corresponding to structural recovery the measured high-temperature annealing XRD patterns and IR spectra of metamict allanite R1 reveal a marked increase in correlation length. The structural recovery of the allanite is also indicated by a detected decrease in the unit-cell volume during annealing. The synchrotron X-ray diffraction results show, that major changes in the crystallinity of metamict allanite occur within the first hour of annealing. A kinetic analysis following Sharp-Hancock points to the fact, that the recrystallization process follows at least two different structural mechanisms of reorganization. During the annealing experiments at 823 and 1073 K, no chemical decomposition of allanite was observed.