No abstract

A survey is presented on some characteristic features of meteoritic (Fe,Ni) 3 P which is an abundant and important minor phase of most iron meteorites. This mineral (named schreibersite/rhabdite) plays a decisive role during the formation of the so-called Widmanstätten pattern. Different transmission as well as scanning electron microscopic techniques have been applied to get more precise information about the real structure of the phosphide crystals, their chemical composition and the metal distribution across the phoshide/ kamacite interface. X-ray crystal structure determinations have been performed for selected (Fe,Ni) 3 P -cystals from various iron meteorites (Toluca, North Chile, Watson, Orange River, Morasko, Agpalilik, Odessa, Canyon Diablo). These experiments revealed a metal ordering, i.e. for the three non-equivalent metal positions a different substitution of Fe by Ni has been found. The perfection of the brittle (Fe,Ni) 3 P samples differs appreciably and seems to be dependent on the thermal history of each individual meteorite. Moreover, inside Ni-rich rhabdite crystals small monocrystalline inclusions of CrN (carlsbergite) have been detected.

The suitability of the electron back-scatter diffraction technique (supported by EDS) in order to study the complex microstructures of iron meteorites is demonstrated on the example of the Watson meteorite. The orientation relationships between the main phases kamacite, taenite and schreibersite/rhabdite as well as effects of the real structure have been investigated. In kamacite bands highly deformed blocks appear which show a contineous change of orientation. Plessitic regions are surrounded by deformed taenite lamellae. Also these lamellae show the typical M-profile of the Ni concentration in cross section. In the center a martensitic microstructure has been proven. The white plessite is characterized by a high number of individual kamacite grains which however are separated mainly by low-angle boundaries. So an orientation clustering occurs. The determination of orientation relationships was only possible for a single plessite region comparing the intensity distribution in pole figures with simulations. Schreibersite is brittle and shows a high number of microcracks. However, the strong deformation of kamacite does not allow us to decide whether an orientation relationship between the phosphides and the surrounding kamacite exists or not.

The orientation relationship (OR) between the bcc and fcc phase in the plessite microstructure of the iron meteorites Watson, Agpalilik and Gibeon has been analysed in a scanning electron microscope using electron back-scattered diffraction (EBSD). A very strong OR exists, independently on the analysed plessite type and the observed spreading of single orientation data. The agreement between the experimental orientation distribution and existing models varies for each meteorite. The black plessite in the Agpalilik corresponds to the Nishiyama-Wassermann model whereas the duplex plessite of the Gibeon meteorite shows an OR close to the Kurdjumov-Sachs model. The Watson meteorite is strongly deformed so that a general OR is difficult to determine due to the blurred experimental orientation distribution.

No abstract

The line intensity of Kossel reflections of various single crystals ( A l , A2, B3, B4 and C16 type of structure) has been studied experimentally as well as theoretically. In contrast to earlier results high excess intensity of experimental Kossel lines can be explained solely by dynamical diffraction effects without an additional consideration of real structure effects. It has been shown that the different maxima of intensities of Kossel lines found above all result from the different line width, depending strongly on the X-ray wavelength used (2 = 0.47 -8.34 A): Sole convolution of the theoretical profiles partly transforms the differences in the line width into variation of height.Es erfolgt eine Untersuchung der Linienintensitat von Kosselreflexen unterschiedlicher Einkristalle (Al, A2, B3, B4 und C16 Strukturtyp) sowohl experimentell als auch..theoretisch. Im Gegensatz zu friiheren Annahmen ergab sich, daR experimentelle Kossellinien hohe UberschuRintensititen besitzen konnen, die sich allein aus der dynamischen Beugungstheorie ergebenohne zusatzliche Berucksichtigung moglicher Realstruktureffekte. Es zeigte sich, daR die unterschiedlichen Maximalintensitaten experimenteller Kossellinien vor allem durch die mit der Wellenlange (i = 0.47 -8.34 A) der charakteristischen Rontgenstrahlung stark variierende Breite der Profile verursacht wird: Lediglich der notwendige FaltungsprozeR transformiert unterschiedliche Linienbreiten teilweise in Hohenunterschiede.

Synchrotron radiation diffraction studies of meteoritic (Fe,Ni)3P crystals have been performed to reveal the ordering of the elements Fe and Ni on the three metal sites M1, M2 and M3 of the unit cell. The delta synthesis technique, which is a two-wavelength method using anomalous dispersion effects, was applied. For (Fe,Ni) phosphide crystals with different Fe:Ni ratios extracted from different meteorites, it was found that Ni occupies the M3 site and also partially the M2 site, avoiding the M1 position, whereas the M1 site is preferentially occupied by Fe. In connection with earlier results known from the literature, this metal distribution seems to be characteristic of this compound, and is independent of thermodynamic formation conditions.

For the evaluation of Kossel experiments on quasicrystals such as the decagonal phase of Al -Co -Ni or Al -Co -Cu-Si a computer program called KOQUA has been written, which allows to simulate Kossel patterns of two-and three-dimensional quasicrystals as well as of every conventional crystal system. In some examples the application of the program is demonstrated.Zur Auswertung von Kosselexperimenten an Quasikristallen (z. B. der dekagonalen Phasen von Al -Co -Ni oder Al -Co -Si) wurde das Programm KOQUA erstellt, welches die Simulation von Reflexsystemen zwei-und dreidimensionaler Quasikristalle, sowie aller klassischen Kristallsysteme gestattet. Anhand einiger Beispiele wird die Anwendung dieser Software demonstriert.