Dr. Nina Daneu was invited to present the method of transmission electron microscopy for the determination of very small amounts of dopants at planar faults and interfaces (CEP) at the Slonano conference which was held in Ljubljana between 26th and 28th of October. The method was developed in cooperation with dr. Thomas Walther (The University of Sheffield). The method was developed for accurate and precise determination of chemical composition of inversion boundaries in ZnO, however, it can also be used in general for the determination of atomic structure and chemical composition of different types of planar faults and interfaces, for example twins in minerals. Theoretical background of the CEP method, its advantages compared to other techniques (like spatial difference technique) and examples of practical application of the CEP method for determination of chemical composition of twins in different minerals like sphalerite, pyrite, spinel, bixbyite and rutile were presented in the frame of the invited lecture. Twins in these minerals represent systems with different planar fault configurations. In sphalerite, the twin (111) boundary contains one atomic layer of oxygen which triggers the local wurtzite stacking and, in addition, some oxygen is adsorbed on the surface of the sulphide sample, which influences the measurements and must be taken in account in the interpretation of the results. (110) twins in pyrite contain about one atomic layer of copper, which results in linear relationship between the matrix-to-dopant ratio and beam radius. (111) twins in spinel contain beryllium, which cannot be determined with EDS. Beryllium, however, replaces magnesium cations near the twin boundary, therefore the depletion of magnesium was determined at the spinel twins. The 'twinned' bixbyite crystals actually contain multiple parallel antiphase boundaries along the {100} planes, which are Si-rich and the amount of Si at these boundaries was determined using the CEP method. Another system were the (301) twins in rutile, which contain a few nanometers thick lamella of a Fe-rich mineral. Using the CEP method, the chemical composition of these boundaries was determined to be ilmenite instead of hematite. The lecture was well accepted. The problem of Be-rich (111) twins in spinel was of special interest to Prof. Ferdinand Hofer, head of the Austrian Centre for Electron Microscopy and Nanoanalysis, who was highly interested for direct determination of beryllium at these boundaries and we discussed about possible future cooperation.
B.04 Guest lecture
COBISS.SI-ID: 25492263Young researcher Janez Zavašnik presented the results of research work on {110} twins and {010} stacking faults in pyrites from St. Katarina near Ljubljana at the 19th Conference on Material and Technology in Portorož. In the course of resolving the atomic structure and chemical composition of {110} twins in pyrite we set several possible structural models for the boundary based on the experimental HRTEM images. We calculated the simulated images at the given conditions and compared them with the experimental images. We ended with two most probable models for the {110} twins which have different translation between the pyrite domains and different ordering of the boundary atoms. Beside twins, {010} stacking faults are also common in pyrite. The stacking faults can be interpreted as marcasite monolayer within pyrite matrix. In contrast to twin boundaries, which are copper-rich, stacking faults do not have different chemical composition than the pyrite matrix.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 25305383Syenite pegmatites from Mt Malosa (Malawi) are known for their large variety of amphibole, pyroxene, and feldspar minerals. All these minerals commonly show abundant twinning after several different laws. Inspection of the available specimens of aegirine (NaFeSi2O6) and microcline (KAlSi3O8) have shown the presence of two new types of twins that have not been reported in the literature. Aegirine is known to form simple 180°contact or lamellar twins parallel to {100} planes. In addition to those we discovered a new law of twinning where the interface plane is (010). Microcline crystals from Mt Malosa are twinned very frequently on {001}, after the Manebach law, on {010} the Karlsbad law and less frequently on {021} the Baveno law. In addition to these, a new law of mirror (010) twinning, associated with the Manebach twins was observed. This twin has a different crystallographic operation (mirror instead of 180° rotation) if compared to the Karlsbad twin, although the composition plane is the same. They form fourlings, which crystallographically differ from Baveno-Manebach fourlings known in orthoclase and hyalophane.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 25142567