Dr. Nina Daneu presented an invited lecture at the Sobolev Institute for Geology and Mineralogy of the Siberian branch of the Russian Academy of Science (RAS) entitled 'Atomic-scale aspects of twinning, epitaxy and phase transformations in minerals'. The lecture included presentation of our previous and current results from the field of twinning in minerals with special focus on twinning in spinel, where we found that the formation of (111) twins in MgAl2O4 is triggered by the incorporation of a small amount of beryllium into the twin boundary structure causing the local hcp stacking within otherwise ccp spinel structure. In addition, we observed the formation of very interesting epitaxial overgrowths of taaffeite on the {111} planes of spinel. These morphological features closely resemble those in natural and synthetic diamonds where their formation and structure is not explained yet. Considering the fact that scientists at the Sobolev Institute for Geology and Mineralogy in Novosibirsk are specialists for diamonds, these facts were of high interest to them and they are interested in the further cooperation in this field of research.
B.04 Guest lecture
COBISS.SI-ID: 27988007In this poster contribution, presented at the International conference for Microscopy, that was held in Prague in September 2014, we described the results of our analyses on the atomic structure of (130) twins in natural chrysoberyl crystals from the locality Rio das Pratinhas in Bahia (Brazil). Chrysoberyl is typically found in metamorphosed granite pegmatites. There were great inconsistencies in the determination of crystal structure of chrysoberyl on the past, therefore we used Rietveld analysis (in cooperation with dr. Matjaž Mazaj from the National Institute for Chemistry) for determination of structural parameters and the space group of chrysoberyl. The choice was supported by similarities with the related twinned spinel and taaffeite structures. The local atomic structure of the twin boundary and the nanosized rutile precipitates was further analyzed by transmission electron microscopy. The most interesting part of this work is that we used a conventional TEM (JEM 2100 with a W electron source) and in the combination with density functional theory calculations (with dr. Matej Komelj) we managed to determine the exact positions of the Be atoms at the twin boundary and we also determined the unit-cell deformaitonof the rutile precipitates within the chrysoberyl matrix. This fact was especially interesting for the participants of the International microscopy conference since the main focus of the presentations was on the use of advanced (very expensive) aberration corrected microscopes that enable direct observation also of lightest elements. In thi work, however, we used a conventional microscope with a thermal electron source. Base on the results of our analyses we proposed a formation temperature of chrysoberyl crystals from this loclaity at 970°C. The results that were presented in the form of a poster contribution at this conference will be published as a full-klenght scientific paper in the April issue of the American Mineralogist: S. Drev, A. Rečnik, M. Komelj, M. Mazaj and N. Daneu: Structural investigation of (130) twins and rutile precipitates in chrysoberyl crystals from Rio das Pratinhas in Bahia (Brazil). American Mineralogist (2015), in press.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 28021287In this contribution at the International Microscopy conference in Prague (September 2014), we presented our experimental results on oxidation of single crystal of ilmenite during heating in air atmosphere. This study is important for two main reasons. Rutile and hematite, which are the main products of low-temperature ilmenite oxidation, frequently occur in nature in the form of oriented intergrowths. In our previous study we investigated such samples from the Mwinilunga locality in Zambia and based on microstructural characterization we proposed a mechanism of their formation. The aim of this study was to confirm those findings. Another important aspect of this study is the fact that nowadays ilmenite is the most important source of TiO2 and titanium and the process of pre-oxidation of ilmenite is frequently used to improve leaching of iron from the raw ilmenite. In our study we used a natural single crystal of ilmenite from Pakistan that was enriched with manganese (Mn-ilmenite). Ilemntie contains ferrous iron and after heating in oxidizing atmosphere at temperatures up to 800°C, ilmenite decomposes to rutile and hematite that contains ferric iron (iron is oxidized). Ferric iron is more easily removed from the material by acid leaching. Our experiments have revealed that oxidation of Fe and Mn starts on the surface, they diffuse out of the crystal and form a thin additional oxide layer on the surface. This causes non-stoichiometry of the matrix ilmenite, which becomes enriched in titanium. Ti cations start to rearrange within the existing oxygen sublattice into rutile lamellae. The transformation is along specific crystallographic shear (CS) planes and rutile is formed indirectly aver a transitional CS-rutile phase. The described transformation is a so-called topotaxial transformation because the rutile lamellae are in a close structural relationship with the matrix ilmenite. One of the most important findings is that the rutile lamellae represent fast diffusion paths for the out-diffusion of the divalent Fe and Mn cations. Since their amount within rutile strongly exceeds the solid solubility in rutile at normal temperature, these cations precipitate from the rutile during cooling and they form ilmenite precipitates within rutile lamellae. The results of this work are important for better and more efficient extraction of TiO2 from the starting ilmenite.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 27939623