In this contribution we describe the results of a top-down analysis of oriented rutile-hematite intergrowths from Mwinilunga locality in Zambia. Based on detailed transmission electron microscopy (TEM) analyses in two orientations we reconstructed a series of topotaxial reactions that led to their formation. Prior to our investigations of rutile twins and oriented rutile intergrowths it was believed that crystallographic contacts between rutile crystals are formed by growth, whereas the non-crystallographic ones by epitaxial growth of rutile on a suitable, structurally related precursor. Based on this and our other studies of rutile twins and rutile-hematite intergrowths we found that both types of interfaces can form by topotaxial recrystallization and that kinetics of transformation has the most important effect on the orientation of rutile exsolutions form the matrix. Natural rutile crystals commonly occur in the form of oriented intergrowths or sagenites. Orientation relationship between rutile domains may be either crystallographic (coherent) with angles between the domains as for the (101) or (301) twins (114.4° or 57.2°) or non-crystallographic, where the angles between rutile domains are 60° or 120° as, for example, the angles between a and b axes in minerals with hexagonal symmetry (hematite, corundum, …). The main challenge of our first investigation of (301) and (101) rutile twins from Brazil was to determine whether twinning in rutile is triggered by growth. In both types of twins, we found a thicker layer or precipitates of structurally related secondary phases like ilmenite (FeTiO3) and corundum (Al2O3) at the contact between the two rutile domains in twinned orientation. Based on the analyses of the samples in two perpendicular low-index orientations with high-resolution transmission electron microscopy (HRTEM) methods and related spectroscopy techniques (energy dispersive spectroscopy – EDS and electron energy loss spectroscopy – EELS) we found that the formation mechanism of twins in rutile is not chemically induced growth but epitaxial or topotaxial growth on some structurally related precursor mineral. Our first two papers on this topic were published in American Mineralogist: ⧫ Daneu N, Schmid H, Rečnik A, Mader W (2007) Atomic structure and formation mechanism of (301) rutile twins from Diamantina (Brazil). Am Min 92:1789–1799 [ID-20920615] ⧫ Daneu N, Rečnik A, Mader W (2014) Atomic structure and formation mechanism of (101) rutile twins from Diamantina (Brazil). Am Min 99:612–624 [ID-27599399] The main disadvantage of rutile twins from Brazil was the absence of the precursor mineral or substrate for the formation of rutile twins therefore we were not able to offer an adequate explanation of their formation. For this reason, we selected samples of oriented rutile-hematite intergrowths form Mwinilunga for our further investigations, where sagenite rutile networks are found on the surface of large hematite single crystals. Based on the coincidence of [101] direction of rutiles with [210] directions of hematite we macroscopically determined the rutile-hematite orientation relationship. Due to 2.73° declination of the c-axes from the [210] of hematite, 144 contacts between rutile domains may form, these can be classified to 4 types: noncrystallographic contacts at 60° or 120°; low-angle boundaries at 5.56° or 174.44°, (101) twins at 114.44° or 65.56° and (301) twins, where the two rutile domains come in contact at an angle of 54.44° or 125.56°. Further HRTEM and scanning TEM (STEM) analyses in combination with EDS analyses have revealed nano-sized ilmenite (FeTiO3; divalent iron) inclusions in the vicinity of rutile lamellae and these were important for understanding the formation mechanism of the intergrowths during very slow cooling (slow kinetics), where perfect atomic-scale orientation relationship between the matrix and the exsoluted phases can be achieved. The results are published in Contributions to
COBISS.SI-ID: 28374567
We studied classic iron-cross twins of pyrite that are well known among mineralogists, however the conditions of their formation were unexplored. To address this question we studied pyrite twins from the Upper-Permian silts of Mt. Katarina near Ljubljana (Slovenia), that represent one of the most typical geological environments for twinned pyrite. Mineralization of pyrite starts with a reduction of the primary red-colored hematite-rich sediment by sulphide-rich fluids that penetrated the strata. A short period of magnetite crystallization is observed prior to pyrite crystallization, which indicates a gradual reduction process. Sulphur isotope analysis of pyrite shows an enrichment in δ34S, suggesting its origin from the neighboring red-bed deposit. Other sulphides, such as chalcopyrite and galena, formed at the end of pyrite crystallization. Remnants of mineralizing fluids trapped at the interfaces between the inclusions and host pyrite show some trace amounts of Pb and Cu, indicating their presence in the solutions throughout the period of pyrite crystallization. Electron microscopy and spectroscopy study of twin boundaries showed that the interpenetration twinning is accomplished through complex 3D intergrowth of primary {110} Cu-rich boundaries, and secondary {100} boundaries that are pure. We show that 1 monolayer of Cu atoms is necessary to stabilize the {110} twin structure. When the source of copper is interrupted, the two crystal domains continue to grow in predefined orientation along {100} interfaces, favorable for pure pyrite. The presence of Cu thus appears to be necessary condition for the formation of iron-cross twins in pyrite. The achievement describes part of research work performed within WP 1: Sulfosalts.
COBISS.SI-ID: 29763879
In this contribution we describe the results of our study of (130) twin boundaries and rutile precipitates in natural chrysoberyl (BeAl2O4) crystals from Rio das Pratinhas in Brazil. The dimensions and orientation of the chrysoberyl orthorombic unit cell were determined by Rietveld analysis and nanosized precipitates of rutile in the crystals were analyzed by high-resolution transmission electron microscopy (HRTEM). Based on the orientation relationship between the rutile precipitates and the chrysoberyl matrix we determined the temperature of the onset of rutile exsolution. The local structure of the (130) twin boundary was refined from experimental HRTEM images in combination with simulations and density functional theory (DFT) calculations. The results imply that the origin of twinning in chrysoberyl is epitaxial (oriented) growth on or topotaxial recrystallization of some precursor mineral. Natural chrysoberyl crystals frequently occur as contact or cyclic twins and the mechanism of their formation was not explained yet. Is it chemically induced twinning as in spinel or oriented growth on some precursor mineral as in rutile? The answers can only be found by detailed atomic-scale analysis of the twin boundary plane. For our analyses we chose well-developed contact twins of chrysoberyl from Brazil. The study of twinning and precipitates was performed in several stages. Based on the results of Rietveld analysis we first refined the chrysoberyl structure and defined the axes of the orthorhombic unit cell in space-group 62 (Pmnb) with the parameters a = 5.4825(1) Å, b = 9.4163(2) Å, c = 4.4308(1) Å. Such unit cell orientation allows direct comparison of the stacking between structurally related minerals form the spinelloid group like spinel and taaffeite, where the 〈111〉 axis of spinel is parallel to the preudohexagonal axis of taaffeite and chrysoberyl. In this setting, the twin boundary index is (130). Accurate and precise determination of the chrysoberyl structure was used as a standard for determination of cell parameters of strongly deformed rutile precipitates in further HRTEM analyses. Chemical analysis of the twin boundary plane by TEM/EDS has shown enrichment by titanium and iron along the boundary. They are not ordered to specific structural sites, but are present in the form of solid solution. This suggests that Fe and Ti diffused towards the boundary, which acts as a barrier and the result is accumulation of diffusing species in its vicinity. This is a clear indication that twinning in chrysoberyl is not chemically induced as in spinel but is related either to nucleation on some precursor mineral in the beginning of crystal growth or topotaxial recrystallization of a structurally related phase, similar as in rutile. Considering the presence of Fe and Ti in chrysoberyl, it is likely that the precursor is a Fe-Ti containing mineral. The local structure of the twin boundary was determined by HRTEM in combination with image simulations and density functional theory (DFT) calculations. The HRTEM images were recorded on conventional LaB6 TEM JEM 2100, where such defocus-thickness conditions were selected, which are sensitive for the positions of Be atoms. Based on HRTEM images we constructed a model of the twin boundary; however, simulated images did not match the experimental images. The model was refined by DFT calculations and the results have shown that the boundary Be atoms occupy interstitial sites, which are not characteristic for the chrysoberyl structure. Such local structure results in a higher stability (lower energy) of the twin boundary. The last part of this study is HRTEM analysis of nanosized rutile precipitates, which cause various optical effects (e.g. chatoyancy) in chrysoberyl. The analyses have shown that the rutiles have needle- or plate-like morphology (up to 5 nm in thickness and 50 nm in length) and extend along two equivalent directions of chrysoberyl. They are in perfect orientation relation
COBISS.SI-ID: 28468775
Mesocrystalline materials, that are grown by oriented attachment of primary nanocrystals, offer unique properties unmatched by any conventional crystalline materials, including: large specific surface area due to fractal branching of subunits, high bearing strength, crystallographically interconnected architecture and short electron and mass transport pathways, that make them suitable for diverse nanotechnology applications. Several theoretical studies have attempted to explain the forces involved in oriented self-assembly of mesocrystals, however until the present day, a proper understanding of the driving force for mesocrystal assembly has not evolved. Rutile (TiO2) is an ideal material for self-assembly stiudies because it can form complex mesocrystal structures in diverse morphologies, while showing an exceptional combination of electronic and optical properties including appropriate band-gap structure, thermodynamic stability and chemical resistance, which makes the material suitable for range of applications in photocatalysis, photovoltaics, sensors, etc. Mesocrystal assembly depends on chemical conditions that drive surface potentials to approach the isoelectric point that causes a reduction of electrostatic barriers and promotes an irreversible aggregation of primary nanoparticles into a hierarchic structure through condensation processes. Following this process, the total energy of the system is decreased by the reduction of surface energy associated with elimination of capping ligands. Finally, elementary nanoparticles are aggregated into mesocrystal assemblies that mimic the structure of single crystals. The main question that remains is what is the nature of attractive forces that align the particles prior to aggregation. In the present study on self-assembly of rutile we have shown that these interactions range from few, up to several tens of nanometers. The only known principal force that works on such long distances are vdW type of interactions, that exhibit not only separation dependent interactions but also torque components in dielectrically anisotropic materials, such as rutile, that could be responsible for aligning fibrous crystals parallel to their long axes. With the decreasing distance, the magnitude of these forces increases, and their spatial anisotropy becomes more important. This effect becomes more pronounced when the precipitates are thermally agitated, and thus more prone to explore different interaction configurations promoting the attraction between the particles into a loosely associated state. As the distance between the fibers becomes shorter, by attraction, crystallographic alignment along low energy planes becomes dominant. Phase analysis of the amorphous material enclosed in the pockets between imperfectly assembled rutile fibers has shown an unusual harmonic ordering resembling that of the adjacent rutile structure. Through a continuous condensation of ligands captured in the interparticle space the nanoparticles are fixed in accordance to their intrinsic electric field. As a consequence, the crystals obtain twisted morphology, following the symmetry rules of the crystal structure, but with a characteristic angular dispersion. Crystal twisting is thus the main macroscopic indication of the mesocrystal assembly process. In addition to lateral assembly in {110} planes we have shown that precipitated rutile crystals tend to assemble also on the {101} planes that lead to mesocrystal twinning. By this we have demonstrated a new mechanism of twinning in rutile. The mechanism of mesocrystal twinning has a great potential in production of rutile–based fractal structures with many applications in nanotechnology. Our study provided the first experimental evidence indicating the presence of electromagnetic fluctuation interactions, carrying key structural information through which oriented attachment of nanocrystals is possible. The achievement is part of research work within WP 4: Rutile-corundum
COBISS.SI-ID: 29423143
In a complex succession of volcanic and volcaniclastic deposits in the Smrekovec Volcanic Complex, coarser-grained rocks contain alteration minerals with higher temperature stability ranges (laumontite-chlorite-(chlorite-smectite)-albite) than those in coarse-grained tuffs (analcime-heulandite-(chlorite-smectite)), or in fine-grained vitroclastic tuffs (heulandite-clinoptilolite-(chlorite-smectite)). The occurrence is related to convective-advective flow of hydrothermal fluids. In the two-phase area gases separated from denser liquids that were unable to continue to ascend and outflowed downward, preferentially through the layers with higher permeability. The achievement is part of research work within WP 5: Silicates.
COBISS.SI-ID: 2505045