Projects / Programmes
Twinning, epitaxy and phase transformations in minerals
| Code |
Science |
Field |
Subfield |
| 1.06.01 |
Natural sciences and mathematics |
Geology |
Mineralogy and petrology |
| Code |
Science |
Field |
| P420 |
Natural sciences and mathematics |
Petrology, mineralogy, geochemistry |
| Code |
Science |
Field |
| 1.05 |
Natural Sciences |
Earth and related Environmental sciences |
twinning, polytipism, polysomatism, phase transformations, transformation faults, epitaxy, atomic structure, minerals, magmatic and metamorphic rocks
Researchers (9)
Organisations (3)
Abstract
This project deals with growth transformation faults, modulated polytypic structures and epitaxial overgrowths in minerals, which contain important information about the geochemical and thermodynamic conditions during their nucleation and growth. Based on our preliminary investigations of synthetic minerals we set a hypothesis that chemically triggered twins and related planar faults represent an initial stage of the phase transformations. The atomic structure of inherent planar faults reveals the actual nature of the new crystal structures. This hypothesis was later also confirmed on growth twins in some natural minerals (sphalerite, spinel, rutile). Like in minerals synthesised under controlled laboratory conditions, where we influence the thermodynamic conditions and dopant additions, we found that also in natural minerals the formation of growth twins is chemically induced. The atomic structure and chemical composition analyses have shown that the formation twin boundaries in sphalerite is triggered by the presence of oxygen, twinning in spinel by the presence of beryllium and twinning in rutile by the presence of iron or aluminium. In all systems which we analysed we found that the presence of certain impurities (dopants) under specific geochemical and thermodynamic conditions causes the growth of a 2D epitaxial layer in structurally favourable low-index planes of the host crystal. Like in the formation of a binary phase, this topotaxial reaction decreases the energy of the whole system, causing exaggerated growth of the crystal in the direction of the chemically induced 2D fault. The local structure and chemical composition of these structural defects is related to the crystal structure of the host crystal, but it also contains structural elements of the binary phase that exists between the host crystal and the dopant. The formation temperature of these so-called transformation faults in crystals is always lower than the temperature of the binary phase formation. Our hypothesis is based on such a solid background that we can even predict the formation of transformation defects (i.e. growth twins) and consequent exaggerated grain growth based only on a phase diagram of a system of interest. Our previous investigations of this inadequately studied natural phenomenon and the expertise of the research group are a solid foundation for the successful accomplishment of the goals within the proposed research project.
Geochemical conditions during the crystallisation of minerals in natural environments are much more complex than the processes of crystal growth under controlled laboratory conditions. The influence of dopants on crystal growth was therefore not considered important in complex processes of phase transformations. One of the reasons for this was also the lack of analytical methods with enough accuracy and precision to distinguish between the solid solubility of a certain dopant in a crystal and its potential influence on the planar fault formation. With the methods, developed within our research group, such analyses are now possible at the atomic level. For example, near the twin boundary in spinel we detect increased concentrations of Cr, V, Si, Ti and Be, but only Be has a decisive role in the twin formation. Be reacts with spinel to form hexagonal chrysoberyl stacking, the basic element of the twin structure. Similar principles can be followed in all other minerals. The research activities within the proposed project include basic building principles of crystal structures that will be studied on selected mineralogical systems, also interesting from a technological viewpoint: (i) rock salt-nickeline, (ii) sphalerite-wurtzite, (iii) perovskite, (iv) spinel-chrysoberyl, (v) rutile-corundum and others. Nanostructural investigations of transformation faults in these systems will allow us to explain their formation mechanism and give valuable information about geochemical environment in which they formed.
Significance for science
The main objective of our project was to study the role of dopants in the formation of thermodynamically stable planar defects in minerals, based on determination of their local structure and composition at the atomic-scale. We have identified three different mechanisms of twin formation in minerals that crystallize in different structure types; (i) chemically induced twinning, (ii) recrystallization twinning, and (iii) self-assembly of primary nanocrystallites into twinned orientation due to short-range attraction forces. The results of our investigations represent an important contribution to understanding thermodynamic and kinetic processes during crystal nucleation and growth at the atomic scale and provide a new insight into the basic principles of crystal growth. Until now, the concept of chemically induced twinning was confirmed in zincite, sphalerite, pyrite and spinel. In these systems we confirmed our hypothesis that the formation of growth twins is chemically induced and not a consequence of accidental attachment of nanocrystals in the initial growth stages, as suggested by previous authors. We demonstrated that there exists a close correlation between the atomic structure of the defect and that of the binary phase that forms between the matrix and the dopant. The formation of chemically induced defects in crystals is energetically favorable, and is the main reason for anisotropic and exaggerated growth of the composite crystal. In the case of twinning in spinel we demonstrated for the first time that the formation energy of such twin boundary is significantly lower than the formation energy of the fault-free crystal. The concept of chemically induced twining can be exploited for self-assembly of complex inorganic structures for a range of technological applications. We have already initiated investigations of producing branched structures based on crystal twinning of rutile and cassiterite that have a range of applications in the development of new efficient sensors and photocatalysts. In addition to our innovative approach in tackling the problem of early stages of phase transformations in minerals, we have also developed a number of new analytical methods, which enable atomic-scale determination of twin boundary structures. Using these methods, we were able to conduct a complete top-down analysis of growth defects in selected minerals. The results of our project published in international journals contribute to our deeper understanding of geochemical processes at the atomic level and show a great potential for developing new, functionally modulated materials, with novel physical properties.
Significance for the country
The concept of chemically-induced phase transformations, developed by our group, is a novel concept, which improves our understanding of phase formation processes at the atomic scale and has a direct impact on development of novel functional materials. Here our research group has a great advantage, because our core expertise is design of new materials for various technological applications. With the knowledge acquired within this project, Slovenian scientists gained a great advantage in the research of better utilization of energy, which still has many open challenges. By understanding the basic building principles of crystal structure assembly, based on studying twinning and topotaxial processes in natural crystals, we acquired the ability to tailor functional materials for virtually any application. For example, based on twinning of rutile it would be possible to grow highly branched inorganic fractal structures that would have many potential applications in nanotechnology. This concept is already being implemented for the synthesis of complex inorganic architectures for novel photo-catalyst and sensor devices, following the mechanisms of self-assembly and self-organization of nanocrystals based on crystal twinning. Development in this field will influence advancement in different fields of nanotechnology, where it is necessary not only to understand the physical principles of functional materials, but also how their properties can be modified to satisfy given technological requirements. In addition to the foreseen positive effects on the development of sustainable technologies, the project on the other hand has important effect in postgraduate education. The project offered a broad range of research topics that were assigned to young researchers for their doctoral studies. Within the given period one PhD student successfully defended his thesis while three others were additionally assigned to different topics on twinning in minerals and related technological challenges. In addition to numerous high-quality publications, we took care of the promotion of our knowledge through attendance of our researchers at scientific conferences and collaboration within international 7th FP EU projects (MACAN, ESTEEM). In addition, Slovenia was additionally promoted through investigations of minerals from domestic localities (twinning in pyrite, wulfenite, cinnabar), which not only solved some problems regarding the formation of ore deposits and geological processes of investigated localities but also contributed to the promotion of Slovenian natural heritage. Part of these investigations was included in well-renowned monographic works »Minerals of lead-zinc ore deposit Mežica« and »Minerals of mercury ore deposit Idrija«.
Most important scientific results
Annual report
2012,
2013,
final report,
complete report on dLib.si
Most important socioeconomically and culturally relevant results
Annual report
2011,
2012,
2013,
final report,
complete report on dLib.si
