Minerals and meteorites

(111) twins in MgAl2O4 spinel

For a long time it was believed that twinning in spinel is a result of accidental attachment of crystallites in the initial stage of crystal growth (during nucleation). In fact, (111) twins in spinel form as a result of beryllium incorporation into the twin boundary plane. The local structure of the twin boundary is closely related to the structure of ternary Mg-Al-Be oxides also known as taaffeites. This type of twinning is chemically-induced or tropochemical twinning.

Chemically induced twins have important influence on crystal growth – after nucleation of the twin boundary, growth of the composite crystal proceeds exaggeratedly first in the direction of the twin boundary, whereas thickening occurs in the next growth stage:

N. Daneu, A. Rečnik, T. Yamazaki, T. Dolenec: Structure and chemistry of (111) twin boundaries in MgAl2O4 spinel crystals from Mogok. Physics Chemistry of Minerals 34 (2007) 233‐247.  doi: 10.1007/s00269-007-0142-1

S. Drev, A. Rečnik, N. Daneu: Twinning and epitaxial growth of taaffeite-type modulated structures in BeO-doped MgAl2O4. CrystEngComm 15 (2013) 2640-2647. doi: 10.1039/C3CE26997C.

Contact:
Asst. Prof. Nina Daneu Scientist

+386 1 4773 734
nina.daneu@ijs.siRESEARCHGATESICRIS
Field of research:
  • Atomic structure of planar defects (HRTEM)
  • Inversion boundaries in ZnO, GaN
  • Twins in minerals (spinel, rutile, pyrite,…)
  • Bioceramics

Personal page

Prof. Aleksander Rečnik Senior scientist

+386 1 4773 815
aleksander.recnik@ijs.siRESEARCHGATESICRIS
Field of research:
  • Atomic-level studies of polytypic phase transitions, twinning, epitaxy, nucleation of planar faults; atomic structure & chemistry of twin boundaries, inversion boundaries, antiphase boundaries, low-angle tilt grain boundaries; special grain boundaries and their relation to exaggerated and anisotropic crystal growth; microstructure development.

Oxidation of ilmenite, FeTiO3

Oxidation of ilmenite to rutile and hematite is a typical example of topotaxial phase transformation – a solid-state reaction, where the initial single crystal is transformed into one or more products along definite crystallographic directions, leading to the formation of oriented 3D intergrowths. The result of ilmenite oxidation are rutile-hematite intergrowths. Two orientations of rutile exsolutions are possible, with the c-axes or [101] directions of rutile parallel to the a-axes of ilmenite (OR-1 and OR-2):

The direction of rutile exsolutions from ilmenite depends on the kinetics; fast oxidaiton leads to the development of OR-1, whereas OR-2 develops during slow transformaiton (low oxygen fugacity). The OR-1 developed during heating of ilmenite at 700-800 °C in air, while OR-2 was observed on natural rutile-hematite intergrowths from Mwinilunga in Zambia.

A. Rečnik, N. Stanković, N. Daneu (2015) Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia). Contrib. Mineral. Petrol. 169:19., doi: 10.1007/s00410-015-1107-x

Stanković N, Rečnik A, Daneu N (2016) Topotaxial reactions during oxidation of ilmenite single crystal. J Mater Sci 51: 958-968. doi: 10.1007/s10853-015-9425-y

Contact:
Asst. Prof. Nina Daneu Scientist

+386 1 4773 734
nina.daneu@ijs.siRESEARCHGATESICRIS
Field of research:
  • Atomic structure of planar defects (HRTEM)
  • Inversion boundaries in ZnO, GaN
  • Twins in minerals (spinel, rutile, pyrite,…)
  • Bioceramics

Personal page

Prof. Aleksander Rečnik Senior scientist

+386 1 4773 815
aleksander.recnik@ijs.siRESEARCHGATESICRIS
Field of research:
  • Atomic-level studies of polytypic phase transitions, twinning, epitaxy, nucleation of planar faults; atomic structure & chemistry of twin boundaries, inversion boundaries, antiphase boundaries, low-angle tilt grain boundaries; special grain boundaries and their relation to exaggerated and anisotropic crystal growth; microstructure development.

Twinning in rutile TiO2

Natural rutile crystals commonly occur in the form of oriented intergrowths or sagenites, where the angles between the rutile domains frequently correspond to crystallographic (coherent) angles of 114.4° or 57.2° as for the (101) or (301) twins. Well-developed rutile twins usually form under hydrothermal conditions as a consequence of epitaxial growth on a suitable substrate mineral like hematite or ilmenite.

(301) rutile twins:

(101) rutile twins:

The main characteristic of epitaxial/topotaxial twins is that the twin interface contains a thicker layer of some secondary phase, precipitates or is devoid of any dopant (clean contact). (301) twin boundaries in rutiles from Diamantina contain a coherent layer of ilmenite, while the (101) twin contacts oriented precipitates of corundum. Evidences for transformation from oxyhydroxide precursor phase were found in both cases.

Daneu N, Rečnik A, Mader W (2014) Atomic structure and formation mechanism of (101) rutile twins from Diamantina (Brazil). Am Min 99:612–624. doi: 10.2138/am.2014.4672

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. doi: 10.2138/am.2007.2634

Contact:
Asst. Prof. Nina Daneu Scientist

+386 1 4773 734
nina.daneu@ijs.siRESEARCHGATESICRIS
Field of research:
  • Atomic structure of planar defects (HRTEM)
  • Inversion boundaries in ZnO, GaN
  • Twins in minerals (spinel, rutile, pyrite,…)
  • Bioceramics

Personal page

Prof. Aleksander Rečnik Senior scientist

+386 1 4773 815
aleksander.recnik@ijs.siRESEARCHGATESICRIS
Field of research:
  • Atomic-level studies of polytypic phase transitions, twinning, epitaxy, nucleation of planar faults; atomic structure & chemistry of twin boundaries, inversion boundaries, antiphase boundaries, low-angle tilt grain boundaries; special grain boundaries and their relation to exaggerated and anisotropic crystal growth; microstructure development.

Analytical electron microscopy of extraterrestrial materials

Meteorites are rare and precious findings because they reveal important information about the early formation of the solar nebula some 4.6 billion years ago, till very recent times, when they fell to the Earth’s surface. Systematic studies by means of advanced techniques of Analytical Electron Microscopy is being conducted on two stony meteorites, found in Slovenia; meteorite Jesenice and Jezersko.

Second research topic is related to the understanding of the early-stage crystallization pathways of the interstellar dust, which is achieved by specially designed micro gravity experiments where dust particles are homogeneously nucleated from supersaturated vapors, thus mimicking astronomically relevant conditions in the laboratory. Combined in-situ nucleation and high-resolution structure-chemistry studies of synthesized dust particles are providing detailed insights into the complexities of interstellar dust formation.

Meteorite Jezersko (Foto: M. Jeršek).

 

Contact:
Prof. Sašo Šturm Head of the department, Senior scientist

+386 1 4773 418
saso.sturm@ijs.siRESEARCHGATESICRIS
Field of research:
  • Material processing:
    • Solid state synthesis of functional ceramics
    • Hydrothermal synthesis of nanomaterials
  • Advanced TEM characterization and method developments:
    • Structural and chemical characterization of materials and correlation to properties at high spatial resolution
    • Quantitative atomic-scale studies of crystal structure, chemistry and bonding of interfaces and nanostructures
    • Quantitative atomic resolution HRTEM and HAADF/ABF/BF STEM image analysis
    • Quantitative EELS analysis
  • In situ transmission electron microscopy:
    • In situ heating and cooling TEM experiments
    • In situ studies of structural phase transitions