MS13-P09 Theoretical polytypism and practical twinning of aragonite crystals Pavel Gavryushkin (Novosibirsk State University, Laboratory of experimental geochemistry and petrology of the Earth's mantle; Sobolev Institute of Geology and Mineralogy, Laboratory of theoretical and experimental study of high-pressure minerals, Novosibirsk, Russia) Alexander Reˇcnik (Department for Nanostructured Materials, Joˇzef Stefan Institute, Ljubljana, Slovenia) Nina Daneu (Department for Nanostructured Materials, Joˇzef Stefan Institute, Ljubljana, Slovenia) ... ... (..., ..., Russia) ... ... (..., ..., Russia) ... ... (..., ..., Russia) ... ... (..., ..., Russia) ... ... (..., ..., Russia) ... ... (..., ..., Russia)email:    In the present work, we show the results of investigation of aragonite (CaCO3) microstructure  with transmission electron microscopy (TEM) under ambient and elevated temperature, complemented by density functional theory calculations. As objects of investigation we choose crystals from Tazouta (Marocco), Cuenca (Spain), and Koge-Dava (Russia) localities. All crystals were well faceted, 1-3 centimetres length. Different localities were chosen to cover different genesis, morphology and chemistry. 
    The ubiquitous twinning by {110} down to unit cell size was found on all studied crystals. In some areas twinning is so dense, that these areas can be considered as disordered polytype of aragonite. The super-structural reflections (0.5 0.5 0) were found on [1-10] projection. We suggest that these reflections arise from the diffraction on numerous twinning planes, locally doubling d(110). Also strong reflections prohibited for aragonite symmetry was found in [1-10] projection. Appearance of prohibited reflections can be explained by the presence of flattened domains with decreased symmetry. 
    Calculated enthalpies of polytypes, produced by ordered twinning by {110}, shows that at 0 K enthalpies of O4-O16 polytypes are even lower than the enthalpy of aragonite. This is consistent with ubiquitous twinning by {110} in both organic and inorganic samples and characterise this twinning as the fundamental feature of aragonite structure. Calculated Gibbs energies indicate that temperature energetically stabilise aragonite relative to other polytypes, which explains absence of ordered polytypes in real crystals.
    On heating of powder sample above 350C, satellites reflections appear in [1-10] zone axis. One of the possible explanations of such changes is the generation and ordering of {110} twin boundaries. It was theoretically shown [1] that mechanical twinning taking place during aragonite grinding [2]  is realised by the shift of {110} layers of aragonite structure. In our work, we suggest another mechanism of {110} twin boundaries generation, more appropriate for high-temperature conditions.

[1] Liu, J., Huang, Z., Pan, Z., Wei, Q., Li, X., and Qi, Y. (2017). Physical review letters, 118(10), 105501.

[2] Shin, Y. A., Yin, S., Li, X., Lee, S., Moon, S., Jeong, J., Kwon, M., Yoo, S. J., Kim, Y.-M., Zhang, T., et al. (2016). Nature communications, 7, 10772, 10772.
Keywords: microstruture, twinning, TEM