Deciphering crystal growth in a sector-zoned interpenetration twin of loparite from Mt Khibiny (Kola Peninsula, Russia) through atomic-scale characterisation of growth sectors and twin boundaries

Abstract. Loparite is a natural perovskite (ABO3) with a complex composition, essentially a solid solution between loparite(-Ce) (Na0.5Ce0.5TiO3), lueshite (NaNbO3), and perovskite (CaTiO3), with small amounts of many other elements. The majority of reported loparite crystals are spinel-type interpenetration twins with compositional zoning. Associated with the high variability of compositions, different crystal structures of loparite were described, and the origin of twinning has not been addressed so far. In this work, we studied a loparite twin composed of two symmetrically intergrown cuboctahedra. Microprobe analyses revealed that the cubic and octahedral growth sectors have slightly different compositions. According to atomic-scale analyses, the cubic sectors with a lower Na:LREE ratio have a disordered orthorhombic structure, while the octahedral sectors with a higher Na:LREE ratio show partial ordering of Na and light rare-earth elements along A-type lattice planes in the [001] direction. The degree of ordering was evaluated by quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Atomic-scale analyses of (111) and {112} twin boundary (TB) contacts have shown that all TBs contain a 1–2 nm thin layer of an Si-rich amorphous phase. Our observations suggest that the loparite twin was established in the nucleation stage of crystallisation, followed by independent crystallisation of both twin domains from the melt. The initial crystal form was cubic; octahedral sectors evolved when the crystal size was still in the nanometre range, as a result of slow crystal growth. Differences in structural ordering between adjacent growth sectors developed during slow cooling due to the compositional variations. Our results imply that compositionally zoned crystals might show different structural ordering within single domains, which should be considered when interpreting bulk diffraction data of compositionally zoned perovskites.