Crystal structure of larnite β-Ca2SiO4 and specific features of polymorphic transitions in dicalcium orthosilicate

The crystal structure of larnite, a natural analog of synthetic β-Ca2SiO4, has been determined: a = 5.5051(3) Å, b = 6.7551(3) Å, c = 9.3108(5) Å, β = 94.513(4) o , sp. gr. P21/n, Z = 4, and R 1 = 0.0532 for 1071 reflections with I > 2σ (I). Larnite was found in skarn xenoliths (Lakargi, Kabardino-Balkaria, Russia). The mineral structure is based on a heteropolyhedral glaserite-like framework of interconnected Ca polyhedra and isolated [SiO4] tetrahedra. Based on an analysis of the layer-by-layer packing of atoms in the structures of larnite and other Ca2SiO4 polymorphs, the structural features and mechanisms of transitions from high-temperature (α, α′ L , and α′ H ) to low-temperature (β and γ) Ca2SiO4 modifications, as well as their relationship with natural glaserite-like orthosilicates (merwinite Ca3Mg[SiO4]2 and bredigite Ca7Mg[SiO4]4), have been considered. The most likely atomic arrangement in hypothetical Ca2SiO4 models has been calculated by the method of atomistic potentials.