TEM investigation of the domain structure and superstructure in hillebrandite, Ca₂SiO₃(OH)₂

Selected-area electron diffraction, dark-field images, and high-resolution transmission electron microscope images show that there are pervasive domains in hillebrandite from Mexico. The domains are in thin layers that are almost parallel to (010). They range from one to several unit cells thick across the b direction, with an average thickness of about 10 nm. The actual repetition period (7.28 Å) along the a crystallographic direction is that of the wollastonite-like chain in the hillebrandite structure. Neighboring superstructure domains are related by stacking faults, with stacking vector of ±1/2a_sup. Nonperiodic arrangements of the superstructure domains give an average structure of hillebrandite with a 3.64 Å repetition period along the a axis and one-half occupancies of Si atoms in the tetrahedral sites in the two Si-O chains.