Homo-epitaxy and twinning produce complex nanostructures in cryogenic calcite

Calcite is famous for the rich diversity of oriented crystal growths. Although these are commonly explained by twinning, there are cases where the relationship between the associated crystals is not related to axis or mirror plane symmetry. Here, using transmission electron microscopy (TEM), we investigate the nanostructures of cryogenic calcite samples from Oknothichya (Hunter's) cave, Baikal area (Russia), and demonstrate the homo-epitaxial intergrowth of {1120}//{1120}_rotated, {0001}//{1100} and {1102}//{1108} calcite interfaces. We document as well the occurrence of extra calcite reflections at positions halfway between the and Bragg reflections in electron diffraction patterns and the doubling of and spacings (corresponding to 2 × 2.50 Å and 2 × 3.85 Å, respectively) in high-resolution TEM images. These features are similar to those associated with c-type reflections, and here we demonstrate that they are unrelated to Ca-Mg ordering as we study pure calcite samples. We also present a complex high-resolution TEM image that provides evidence for the cooperation of {1014} calcite twins with homo-epitaxial intergrowth at {1120}//{1120}_rotated and {1102}//{1108} interfaces in a subglacially formed calcite from Elephant Moraine (Antarctica). We develop structure models for all the recognized interfaces and report that they are energetically favorable.