Electron diffraction patterns of a calcite crystal from a sea urchin shell. (a) Selected area is about 1 mm diameter, weak diffraction can be seen that is additional to that of the host crystal. (b) Indexed, consistent with viewing direction [010]. (c) Smaller selected area of 100 nm diameter, enhancing reflexions from a small {018} twin domain which gives trhe appearance of a fivefold superstructure.

Electron diffraction of the rhombohedral carbonate minerals can show additional diffraction spots which have been ascribed to various metastable Ca-Mg ordering schemes that remain unknown as macroscopic minerals. We have found that such reflexions can be produced by nanoscale twin domains which appear to be widespread in both biogenic and abiogenic carbonates. Because of the many metrical pseudosymmetries in the calcite structure, such twins can produce diffraction resembling that of commensurate modulated structures. Twin nanodomains on {104}, in particular, can produce the diffraction patterns of any of the supposed "g", "m" and "n" superstructures, provided only that the usual carbonate orientational order is lost in the twin. Thus, these superstructures may not actually exist, and controversies surrounding their occurrence may not be irrelevant. {018} twins are also common, and diffract similarly to a fivefold superstructure.