*Article* **Diffraction Features from (1014) Calcite Twins Mimicking Crystallographic Ordering**

**Péter Németh 1,2**


**Citation:** Németh, P. Diffraction Features from (1014) Calcite Twins Mimicking Crystallographic Ordering. *Minerals* **2021**, *11*, 720. https://doi.org/10.3390/ min11070720

Academic Editor: Thomas N. Kerestedjian

Received: 3 June 2021 Accepted: 1 July 2021 Published: 4 July 2021

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**Abstract:** During phase transitions the ordering of cations and/or anions along specific crystallographic directions can take place. As a result, extra reflections may occur in diffraction patterns, which can indicate cell doubling and the reduction of the crystallographic symmetry. However, similar features may also arise from twinning. Here the nanostructures of a glendonite, a calcite (CaCO<sup>3</sup> ) pseudomorph after ikaite (CaCO<sup>3</sup> ·6H2O), from Victoria Cave (Russia) were studied using transmission electron microscopy (TEM). This paper demonstrates the occurrence of extra reflections at positions halfway between the Bragg reflections of calcite in *0kl* electron diffraction patterns and the doubling of *d*<sup>104</sup> spacings (corresponding to 2·3.03 Å) in high-resolution TEM images. Interestingly, these diffraction features match with the so-called carbonate *c-type* reflections, which are associated with Mg and Ca ordering, a phenomenon that cannot occur in pure calcite. TEM and crystallographic analysis suggests that, in fact, (1014) calcite twins and the orientation change of CO<sup>3</sup> groups across the twin interface are responsible for the extra reflections.

**Keywords:** electron diffraction; *c-type* reflections; ordering; twinning; calcite; glendonite; TEM
