Diffraction Features from (101¯4) Calcite Twins Mimicking Crystallographic Ordering

Péter Németh

doi:10.3390/min11070720

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 (CaCO3) pseudomorph after ikaite (CaCO3·6H2O), 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 d104 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, (101¯4) calcite twins and the orientation change of CO3 groups across the twin interface are responsible for the extra reflections.

Highlights

Transmission electron microscopy (TEM) is an excellent method for characterizing the nanostructures of crystalline materials
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 d104 spacings in high-resolution transmission electron microscopy (TEM) images
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

Summary

Introduction

Transmission electron microscopy (TEM) is an excellent method for characterizing the nanostructures of crystalline materials. A wide range of imaging and diffraction techniques provide unique information about the crystal structure of the studied samples, such as the local elemental composition, electronic structure and bonding down to the atomic level [1]. TEM is practically an indispensable technique for investigating crystal defects and studying phase transitions. The interpretation of TEM data can be challenging, and ambiguities can arise for cases that look simple [4]. The recognition of twins from TEM data can be difficult because their diffraction features can be confused with ordering and superstructures [5,6,7]

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