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

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.

Links between Ikaite Morphology, Recrystallised Ikaite Petrography and Glendonite Pseudomorphs Determined from Polar and Deep-Sea Ikaite

“…Several ikaite break down models have been presented in [20,29,47] and demonstrated that Amorphous Calcium Carbonate (ACC) played an important role during the ikaite to calcite transition, and it is suggested that carbonate and calcium ions change orientation during loss of structural water [48]. Both these mechanisms are consistent with the observation illustrated in Figure 2a,(e3) of a possible structural element rarely observed in the glendonite (Figure 2f) forming along inner structures preserved in the pseudomorph.…”

Tracing structural relicts of the ikaite-to-calcite transformation in cryogenic cave glendonite

“…The X-ray diffraction patterns of samples from Victoria cave are consistent with ordinary calcite (Németh 2021). No structural remnants of ikaite were found during TEM analysis.…”

“…Based on ICP-OES measurements Németh (2021) reported that the glendonite powder from Victoria cave contains 39.99 m% Ca and 0.045 m% Mg, i.e., the sample is practically pure CaCO 3 . In order to study possible nanoscale chemical homogeneity, the intensity of HAADF-STEM images, also called Z contrast images, was used.…”

“…2b). A detailed description of these (1014) calcite twins, which could be misinterpreted as structural ordering, was recently reported in Németh (2021).…”

Editorial for the Special Issue “Modularity and Twinning in Mineral Crystal Structures”

“…Instead, classical twinning is central in the articles contributed by Németh [4], Bindi and Morana [5], and Makovicky [6]. Via the analysis of electron diffraction patterns and high-resolution TEM images, the article [4] shows that the extra reflections occurring halfway between the Bragg reflections of glendonite, a calcite (CaCO3) pseudomorph after ikaite (CaCO 3 .6H 2 O), are the consequence of microtwinning by reticular pseudomerohedry based on a pseudo-orthorhombic C-centered sublattice. The same diffraction features correspond to the so-called carbonate c-type reflections associated with Mg and Ca ordering, a phenomenon that cannot occur in pure calcite, as is the studied sample.…”