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(1014) Calcite Twins Mimicking Crystallographic Ordering

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(1014) Calcite Twins Mimicking Crystallographic Ordering minerals Article Diffraction Features from (1014) Calcite Twins Mimicking Crystallographic Ordering Péter Németh 1,2 1 Research Centre for Astronomy and Earth Sciences, Institute for Geological and Geochemical Research, Eötvös Loránd Research Network, Budaörsi Street 45, 1112 Budapest, Hungary; [email protected] 2 Department of Earth and Environmental Sciences, University of Pannonia, Egyetem út 10, 8200 Veszprém, Hungary Abstract: During phase transitions the ordering of cations and/or anions along specific crystallo- graphic 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, (1014) calcite twins and the orientation change of CO3 groups across the twin interface are responsible for the extra reflections. Citation: Németh, P. Diffraction Keywords: electron diffraction; c-type reflections; ordering; twinning; calcite; glendonite; TEM Features from (1014) Calcite Twins Mimicking Crystallographic Ordering. Minerals 2021, 11, 720. https://doi.org/10.3390/ 1. Introduction min11070720 Transmission electron microscopy (TEM) is an excellent method for characterizing the nanostructures of crystalline materials. A wide range of imaging and diffraction techniques Academic Editor: Thomas provide unique information about the crystal structure of the studied samples, such as the N. Kerestedjian local elemental composition, electronic structure and bonding down to the atomic level [1]. Thanks to the latest developments, not only has the resolution limit been pushed below Received: 3 June 2021 0.5 Å, but nowadays even the behavior of materials and devices at the atomic scale can Accepted: 1 July 2021 Published: 4 July 2021 be mapped [2,3]. TEM is practically an indispensable technique for investigating crystal defects and studying phase transitions. However, the interpretation of TEM data can be Publisher’s Note: MDPI stays neutral challenging, and ambiguities can arise for cases that look simple [4]. In particular, the with regard to jurisdictional claims in recognition of twins from TEM data can be difficult because their diffraction features can published maps and institutional affil- be confused with ordering and superstructures [5–7]. iations. Here TEM is used to study the nanostructure of glendonite, which is a synonym for calcite pseudomorph after ikaite (CaCO3·6H2O), as a result of transformation. Glendonite is commonly associated with cold paleotemperature [8–10]. However, issues have been raised with regard to using it as an indicator for cold temperature [11] because, in a laboratory setting, ikaite formation was reported even above 20 ◦C[11,12]. Ikaite rapidly Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. disintegrates into a mush of water and recrystallizes to calcite during slight warming or This article is an open access article pressure release, resulting in a highly porous crystal mesh [13–16]. Aragonite (unit cell distributed under the terms and parameters a = 4.9611 Å, b = 7.9672 Å, c = 5.7404 Å; space group: Pmcn) has also been conditions of the Creative Commons observed from ikaite transformation in marine and alkaline lake environments [17,18]. Attribution (CC BY) license (https:// Studies on the transformation of synthetic ikaite suggest that it can also transform to creativecommons.org/licenses/by/ amorphous calcium carbonate [19] and vaterite (unit cell parameters a = 4.13 Å, b = 7.15 Å, 4.0/). c = 8.48 Å; space group: Pbnm) (e.g., [20]). Macroscopic calcite pseudomorphs after ikaite are Minerals 2021, 11, 720. https://doi.org/10.3390/min11070720 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 10 Minerals 2021, 11, 720the transformation of synthetic ikaite suggest that it can also transform to amorphous cal- 2 of 9 cium carbonate [19] and vaterite (unit cell parameters a = 4.13 Å, b = 7.15 Å, c = 8.48 Å; space group: Pbnm) (e.g., [20]). Macroscopic calcite pseudomorphs after ikaite are tradi- tionally called glendonite, although other names also are used [8]. According to [11], the traditionally called glendonite, although other names also are used [8]. According to [11], pseudomorph replacement of ikaite by calcite occurs through a coupled dissolution–re- the pseudomorph replacement of ikaite by calcite occurs through a coupled dissolution– precipitation mechanism at the ikaite–calcite interface. Since glendonites preserved the reprecipitation mechanism at the ikaite–calcite interface. Since glendonites preserved the parent phase morphology, structural relicts indicating phase transition can be expected parent phase morphology, structural relicts indicating phase transition can be expected [21]. [21]. In fact, the TEM data here reported indicated an interesting lamellar structure with In fact, the TEM data here reported indicated an interesting lamellar structure with the the occurrence of diffraction features that are inconsistent with ordinary calcite and that occurrence of diffraction features that are inconsistent with ordinary calcite and that could could mistakenly be associated with crystallographic ordering and superstructures. mistakenly be associated with crystallographic ordering and superstructures. Understanding the structural details of Ca-Mg carbonates is an important topic since Understanding the structural details of Ca-Mg carbonates is an important topic since they are rock-forming minerals. In particular, calcite is the dominant carbonate phase on they are rock-forming minerals. In particular, calcite is the dominant carbonate phase on the Earth’s surfacethe Earth’s and it is surface the third and most it is theabundant third most minera abundantl after quartz mineral and after feldspars. quartz andIt feldspars. It abundantly precipitatesabundantly at precipitatesambient conditions at ambient and conditions is one of andthe most is one important of the most biominer- important biominerals. als. Calcite is rhombohedralCalcite is rhombohedral (unit cell parameters (unit cell parameters a = 4.989 Åa =and 4.989 c = Å17.06 and Å;c = space 17.06 group: Å; space group: R3c). R3c). In this paperIn this, three paper, independent three independent indices hkl indices are usedhkl are to usedlabel toreflections label reflections and d-spac- and d-spacings and ings and four indicesfour indices hkil (hkili = −((ih= +− k))(h are+ k ))used are usedwhenever whenever crystallographic crystallographic planes planes and anddi- directions are (1014) 3 rections are discussed.discussed. Ca Ca atoms atoms lie lie in in the the (1014) plane plane alternating alternating with CO 3 triangulartriangular groups along groups along thethe [0001] [0001] ( (FigureFigure 11a).a). The orientation change of the carbonate groups groups im- implies a c glide plies a c glide that isis revealedrevealed byby systematicsystematic absencesabsences ((ll= = 22nn ++ 11 forfor 0kl0klreflections) reflections) in in the the corresponding correspondingreciprocal reciprocal lattice lattice (Figure (Figure1 b).1b). In In dolomite, dolomite, (10 (1014)14) Mg Mg and and Ca Ca cation cation layers layers alternate along alternate along[0001]; [0001] as; as a result,a result, the theR 3Rc3symmetryc symmetry is reducedis reduced to Rto3 Rand3 andb-type b-typereflections reflec- (l = 2n + 1 for tions (l = 2n + 10kl forreflections) 0kl reflections occur) occur (e.g., (e.g., [22 []).22] Although). Although such suchb-type b-typereflections reflections do do notnot occur in pure occur in pure conventionalconventional calcite, calcite, they they are are present present in in a a disordered disordered phase phase of of calcite calcite (space (space group: R3m) group: R3m) asas a a consequence consequence of of the the rotational rotational disorder disorder of of the the CO CO3 groups.3 groups. This This poly- polymorph was morph was reportedreported at high at high temperature temperature (>1260 (>1260 K) and K) andat high at high pCO pCO2 [232]. [The23]. reflections The reflections halfway halfway betweenbetween those thoseof the ofa-type the a-type(l = 2n( lfor= 2then for0kl thereflections0kl reflections)) and b-type and areb-type the soare- the so-called called c-type reflectionsc-type reflections (Figure (Figure 1c), which1c), which have havebeen beenassociated associated with withordering ordering and andat- attributed to tributed to variousvarious superstructures superstructures in Mg in Mg-bearing-bearing calcite calcite and and dolomite dolomite [22,24,25] [22,24,25. In]. Incon- contrast to these trast to these explanations,explanations, Larsson Larsson and and Christy Christy [5] [5 show] showeded that that the the c-typec-type reflectionsreflections can can arise from arise from submicronsubmicron-sized-sized calcite calcite twins. twins. Shen Shen
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