International
Tables for
Crystallography
Volume D
Physical properties of crystals
Edited by A. Authier

© International Union of Crystallography 2006
International Tables for Crystallography (2006). Vol. D. ch. 3.3, pp. 434-435

Section 3.3.10.5.1. Aragonite, CaCO3

Th. Hahna* and H. Klapperb

a Institut für Kristallographie, Rheinisch–Westfälische Technische Hochschule, D-52056 Aachen, Germany, and bMineralogisch-Petrologisches Institut, Universität Bonn, D-53113 Bonn, Germany
Correspondence e-mail:  hahn@xtal.rwth-aachen.de

3.3.10.5.1. Aragonite, CaCO3

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The earliest structural model of a twin boundary was derived for aragonite by Bragg (1924[link]), reviewed in Bragg (1937[link], pp. 119–121) and Bragg & Claringbull (1965[link], pp. 131–133). Aragonite is orthorhombic with space group Pmcn. It exhibits a pronounced hexagonal pseudosymmetry, corresponding to a (hypothetical) parent phase of symmetry [P6_3/mmc], in which the Ca ions form a hexagonal close-packed structure with the CO3 groups filling the octahedral voids along the [6_3] axes. By eliminating the threefold axis and the C-centring translation of the orthohexagonal unit cell, the above orthorhombic space group results, where the lost centring translation now appears as the glide component n. Of the three mirror planes parallel to [\{11{\bar 2}0\}_{\rm hex}] and the three c-glide planes parallel to [\{10{\bar 1}0\}_{\rm hex}], one of each set is retained in the orthorhombic structure, whereas the other two appear as possible twin mirror planes [\{110\}_{\rm orth}] and [\{130\}_{\rm orth}]. It is noted that predominantly planes of type [\{110\}_{\rm orth}] are observed as twin boundaries, but less frequently those of type [\{130\}_{\rm orth}].

From this structural pseudosymmetry the atomic structure of the twin interface was easily derived by Bragg. It is shown in Fig. 3.3.10.6[link]. In reality, small relaxations at the twin boundary have to be assumed. It is clearly evident from the figure that the twin operation is a glide reflection with glide component [\textstyle{1\over2}{\bf c}] (= twin displacement vector t).

[Figure 3.3.10.6]

Figure 3.3.10.6 | top | pdf |

Structural model of the (110) twin boundary of aragonite (after Bragg, 1924[link]), projected along the pseudo-hexagonal c axis. The ortho­rhombic unit cells of the two domains with eigensymmetry Pmcn, as well as their glide/reflection planes m and c, are indicated. The slab centred on the (110) interface between the thin lines is common to both partners. The interface coincides with a twin glide plane c and is shown as a dotted line (twin displacement vector [{\bf t} = 1/2 {\bf c}]). The model is based on a hexagonal cell with [\gamma = 120^\circ], the true angle is [\gamma = 116.2^\circ]. The origin of the orthorhombic cell is chosen at the inversion centre halfway between two CO3 groups along c.

References

First citation Bragg, W. L. (1924). The structure of aragonite. Proc. R. Soc. London Ser. A, 105, 16–39.Google Scholar
 First citation Bragg, W. L. (1937). Atomic structure of minerals. Ithaca, NY: Cornell University Press.Google Scholar
 First citation Bragg, W. L. & Claringbull, G. F. (1965). The crystalline state, Vol. IV. Crystal structures of minerals, p. 302. London: Bell & Sons.Google Scholar








































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