Anomalous mixed crystals: a peculiar case of adsorption/absorption

TL;DR: In this article, a case study of adsorption/absorption in growing calcite is presented, where the impurity is adsorbed and incorporated in a growing crystal, giving rise to a bi-phased system: the host dominant crystal and the guest impurity crystallized as thin lamellae in selected growth sectors.

Abstract: The paper deals with the peculiar case of adsorption/absorption in which the impurity is adsorbed and, successively, incorporated in a growing crystal, giving rise to a bi-phased system: the host dominant crystal and the guest impurity crystallized as thin lamellae in selected growth sectors. The historical path is revisited, since the pioneering papers by Johnsen, Gaubert, Neuhaus and Seifert, through the works of Kern's and Hartman's Schools who verified the theoretical hypotheses on the growth mechanisms, initially put forward by Bunn, Royer and Kleber. An important attention is paid to the interesting case study represented by the system NaCl/CdCl2, both for the pure adsorption of CdCl2 on the NaCl crystal faces and for the formation of a three dimensional mixed salt of composition CdCl2·2 NaCl·3H2O. The core of the work is devoted to two highlighting examples of adsorption/absorption. In the first one, varying concentrations of lithium ions present as impurity in aqueous CaCO3 solutions, act as habit modifiers of the growing calcite crystals (adsorption) and, contemporarily, enter the calcite lattice in selected growth sectors, as Li2CO3 iso-oriented lamellae (absorption). All this has been proved by X-ray powder diffraction diagrams (XRPD), cathode-luminescence and atomic force microscopy (AFM) measurements. In the second example, the well known example of habit modification of NaCl crystals growing from aqueous solution in the presence of formamide is recovered and newly investigated in the light of up to date experimental and theoretical analyses. NaCl crystals were obtained from water-formamide (H-CO-NH2) solutions, either by slow evaporation or by programmed cooling of saturated solutions, the formamide concentration ranging from 0 to 100%. Accordingly, the crystal morphology changes from {100} (pure aqueous solution) to {100} + {111} (water-formamide solutions) to {111} (pure formamide solution). X-ray powder diffraction diagrams prove that formamide is not only adsorbed on the {111}NaCl octahedron but is also selectively captured within the {111} growth sectors. The 2D-lattice coincidences between the d101 layers of formamide and the NaCl – d111 ones suggest that formamide can be adsorbed as ordered epitaxial layers; further, the equivalence between the thickness of the elementary layers and indicates that formamide is allowed to be buried (absorption) in the growing crystal. Hence, one can ultimately state that formamide is not only an habit modifier of NaCl crystals, but that “anomalous NaCl/formamide mixed crystals” form, limited to the {111} NaCl growth sectors.