Anomalous mixed crystals: a peculiar case of adsorption/absorption

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/CdCl 2 , both for the pure adsorption of CdCl 2 on the NaCl crystal faces and for the formation of a three dimensional mixed salt of composition CdCl 2 ·2 NaCl·3H 2 O. 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 CaCO 3 solutions, act as habit modifiers of the growing calcite crystals (adsorption) and, contemporarily, enter the calcite lattice in selected growth sectors, as Li 2 CO 3 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-NH 2 ) 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 d 101 layers of formamide and the NaCl -d 111 ones suggest that formamide can be adsorbed as ordered epitaxial layers; further, the equivalence between the thickness of the elementary layers d 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.

About the Genetic Mechanisms of Apatites: A Survey on the Methodological Approaches

“…Furthermore, investigation about the surface profiles, carried out through the Hartman-Perdok method applied to both HAp and monetite (CaHPO4), was also proved to be necessary as a We suggest explaining the whole morphological and structural modification in the light of the cooperative effect [67,69,99] between Ca-carbonates in solution and Ca-apatite surfaces, considering both the modification of growth morphology and the stabilization of carbonate-containing apatites as the consequence of the epitaxy as the key mechanism during crystal growth.…”

“…According to our experience about adsorption phenomena in crystal growth, selective adsorption/absorption occurs only when the adsorbing surfaces are crystallographically different [66,67]: hence, something should be wrong when neglecting that monoclinic HAp structure could be "necessarily" more rich than the hexagonal one, at least for the variety of the surface profiles exposed to the mother phase.…”

Role of Hydrodynamics, Li+ Addition and Transformation Kinetics on the Formation of Plate-Like {001} Calcite Crystals

“…In all the systems the formation of Li 2 CO 3 was not been observed, which could be expected due to the solution undersaturation with respect to lithium carbonate (S Li2CO3 < 0.39). The literature data showed that in similar systems no precipitation of Li 2 CO 3 could be expected due to the lack of homogeneous distribution of Li + during calcium carbonate formation [44]. The results of the distribution of polymorphs in the initially formed precipitate, but also the precipitate isolated during the aging process and at different lithium concentrations are shown in Figure 1.…”

“…Thus, the cal obtained in the model system (no lithium ions) for all three stirring modes an days of aging are the rhombohedrons with well-expressed {104} faces: the cry MECH and MAG systems are more aggregated. However, the increasing Li tion caused a gradual change of dominant shape of crystals: in the systems wit Li + concentration applied, c(Li + ) = 0.1 mol dm −3 , prismatic calcite crystals with truncation of all six {104} rhombohedral faces, thus resulting in specific plate-like crystal shape [44]. At the highest Li + concentrations applied, c(Li + ) = 1.0 mol dm −3 , a mixture of plate-like and rhombohedral truncated crystals can still be observed in MECH systems, while in MAG and US systems only plate-like crystals predominates.…”

Calcium Carbonate Polymorphs Growing in the Presence of Sericin: A New Composite Mimicking the Hierarchic Structure of Nacre

“…As demonstrated in many previous papers, the 2D-epitaxial growth may be considered a common thread for the modification of a crystal growth habit and phase stabilization [40][41][42][43][44]. In the case of sericin and calcium carbonate polymorphs, we started from the considerations by Mann [8] about the matching of the Ca-Ca distances in the 001 plane of aragonite along the directions [010] and [100].…”