Calcium hydroxyapatite (HAp) has been crystallized from aqueous solutions in the presence of citrate ions, in two temperature intervals. At lower temperature, where citrate could form the stable 3D-ordered phase Ca-citrate-tetrahydrate (Ca-Cit-TH), only the monoclinic (P21/c) HAp polymorph occurs and assumes the shape of fence-like aggregates, built by sharply [010] elongated lamellae dominated by the pinacoid {001}. This pronounced anisotropic growth habit is compared with the usually considered rod-like pseudo-hexagonal occurring in pure aqueous solution growth. The habit change is interpreted by assuming that 2D islands of Ca-citrate-tetrahydrate can be adsorbed as epi- monolayers of thickness d001 onto the different growth forms: {001}, {100}, {102¯}, {010}, and {101¯} of HAp. A comparison is made among the corresponding coincidence lattices, in order to explain on reticular basis the selective adsorption of citrate on the {001} HAp form. The role exerted by the 2D-epitaxially adsorbed Ca-Cit-TH as a “mortar” in the monoclinic HAp “brick” assembly is outlined as well.

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Habit Change of Monoclinic Hydroxyapatite Crystals Growing from Aqueous Solution in the Presence of Citrate Ions: The Role of 2D Epitaxy

Crystal
growth experiments carried out in aqueous solutions, supersaturated
with respect to calcite (CaCO3) at room temperature and
pressure, show that the lithium ion added to growth solutions not
only acts as a habit modifier of calcite, but promotes the nucleation
and growth of the two other CaCO3 polymorphs: aragonite
and vaterite. This behavior is interpreted on the grounds of foreign
adsorption going beyond Langmuir’s isotherm model: two-dimensional
(2D)-adsorbed nuclei of Li2CO3 (mineral zabuyelite)
can change, under varying Li+ concentrations in solution,
the growth habit of calcite, promoting as well both nucleation and
growth of aragonite and vaterite, through the mechanism of 2D-epitaxy.
Calculation proves how the epitaxy aragonite/2D-zabuyelite reduces
the average value of the specific surface energy of aragonite, allowing
the nucleation of aragonite and then the coexistence of calcite and
aragonite in the same batch. Finally, it is outlined that the 2D lattices
fulfilling the epitaxy constraints for the couples calcite/zabuyelite
and aragonite/zabuyelite show, as a common feature, a remarkable pseudohexagonality.

Impurity Effects on Habit Change and Polymorphic Transitions in the System: Aragonite-Calcite-Vaterite

The overgrowth of gypsum (CaSO4·2H2O) on calcite (CaCO3), used as a substrate, has been obtained in order to verify the hypothesis of a crystallographic fit between the two phases. Lattice coincidences show that many orientations of gypsum with respect to calcite can occur. Experimentally, only two among these are fulfilled during the early stages of growth, the selection criterion reasonably being the adhesion energy. Moreover, it has been shown that the presence of calcium carbonate as a specific surface impurity in solution modifies the growth habit of gypsum acting on the growth rate and surface morphology of some gypsum forms. At high concentration, the calcium carbonate in solution promotes the twinning of gypsum according to the Montmartre law.

Calcite passivation by gypsum: the role of the cooperative effect

The whole series of the face-centered cubic (fcc) alkali halides, from LiF to RbI, has been crystallized from aqueous solutions in the presence of variable amounts of formamide (HO–C–NH2) Scanning electron microscope images show that the corresponding morphology of the grown crystals is not homogeneous within the whole crystal series, and that the {111} octahedron does appear and compete with the {100} cube only in three separated crystal domains: (NaF), (LiBr, RbF, NaCl) and (KCl, NaI, RbCl, KBr) Starting from our preceding finding about the formation of NaCl/formamide anomalous mixed crystals and recollecting the peculiar {100} + {111} habit change of NaCl growing from pure aqueous solution, we propose that formamide can be adsorbed as a 2D epitaxial layer on the {111} surfaces of the fcc alkali halides according to a three-modal distribution

A three-modal epitaxial adsorption of formamide (HO–C–NH2) on the {111} surfaces of f.c.c. alkali halide crystals growing from aqueous solutions

KCl f.c.c. crystals generally exhibit a {100} habit when growing from pure aqueous solutions, a richer {100} + {111} morphology being obtained only under well-defined growth temperature and supersaturation. When increasing amounts (less than 2000 ppm) of Pb are put in a supersaturated solution, the KCl growth morphology undergoes a progressive change: {100} → {100} + {111} → {111}. Detailed growth patterns have been investigated by means of SEM and AFM, while EDS and XRF analyses allowed us to ascertain that Pb is not only adsorbed on the growing KCl surfaces but also selectively absorbed within the {111} growth sectors. Starting from recent and analogous findings, we tried to interpret the morphological change by means of a geometric and structural model of epitaxy between the {100} and the {111} forms of KCl and the most important forms of those compounds that could be adsorbed on them: PbCl2 (cotunnite), PbCl(OH) (laurionite–paralaurionite) and KCl·PbCl2 (challacolloite). Excellent lattice coincidences have been found, thus proving that the {111} KCl octahedron is largely privileged for adsorption/absorption to occur with respect to the {100} KCl cube. Based on this ground, simple kinetic considerations can be proposed to satisfactorily explain the observed morphology change.

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D. Aquilano

Papers

{100} → {111} morphological change in KCl crystals grown from Pb2+-doped aqueous solutions