Dino Aquilano

Calcite is famous for the rich diversity of oriented crystal growths. Although these are commonly explained by twinning, there are cases where the relationship between the associated crystals is not related to axis or mirror plane symmetry. Here, using transmission electron microscopy (TEM), we investigate the nanostructures of cryogenic calcite samples from Oknothichya (Hunter's) cave, Baikal area (Russia), and demonstrate the homo-epitaxial intergrowth of {11 2 0}//{11 2 0} _rotated , {0001}//{ 1 100} and {1 1 02}//{1 1 08} calcite interfaces. We document as well the occurrence of extra calcite reflections at positions halfway between the hh\overline{ 2h} 0 and 0h\overline {h} 2h Bragg reflections in electron diffraction patterns and the doubling of {d_{({11\bar 20} )}} and {d_{({01\bar 12} )}} spacings (corresponding to 2 × 2.50 Å and 2 × 3.85 Å, respectively) in high-resolution TEM images. These features are similar to those associated with c -type reflections, and here we demonstrate that they are unrelated to Ca–Mg ordering as we study pure calcite samples. We also present a complex high-resolution TEM image that provides evidence for the cooperation of {10 1 4} calcite twins with homo-epitaxial intergrowth at {11 2 0}//{11 2 0} _rotated and {1 1 02}//{1 1 08} interfaces in a subglacially formed calcite from Elephant Moraine (Antarctica). We develop structure models for all the recognized interfaces and report that they are energetically favorable.

The shape evolution of a mineral inclusion in diamond is discussed through thermodynamic and kinetic considerations.

The mechanisms of crystal growth from solution are more complicated than those from vapor or melt growth. These mechanisms are important in biomineralization (bones, teeth, shells) and in the characterization of optically non-linear crystals, perfectly crystallized proteins, nucleic acids and complex viruses. This motivated studies on new basic phenomena, including the crystal interfaces of biological macromolecules and those of low-solubility inorganic salts. Generalization of the Gibbs–Thomson effect has been questioned when describing isotropic (circular) or anisotropic (polygonized) spiral steps spreading onto a crystal face. This work focuses on the brushite crystal (CaHPO₄·2H₂O) since it can grow freely or as a deposit on a gypsum (CaSO₄·2H₂O) substrate. Brushite looks like a 2D crystal built by a flat {010} form limited by {h0l} polar and thin faces running as straight steps. Moreover, it is worth investigating the {010} gypsum to {010} brushite transformation since mineralogy and geochemistry play an important role in the crystal growth. Here, we study the freely growing brushite and, subsequently, its {010} form arising from gypsum. The epitaxy {010} gypsum/{010} brushite is also examined because brushite twins are involved. A new way of thinking about the epi-relations is proposed, along with more rigorous constraints; this new approach emphasizes the physical importance of the adhesion energy connected to every epitaxy.

Determining the mechanisms of the formation of giant crystals is a challenging subject. Gypsum, calcium sulfate dihydrate (CaSO₄·2H₂O), is known to form crystals larger than one meter in several locations worldwide. These selenite crystals grow at different temperatures, either in sedimentary or hydrothermal systems. The famous selenite crystals of the geode of Pulpí (Almería, Spain) are known to have grown at a temperature T = 20 ± 5 °C and have been proposed to form in a subaqueous environment by a self-feeding mechanism triggered by anhydrite dissolution and the ripening of microcrystalline gypsum, enhanced by oscillations in temperature. This paper reports the monitored crystallization of gypsum crystals, from anhydrite powder dissolution, inside airtight evaporation-free reactors under oscillating low temperatures (15 °C < T < 25 °C). These crystals are clearly smaller than the ones in the Pulpí mine but exhibit similar habits (i.e., single blocky crystals and twins following the 100 twinning law). The growth rate of gypsum single crystals has been measured to be between 3.8 and 35.3 µm/day. Noteworthy, we document the occurrence of the 100 contact twinning law of gypsum, which is the most widespread twinning law in natural environments but never univocally reported in laboratory experiments. The selection of the 100 contact twinning law has been correlated to the low supersaturation values obtained in the experiment, where the concentration in these long-duration experiments can be safely assumed to be the equilibrium concentration, i.e., 0.3 (at 25 °C) ≤ SI ≤ 0.4 (at 15 °C). We discuss the relevance of our experiment for forming the gypsum crystals of Pulpí in the framework of the geological history of Pulpí mineralization. These laboratory model experiments contribute to a deeper understanding of mineral nucleation and growth processes in natural environments.

The two most abundant CaCO₃ polymorphs, calcite and aragonite, are universally recognized for the richness of their morphology to which different twins make relevant contributions. The epitaxial transformation calcite ↔ aragonite has long been debated. While the twinning has been thoroughly treated, the homo-epitaxy occurring within each of these minerals has, inexplicably, been overlooked to date, both experimentally and theoretically. Twinning can be deceptive to the point where it can be mistaken for homo-epitaxy, thus making the proposed growth mechanism in the crystal aggregate wrong. Within the present work, the first aim is a theoretical investigation of the homo-epitaxies among the three {10.4}-cleavage, {01.2}-steep and {01.8}-flat rhombohedra of calcite. Accordingly, the specific adhesion energies were calculated between facing crystal forms, unequivocally showing that the {01.2}/{01.8} homo-epitaxy competes with the generation of both {01.2} and {01.8} contact twins. Secondly, the calculation of the specific adhesion energy was extended to consider homo-epitaxy for the {10.4} rhombohedron. The two-dimensional geometric lattice coincidence has been tried for the {00.1} pinacoidal form as well.

Calcite and aragonite are the two most abundant among the CaCO₃ polymorphs and are also well known for their rich morphology and habit, to which twinning relevantly contributes. Moreover, the calcite → aragonite transformation has been debated for a long time, even though the homo-epitaxies occurring within each polymorph have been overlooked, to date, from both experimental and theoretical points of view. Twinning is common and unfortunately can be deceptive as it can be mistaken for homo-epitaxy, thus leading to confusion regarding the growth mechanisms of many crystal aggregates. Here, experimental and theoretical studies were carried out to investigate the twinning and homo-epitaxy that operate in aragonite: (i) the connection between twinning and homo-epitaxy for contact and penetration aggregates made by the lateral {010} and {110} aragonite forms; (ii) the homo-epitaxial relationships among the {001} pinacoid and both the {010} pinacoid and the prismatic {110} aragonite forms. This work attempts to provide a new approach to monitoring the growth mechanisms of aragonite in cases where it is obtained not as a single crystal but as an aggregate. An analogous problem will be explored in our forthcoming work on calcite.