Vaterite mesocrystals with hexagonal morphology and uniform size have been successfully synthesized in the presence of a N-trimethylammonium derivative of hydroxyethyl cellulose via aggregation-mediated crystallization using a simple gas-diffusion method. The uniform hexagonal plates display sharp facets and edges, even though they are formed by the aggregation of nanocrystals. The results demonstrate that each vaterite plate can be explained as consisting of aggregates of nanoparticles that share the same three-dimensional orientation. A mechanism for the formation of hexagonal vaterite mesocrystals made of primary nanoparticles and hexagonal units is also presented. An understanding of the mesoscale transformation process will be helpful in controlling the aggregation-driven formation of complex higher-order structured materials and will provide new insights into biomineralization mechanisms. For example, the spines of sea urchins can be discussed within the framework of the mesocrystal concept. This study could provide an additional tool for designing advanced materials and could be used for the synthesis of more complex crystalline three-dimensional structures.

Uniform hexagonal plates of vaterite CaCO3 mesocrystals formed by biomimetic mineralization

Crystalline hexagonal-shaped superstructures of calcium carbonate, synthesized in the presence of ammonia, are shown to be assembled by a three-dimensional oriented attachment of vaterite nanoparticles. This unusual crystallographic lock-in mechanism enables the formation of complicated rounded structures with a crystallographic orientation from nanosized building blocks, which has so far only been found for transition metal systems.

Superstructures of Calcium Carbonate Crystals by Oriented Attachment

https://www.ugr.es/~grupo179/pdf/ruiz%20agudo%2010a.pdf

The role of background electrolytes on the kinetics and mechanism of calcite dissolution

Vaterite with different morphologies, a thermodynamically unstable polymorph of calcium carbonate, was successfully synthesized by a simple injection-precipitation method. The phase composition and morphology of the products were characterized by the XRD, FT-IR, SEM, and TEM techniques. Experiments were performed at 37 and 25 °C using pH values of 1.5, 3.0, and 6.9. At 37 °C, precipitated vaterite has spindle-like morphology at the low pH 1.5 of the initial CaCl 2 solution, and shows coexistence of the spindle-, spheroid-, and cauliflower-like morphologies at the intermediate pH 3.0, whereas it is spheroidal at pH 6.9. SAED analyses revealed that the spindle-like vaterite superstructures were self-assembled by the oriented aggregation of vaterite micro-crystals along the crystallographic c direction. At 25 °C, however, the low pH (1.5) led to coexistence of cauliflower- and spheroid-shaped vaterite, whereas spherulite-like vaterite was always obtained at either pH 3.0 or 6.9. We show that simple inorganic precipitation processes lead to complex and unusual morphologies with hierarchical structure. Therefore, care must be taken when morphological criteria are claimed as proof for biogenic origin of minerals. Moreover, time studies of the polymorphic transition revealed that, although solution-mediated dissolution of precursor vaterite and reprecipitation of secondary calcite always occur, Ostwald ripening finally contributes to the formation of rhombohedral calcite. Therefore, the polymorphic transition sequentially proceeds from vaterite through irregular calcite aggregates to stable calcite rhombohedra.

Controlled crystallization of unstable vaterite with distinct morphologies and their polymorphic transition to stable calcite

http://digital.csic.es/bitstream/10261/211471/1/GarciaRuiz_ProgresCrystal62_227.pdf

Three study cases of growth morphology in minerals: Halite, calcite and gypsum

Calcite crystals were nucleated and grown from supersaturated aqueous solutions in the presence of variable concentrations of lithium The diagram of supersaturation vs [Li+]/[Ca2+] concentration r

Morphology of Calcite (CaCO3) Crystals Growing from Aqueous Solutions in the Presence of Li+ Ions. Surface Behavior of the {0001} Form

In this study, we describe a method to obtain experimental values of the surface energy of calcite. A zenithal imaging device was used to acquire pictures of droplets of CaCO3 saturated aqueous solution on the surface of a calcite crystal sample. Pictures were used to measure the contact angle between the droplets and the {10.4} calcite surfaces. The method is discussed along with its geometrical ground, as well as the theoretical foundation of the contact angle calculation. A comparison is made with the literature data; a good agreement is found between our experimental values and those obtained from the more recent ab initio calculations.

Experimental Value of the Specific Surface Energy of the Cleavage {10.4} Calcite Rhombohedron in the Presence of Its Saturated Aqueous Solution

Calcite crystals have been obtained from supersaturated solutions diffusing both in agarose and Na-metasilicate gels. Crystal morphology in agarose is characterized mainly by spherulites grown around solid gel impurities and terminated by the cleavage f 10 %

Emanuele Costa

Papers

Morphology of Calcite (CaCO3) Crystals Growing from Aqueous Solutions in the Presence of Li+ Ions. Surface Behavior of the {0001} Form

Experimental Value of the Specific Surface Energy of the Cleavage {10.4} Calcite Rhombohedron in the Presence of Its Saturated Aqueous Solution

steep rhombohedra in gel grown calcite crystals