Amorphous calcium carbonate | EPFL Graph Search

Amorphous calcium carbonate (ACC) is the amorphous and least stable polymorph of calcium carbonate. ACC is extremely unstable under normal conditions and is found naturally in taxa as wide-ranging as sea urchins, corals, mollusks, and foraminifera. It is usually found as a monohydrate, holding the chemical formula CaCO3·H2O; however, it can also exist in a dehydrated state, CaCO3. ACC has been known to science for over 100 years when a non-diffraction pattern of calcium carbonate was discovered by Sturcke Herman, exhibiting its poorly-ordered nature. ACC is an example of crystallization by particle attachment (CPA), where crystals form via the addition of particles ranging from multi-ion complexes to fully formed nanocrystals. Research of such systems have diverse application; however, the current lack of unambiguous answers to fundamental questions (i.e. solubility product, interfacial forces, structure, etc.) causes them to be topics of study in fields ranging from

Marine and freshwater micropearls: biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the genus Tetraselmis (Chlorophyta)

Michaël Bensimon, Montserrat Filella

Unicellular algae play important roles in the biogeochemical cycles of numerous elements, particularly through the biomineralization capacity of certain species (e.g., coccolithophores greatly contributing to the "organic carbon pump" of the oceans), and unidentified actors of these cycles are still being discovered. This is the case of the unicellular alga Tetraselmis cordiformis (Chlorophyta) that was recently discovered to form intracellular mineral inclusions, called micropearls, which had been previously overlooked. These intracellular inclusions of hydrated amorphous calcium carbonates (ACCs) were first described in Lake Geneva (Switzerland) and are the result of a novel biomineralization process. The genus Tetraselmis includes more than 30 species that have been widely studied since the description of the type species in 1878. The present study shows that many other Tetraselmis species share this biomineralization capacity: 10 species out of the 12 tested contained micropearls, including T. chui, T. convolutae, T. levis, T. subcordiformis, T. suecica and T. tetrathele. Our results indicate that micropearls are not randomly distributed inside the Tetraselmis cells but are located preferentially under the plasma membrane and seem to form a definite pattern, which differs among species. In Tetraselmis cells, the biomineralization process seems to systematically start with a rod-shaped nucleus and results in an enrichment of the micropearls in Sr over Ca (the Sr/Ca ratio is more than 200 times higher in the micropearls than in the surrounding water or growth medium). This concentrating capacity varies among species and may be of interest for possible bioremediation techniques regarding radioactive Sr-90 water pollution. The Tetraselmis species forming micropearls live in various habitats, indicating that this novel biomineralization process takes place in different environments (marine, brackish and freshwater) and is therefore a widespread phenomenon.

COPERNICUS GESELLSCHAFT MBH2018

In Situ Liquid SAXS Studies on the Early Stage of Calcium Carbonate Formation

Mohammad Reza Andalibi, Agnese Carino, Andrea Testino

Calcium carbonate is a model system to investigate the mechanism of solid formation by precipitation from solutions, and it is often considered in the debated classical and nonclassical nucleation mechanism. Despite the great scientific relevance of calcium carbonate in different scientific areas, little is known about the early stage of its formation. Therefore, contactless devices are designed that are capable of providing informative investigations on the early stages of the precipitation pathway of calcium carbonate in supersaturated solutions using classical scattering methods such as wide‐angle X‐ray scattering (WAXS) and small‐angle X‐ray scattering (SAXS) techniques. In particular, SAXS is exploited for investigating the size of entities formed from supersaturated solutions before the critical conditions for amorphous calcium carbonate (ACC) nucleation are attained. The saturation level is controlled and kept constant by mixing four diluted solutions (i.e., NaOH, CaCl2, NaHCO3, H2O) at constant T and pH. The scattering data are collected on a liquid jet generated about 75 s after the mixing point. The data are modeled using parametric statistical models providing insight about the size distribution of denser matter in the liquid jet. Theoretical implications on the early stage of solid formation pathway are inferred.

2019

Precipitation Pathways: In Situ Liquid SAXS Studies on the Early Stage of Calcium Carbonate Formation (Part. Part. Syst. Charact. 6/2019)

Mohammad Reza Andalibi, Agnese Carino, Andrea Testino

In article number 1800482, Andrea Testino and co‐workers investigate calcium carbonate clusters in‐situ by small angle X‐ray scattering using a liquid jet setup at constant pH, temperature, and saturation level. The saturation is kept constant and below the critical condition for amorphous calcium carbonate formation. Data analysis allows for extracting information about entities in the liquid jet characterized by a higher density: highly hydrated small clusters (

2020

Marine and freshwater micropearls: biomineralization producing strontium-rich amorphous calcium carbonate inclusions is widespread in the genus Tetraselmis (Chlorophyta)

Michaël Bensimon, Montserrat Filella

COPERNICUS GESELLSCHAFT MBH2018