Search results for "CARBONATE"

Sedimentology, petrography and geochemistry of a limestone breccia (Pietra di Billiemi) from NW Sicily, Italy: implications for evolution of the Teth…

2009

In this study, the Pietra di Billiemi, a famous dimension stone, is investigated because it records the tectonic evolution of the south Tethys continental margin and preserves a record of major environmental changes occurring near the Triassic/Jurassic boundary. The Pietra di Billiemi is a grey, coarse-grained and clast-supported limestone breccia cropping out in an area of the Palermo Mountains representing a segment of the Apennine–Maghrebian chain in western Sicily. The rock consists of metre-sized to centimetre-sized angular clasts, derived from Upper Triassic sponge boundstones and rudstones, with a differently coloured, silt-grained matrix. Fitted fragments are observed commonly which…

Stable amorphous calcium carbonate is the main component of the calcium storage structures of the crustacean Orchestia cavimana.

2002

Amorphous calcium carbonate (ACC) is the least stable form of the six known phases of calcium carbonate. It is, however, produced and stabilized by a variety of organisms. In this study we examined calcium storage structures from the terrestrial crustacean Orchestia cavimana, in order to better understand their formation mode and function. By using X-ray diffraction, infrared and Raman spectroscopy, thermal analysis and elemental analysis, we determined that the mineral comprising these storage structures is amorphous calcium carbonate with small amounts of amorphous calcium phosphate (5%). We suggest that the use of amorphous calcium carbonate might be advantageous for these storage struct…

The enzyme carbonic anhydrase as an integral component of biogenic Ca-carbonate formation in sponge spicules

2013

The inorganic scaffold of the spicules, the skeletal elements of the calcareous sponges, is formed of calcium carbonate (CaCO3). The growth of the approximately 300-μm large spicules, such as those of the calcareous sponge Sycon raphanus used in the present study, is a rapid process with a rate of about 65 μm/h. The formation of CaCO3 is predominantly carried out by the enzyme carbonic anhydrase (CA). The enzyme from the sponge S. raphanus was isolated and prepared by recombination. The CA-driven deposition of CaCO3 crystallites is dependent on temperature (optimal at 52 °C), the pH value of the reaction assay (7.5/8.0), and the substrate concentration (CO2 and Ca2+). During the initial pha…

Dilution-Triggered SMM Behavior under Zero Field in a Luminescent Zn2Dy2 Tetranuclear Complex Incorporating Carbonato-Bridging Ligands Derived from A…

2013

The synthesis, structure, magnetic, and luminescence properties of the Zn2Dy2 tetranuclear complex of formula {(mu(3)-CO3)2[Zn(mu-L)Dy(NO3)}(2)}center dot 4CH(3)OH (1), where H2L is the compartmental ligand N,N',N"-trimethyl-N,N"-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine, are reported. The carbonate anions that bridge two Zn(mu-L)Dy units come from the atmospheric CO2 fixation in a basic medium. Fast quantum tunneling relaxation of the magnetization (QTM) is very effective in this compound, so that single-molecule magnet (SMM) behavior is only observed in the presence of an applied dc field of 1000 Oe, which is able to partly suppress the QTM relaxation process. At variance,…

Mechanistic analysis and experimental verification of bicarbonate-controlled enteric coat dissolution: Potential in vivo implications

2019

Enteric coatings have shown in vivo dissolution rates that are poorly predicted by traditional in vitro tests, with the in vivo dissolution being considerably slower than in vitro. To provide a more mechanistic understanding of this, the dependence of the release properties of various enteric-coated (EC) products on bulk pH and bicarbonate molarity was investigated. It was found that, at presumably in vivo-relevant values, the bicarbonate molarity is a more significant determinant of the dissolution profile than the bulk pH. The findings also indicate that this steep relationship between the dissolution of enteric coatings and bicarbonate molarity limits those coatings' performance in vivo.…

Enzyme-accelerated and structure-guided crystallization of calcium carbonate: Role of the carbonic anhydrase in the homologous system

2014

Abstract The calcareous spicules from sponges, e.g. from Sycon raphanus, are composed of almost pure calcium carbonate. In order to elucidate the formation of those structural skeletal elements, the function of the enzyme carbonic anhydrase (CA), isolated from this species, during the in vitro calcium carbonate-based spicule formation, was investigated. It is shown that the recombinant sponge CA substantially accelerates calcium carbonate formation in the in vitro diffusion assay. A stoichiometric calculation revealed that the turnover rate of the sponge CA during the calcification process amounts to 25 CO2 s−1 × molecule CA−1. During this enzymatically driven process, initially pat-like pa…

2016

AbstractAuthigenic carbonate build-ups develop at seafloor methane-seeps, where microbially mediated sulphate-dependent anaerobic oxidation of methane facilitates carbonate precipitation. Despite being valuable recorders of past methane seepage events, their role as archives of atmospheric processes has not been examined. Here we show that cyclic sedimentation pulses related to the Indian monsoon in concert with authigenic precipitation of methane-derived aragonite gave rise to a well-laminated carbonate build-up within the oxygen minimum zone off Pakistan (northern Arabian Sea). U–Th dating indicates that the build-up grew during past ~1,130 years, creating an exceptional high-resolution a…

Water-Controlled Crystallization of CaCO3, SrCO3, and MnCO3 from Amorphous Precursors

2018

Calcium carbonate is the most abundant biomineral, whose amorphous form is stabilized in nature by a variety of organic additives and water. It is used to manipulate the morphology of new materials and to make strong inorganic/organic hybrid materials. However, the crystallization pathways (e.g., nucleation and growth, two-step nucleation pathways involving disordered, amorphous, or dense liquid states preceding the appearance of crystalline phases) remain often unclear. We have synthesized three amorphous carbonates, CaCO3 (ACC), SrCO3 (ASC), and MnCO3 (AMnC), that do not require any stabilization by additives to study their crystallization kinetics and mechanisms in the presence of water.…

2019

Here we explore the carbon and oxygen isotope compositions of the co-existing carbonate and phosphate fractions of fish tooth enameloid as a tool to reconstruct past aquatic fish environments and harvesting grounds. The enameloid oxygen isotope compositions of the phosphate fraction (δ18OPO4) vary by as much as ~4‰ for migratory marine fish such as gilthead seabream (Sparus aurata), predominantly reflecting the different saline habitats it occupies during its life cycle. The offset in enameloid Δ18OCO3-PO4 values of modern marine Sparidae and freshwater Cyprinidae from the Southeast Mediterranean region vary between 8.1 and 11.0‰, similar to values reported for modern sharks. The mean δ13C …

A hydrated crystalline calcium carbonate phase: Calcium carbonate hemihydrate.

2019

Hydrous CaCO 3 gets a new structure Calcium carbonate (CaCO 3 ) forms important minerals on Earth and is a model system for understanding crystal nucleation. Three different structures of CaCO 3 are known, along with two structures that are hydrated. Zou et al. found a third hydrated CaCO 3 structure formed from amorphous CaCO 3 in the presence of magnesium ions. The discovery illustrates the importance of amorphous precursors for producing new materials. Science , this issue p. 396