Solid‐State Transformation of Amorphous Calcium Carbonate to Aragonite Captured by CryoTEM
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Abstract:
Early-stage reaction mechanisms for aragonite-promoting systems are relatively unknown compared to the more thermodynamically stable calcium carbonate polymorph, calcite. Using cryoTEM and SEM, the early reaction stages taking place during aragonite formation were identified in a highly supersaturated solution using an alcohol-water solvent, and an overall particle attachment growth mechanism was described for the system. In vitro evidence is provided for the solid-state transformation of amorphous calcium carbonate to aragonite, demonstrating the co-existence of both amorphous and crystalline material within the same aragonite needle. This supports non-classical formation of aragonite within both a synthetic and biological context.Keywords:
Amorphous calcium carbonate
Supersaturation
Vaterite
Amorphous calcium carbonate
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In this report, controls of polymorphism and morphology of calcium carbonate compounds were studied by suspending amorphous calcium carbonate hydrate (ACC) powder in water or MgCl2 solution. Characteristics of calcium carbonate compounds formed from ACC were determined by means of X-ray diffraction, thermal analysis (TG-DTA) and scanning electron microscopic observation. ACC was synthesized by adding 0.1mol⋅dm-3 CaCl2 solution into a mixed solution of 0.1mol⋅dm-3 NaOH and 0.1mol⋅dm-3 Na2CO3 at 0°C. The calcium carbonate compounds formed easily by suspending ACC in water and was affected remarkably by temperature (0-80°C) and pH (1.7-14.0). Thus ACC changed into hexagonal plate-like calcium carbonate hexahydrate at 0°C, rhombohedral calcite at 15-20°C, spherical vaterite at 30-50°C and needle-like aragonite above 80°C after aging for 1h. ACC was also changed into vaterite in the pH region of 7.0-9.5 and to basic calcium carbonate above pH 12.8 at 20-40°C. On the other hand, spherical calcium carbonate monohydrate was formed from ACC in MgCl2 solution (0.03-0.50mol⋅dm-3) and then changed finally to aragonite after long aging. For example, the amount of spherical monohydrate with a diameter of 30μm reached a maximum after aging for 3d, and it changed to needle-like aragonite with an average length of 50μm after 10d in 0.10mol⋅dm-3 MgCl2 solution. Accordingly, ACC changed easily to calcium carbonate anhydrides (calcite, aragonite, vaterite), calcium carbonate hydrates (monohydrate, hexahydrate) and basic calcium carbonate, when ACC was suspended in solutions of different conditions such as temperature, pH and concentration of MgCl2.
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Abstract Calcium carbonate polymorphism is one of the most challenging problems in biomineralization. Calcium carbonate exists in three crystalline forms: calcite, aragonite and vaterite. Otoliths of teleost fishes are made of vaterite and aragonite, located in different sacs and never mixed together. This paper presents a biochemical characterization of the intracrystalline macromolecules associated to the vateritic and aragonitic phases of otoliths. Experimental evidences from calcium carbonate overgrowth on the surface of otoliths and in vitro crystallization on the chitin‐silk fibroin assembly organic matrix suggest that the intracrystalline macromolecules associated to the otolith influence the aragonite−vaterite polymorphism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)
Vaterite
Amorphous calcium carbonate
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Abalone shell is natural inorganic/organic hybrid material, which is biologically constructed by using two calcium carbonates, aragonite and calcite. Aragonite can provide an enhanced mechanical support for the shell, and the stable phase, calcite, acts as the outer layer. In the current study, large-area aragonite film is deposited onto a calcite film in the presence of magnesium ion to biomimetically construct an aragonite−calcite complex structure. A calcite film is fabricated on a silicon wafer by a controlled phase transformation of amorphous calcium carbonate film at 120 °C, which is used as a substrate to induce the deposition of the aragonite layer with the addition of magnesium ion. We demonstrated the importance of the transition process from calcite to aragonite during the formation of the aragonite−calcite complex layer. This study could be used to mimic a sharp transition process of calcite to aragonite in vitro without any organic macromolecules.
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SUMMARY Solutions of calcium bicarbonate were allowed to lose carbon dioxide and evaporate to dryness under controlled temperature conditions. With filtered solutions prepared from spar calcite, precipitates were 100% calcite in the 2° to 100°C temperature range. When, in analogous experiments, coralline aragonite was the starting material, the precipitates were 100% calcite. Essentially the same was true when carbonate rocks from karst areas were used to prepare the experimental solutions. An artificially prepared mixture (maximum crystal size of about 7 u) of 70% aragonite and 30% calcite was also used in the study. The precipitates from this starting material were apparently affected by seed nuclei which passed through the filter. The stability of calcium carbonate seed nuclei appears to vary with temperature. Natural calcium bicarbonate solutions from caves yielded only calcite at 25°C. Calcite should be the dominant or only polymorph of CaCO 3 formed by the loss of carbon dioxide and evaporation of natural calcium bicarbonate solutions if temperature is the controlling factor. Since appreciable amounts of aragonite are found in many cave deposits, factors other than temperature must influence the polymorphs formed. POBEGUIN (1955) proposed that rapid evaporation and slow diffusion of solutions favor aragonite. If so, layers of aragonite and calcite in speleothems may reprsent alternate wet and dry paleoclimates. During these periods, rate of introduction of solution and rate of evaporation would change markedly.
Bicarbonate
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Cementitious
Amorphous calcium carbonate
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Abstract The seeding crystallization of calcite and aragonite in seawater was evaluated theoretically and experimentally. The level of supersaturation with respect to calcium carbonate, which is the driving force for the crystal growth, was found to be influenced by the pH value of seawater, the temperature and the seed morphology. It was proven experimentally and theoretically that the level of supersaturation in seawater with respect to calcium carbonate is more sensitive to pH than to temperature. The growth process of calcite or aragonite cannot start if the pH value of seawater is not adjusted to be higher than 8.0 in the basic medium. An initial pH value of 8.2 is found to be enough to initiate the growth process of both calcite and aragonite seeds. Calcite seeds were found to be subjected to higher levels of supersaturation than aragonite. Keywords: Seeding crystallizationSeawaterCalcium carbonateScale controlpHCalciteAragonite
Supersaturation
Seed crystal
Artificial seawater
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Aragonite (along with calcite) is one of the most common polymorphs of the crystalline calcium carbonate that forms the skeletal structures of organisms, but it has relatively low preservation potential. Under ambient conditions and in the presence of water, aragonite transforms into calcite, the stable polymorph. Aragonite is also more soluble therefore, in water-permeable siliceous limestones (opokas) that are typical of Upper Cretaceous deposits of Poland and Ukraine, the primary aragonitic skeletons are either entirely dissolved and found as moulds and casts or transformed into secondary calcite, whereas the primary calcitic shells remain well preserved. Contrary to the common notion of the lack of aragonite in such porous carbonate deposits, we show that relics of aragonite can be preserved as a nacreous lining on cephalopod moulds or as thin, lenticular structures entrapped in neomorphic calcite. Based on the observed intermediate steps of aragonite alteration, we propose an extended model of nacre diagenesis. Among the originally aragonitic biota, only nautilids and ammonites have retained relics of pristine skeletons. Such selective preservation of only some aragonitic structures (nacre but not the prismatic aragonitic layers) points to the role of microstructural and biochemical differences between cephalopod shell layers that may set a threshold for the dissolution, dissolution/precipitation or preservation of original biomineral structures.
Cephalopod
Amorphous calcium carbonate
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Summary Bivalves lay down two forms of calcium carbonate in their shells, aragonite and calcite. Shells may be wholly aragonitic, or may contain both aragonite and calcite, in separate monomineralic layers. Shells are built up of several layers of distinct aggregations of calcium carbonate crystals. These aggregations are referred to as shell structures and their general features are described. Aragonite occurs as prismatic, nacreous, crossed‐lamellar, complex crossed‐lamellar and homogeneous structures. Calcite occurs as prismatic or foliated structures. Myostracal layers (calcium carbonate laid down below sites of muscle attachment) are always aragonitic. The ligament and byssus when calcined are also invariably aragonitic. A summary of the occurrence of calcite and aragonite and the associated shell structures is given. Calcite is found only in the outer layer of superfamilies belonging to the subclass Pterio‐morphia with the exception of two species only from the Heterodont superfamily Chamacea. Generally within a superfamily shell structure and mineralogy are very constant. In all superfamilies these combinations have existed for many millions of years. It is therefore demonstrated that the prime control on shell mineralogy is genetic. Possible controls on mineralogy by the mantle cells, nature of the extrapallial fluid, nature of the periostracum and the organic matrix of the shell layers are discussed. It is known that environmental factors may modify the basic mineralogy/shell structure pattern within a superfamily. Thus there is an inverse relationship between the percentage of calcite in the shell and the mean temperature of the environment inhabited by the bivalve. A critical examination of published data shows that the evidence is convincing only in the superfamily Mytilacea. The species Mytilus californianus, which shows the greatest temperature effects, is peculiar amongst the Mytilacea in having an inner calcite layer as well as an outer one. Conflicting evidence for an inverse relationship between salinity and aragonite content is reviewed. The differences of opinion cannot be resolved without experimental work. We are grateful to the following for much useful discussion, and encouragement in many ways: Dr J. R. Baker, Dr G. E. Beedham, Dr B. C. M. Butler, Dr A. Hallam, Dr J. D. Hudson, Dr R. P. S. Jefferies, Mr J. Macrae, Dr W. S. McKerrow, Mr N. J. Morris, Mr C. P. Palmer, Mr N. Tebble, Dr E. R. Trueman and Professor A. Williams. Our best thanks are to Mr R. Cleevley for critically reviewing the manuscript. The following have rendered us considerable technical assistance: the staff of the electromicroscopy unit of the British Museum (Natural History), under the direction of Mr B. Martin; the technical staff of the Department of Geology, King's College, London and of the Department of Geology and Mineralogy, Oxford; Mrs J. M. Hall, and Mr G. Burton.
Amorphous calcium carbonate
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