Calcium carbonate, crystal structure

Consider how this mechanism of interface failure could apply to a natural laminated material like mother of pearl. This nacreous substance is formed by the deposition of thin calcium carbonate crystals by the mollusc. Such plate-hke crystals are typically 10 pm long and 0.3 pm thick, separated by a tenuous layer of polymeric protein material which forms only a few percent of the composite volume. Yet, despite this low polymer content, nacre displays significant plastic deformation when stretched, with strains up to 0.018, accompanied by white deformation bands in the composite structure. " ... 

While the presence of a filler affects the way a matrix crystallizes, the opposite is also true. In studies of in situ formation of calcium carbonate in different copolymers, different crystalline forms of calcium carbonate were found.Calcium carbonate crystallized without a polymer had a rhombohedral morphology. When crystallized in the presence of polyethylene oxide its morphology remained rhombohedral because the polymer does not interact with the crystal of calcium carbonate as it forms. The calcium carbonate crystals which formed in the presence of methacrylic acid copolymer had an elongated structure not found in the other two cases. 

Ionic crystals are not limited to monatomic ions polyatomic ions—ions consisting of two or more atoms—also form crystal structures. Atoms within a polyatomic ion are held together by covalent bonds (Section 12.3). Figure 12.3 is a model of a calcium carbonate crystal. The formula of a carbonate ion, one of which is circled in Figure 12.3, is The carbon atom is surrounded by three covalently bonded... 

Spherical vaterite crystals were obtained with 4-mercaptobenzoic acid protected gold nanoparticles as the nucleation template by the carbonate diffusion method [51]. The crystallization of calcium carbonate in the absence of the 4-MBA capped gold nanoparticles resulted in calcite crystals. This indicates that the polymorphs of CaCOj were controlled by the acid-terminated gold nanoparticles. This result indicates that the rigid carboxylic acid structures can play a role in initiating the nucleation of vaterite as in the case of the G4.5 PAMAM dendrimer described above. 

Marble. The word marble is used as the common name for two types of monomineral rocks one derived from limestone and therefore composed of calcium carbonate, the other derived from dolomite and composed of calcium magnesium carbonate. Extremely high pressures and heat during past geological times modified the structure of both limestone and dolomite, compacting them into a characteristic crystal structure. Most marble is white however, minor and trace amounts of metallic impurities cause the formation of stains in a variety of colors, hues, and patterns, or of colored marble. 

The bones and teeth of humans and other vertebrate animals, for example, consist mainly of a composite material made up of an organic substance, collagen, and a biomineral, calcium carbonate phosphate (see Textboxes 32 and 61). The latter, which makes up about two-thirds of the total dry weight of bone, is composed of calcium phosphate containing between 4-6% calcite (composed of calcium carbonate) as well as small amounts of sodium, magnesium, fluorine, and other trace elements. The formula Ca HPChXPChMCChXOH) approximately represents its composition its crystal structure is akin to that... 

Non-lattice sites may play an important role in the incorporation of large foreign ions in crystal structures during coprecipitation Pingitore (Chapter 27) discusses the importance of these sites in the formation of coprecipitates of calcium carbonate containing Srz+ or Ba. White and Yee (Chapter 28) discuss the diffusion of alkali ions into defect structures in the surfaces of glasses and crystalline feldspars. 

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

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