Calcium , crystal structure

Table 21-IV shows some properties of the metals and their crystal forms. Since different crystal forms are involved in the series, trends in the properties are obscured. Figure 21-2 shows scale representations of the crystal structures of metallic beryllium, calcium, and barium.

Deng, L., et al. (2002b). The crystal structure of the calcium-regulated photoprotein obelin from Obelia geniculata. Luminescence 17 87-88. 

The tannins influence calcium salt crystal structures to produce the easily dispersible calcite (cubic structure) rather than the scaleforming aragonite (rhombic structure). 

Calcium salt crystal structures producing calcite 406... 

FIGURE 2.8 A micrograph ot bone, which owes its rigidity to calcium phosphate. The overlay shows part of the crystal structure of calcium phosphate. Phosphate ions are polyatomic ions however, as shown in the inset, they are nearly spherical and fit into crystal structures in much the same way as monatomic ions of charge —3. 

Crystals of the intermetallic compound magnesium stannide, MgjSn, have been prepared and investigated by means of Laue and spectral photographs with the aid of the theory of space-groups. The intermetallic compound has been found to have the calcium fluoride structure, with dwo = 6.78 0.02 A. U. The closest approach of tin and magnesium atoms is 2.94 0.01 A. U. 

Fig. 8.1 Crystal structures of some calcium-(B,C,N) compounds (a) CaBe (LaBg type) (b) CaB4 xCx (LaB4 type) (c) CaB2C2 (corresponding with LaB2C2) (d) Can(CN2)2N6 (e) Ca3(BN2)N (f) Ca3Cl2C3 (g) Ca3Cl2(CBN).

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... 

Figure 5.18.1 The NaCl crystal structure consisting of two interpenetrating face-centered cubic lattices. The face-centered cubic arrangement of sodium cations (the smaller spheres) is readily apparent with the larger spheres (representing chloride anions) filling what are known as the octahedral holes of the lattice. Calcium oxide also crystallizes in the sodium chloride structure.

A comparison of the crystal structures, NMR and IR spectra of various Yb(ll) and calcium complexes demonstrated that they were strikingly similar, a reflection of the nearly identical radii of Ybz+ and Ca2+.25 Nevertheless, the dibenzylytterbium(ll) analog of 127 produces polystyrene of high syndiotacticity (r= 94.9%, rr= 90.0%), whereas 127 itself yields only atactic or slightly syndiotactic polymer. A difference in Yb-L and Ca-L bond strengths, despite their similar lengths, has been proposed as the source of the difference.315... 

As discussed in more detail in Section 2.02.4.2.2.(iii), almost all the metallocenes of calcium, strontium, or barium are bent, even when unsolvated. The only exception to this so far is the sterically crowded (CsPr Ba 152 (Figure 79),348 whose X-ray crystal structure reveals a linear geometry with Ba-C = 2.997(4)A. All three... 

Consider now the bonds to each O2- ion in the perovskite structure. First, there are two bonds to Ti4+ ions that have a character of 4/6 each, which gives a total of 4/3. However, there are four Ca2+ ions on the corners of the face of the cube where an oxide ion resides. These four bonds must add up to a valence of 2/3 so that the total valence of 2 for oxygen is satisfied. If each Ca-O bond amounts to a bond character of 1/6, four such bonds would give the required 2/3 bond to complete the valence of oxygen. From this it follows that each Ca2+ must be surrounded by 12 oxide ions so that 12(1/6) = 2, the valence of calcium. It should be apparent that the concept of electrostatic bond character is a very important tool for understanding crystal structures. 

Jiang, Y., Lee, A., Chen, J. et al. Crystal structure and mechanism of a calcium-gated potassium channel. Nature 417, 515-522, 2002. 

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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