Crystal Chemistry

Crystal Chemistry - Principles of Crystal Chemistry Our...

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Unformatted text preview: Principles of Crystal Chemistry Our discussion of bonding led us to conclude that the bond character is determined primarily by the electronic configurations and electronegativity values of the combining elements, whereas the crystal structure (which controls the shape and system of the crystal) is a function of the plane lattice and the coordination number(s) (C.N.). I- Coordination Number 1- Is the number of nearest neighbors in a crystal structure. It depends on the radius ratio + type of hybridization in covalent ones (Table 1) and the radius ratio in ionic structures (Table 2). 2- Whereas the C.N. is strongly controlled by the radius ratio, the radius of an ion (or atom) may (and usually does) change as a function of the C.N. (Fig. 1; Table 3). Other factors influencing the size of an ion are its charge, the atomic number of its element, and the shielding effect of some electrons, especially those of the penultimate shell. Of particular interest is the lanthanide contraction . 3- Coordination numbers are usually 2, 3, 4, 6, 8, or 12. C.N.s of 5, 7, 9, 10, or 11 are much less common, but can occur in some cases! Some Examples of C.N. (Fig. 2) Halite: NaCl Na : Cl = 6 : 6 Sphalerite: ZnS Zn : S = 4 : 4 Fluorite: CaF 2 Ca : F = 8 : 4 II- Structure Controls for minerals with metallic bonding: Bonding takes place between electropositive elements of similar electronegativities. Radii of the combining elements are also similar; radius ratio (r A /r B ) 2245 1. Three types of packing (Fig. 3): 1. Hexagonal close packing HCP: layers arranged in the pattern: ABAB. C.N. = 12. 2. Cubic close packing CCP: layers arranged in the pattern: ABCABC. C.N. = 12. 3. Body centered cubic packing: lower density structure; C.N. is always 8. Close packing results in the high density observed for metals and alloys. III- Structure controls for minerals with covalent bonding: Hybridization of the orbitals forces this type of bonding to have a strong directional character. Because of hybridization, the covalent radius of an element will be different from its ionic or metallic radii. 2 The structure of the mineral will be controlled by (i) the covalent radii of the elements; (ii) the type of hybrid resulting (Table 1); same factors that affect the C.N.! 3 IV- Structure controls for minerals with ionic bonding: Paulings Rules for ionic compounds: Rule 1:- Cation anion distance in a structure determined by radius sums, C.N.s determined from r + /r- (Table 1). For this rule, the following points are very important: 1- The r + /r- limiting values for the different coordination numbers are derived by simple rules of geometry and trigonometry (Figs 4 & 5)....
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Crystal Chemistry - Principles of Crystal Chemistry Our...

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