Sodium chloride coordination environments

The sodium chloride (NaCl also called the rock salt structure) and zinc blende (ZnS) structures are based on a face-centered cubic lattice. In both structures the anions sit on the lattice points that lie on the corners and faces of the unit cell, but the two-atom motif is slightly different for the two structures. In NaCl the Na ions are displaced from the CF ions along the edge of the unit cell, whereas in ZnS the Zn ions are displaced from the ions along the body diagonal of the unit cell. This difference leads to different coordination numbers. In sodium chloride, each cation and each anion are surrounded by six ions of the opposite type, leading to an octahedral coordination environment. In zinc blende, each cation and... 

Aluminum occurs widely in nature in silicates such as micas and feldspars, complexed with sodium and fluorine as cryolite, and in bauxite rock, which is composed of hydrous aluminum oxides, aluminum hydroxides, and impurities such as free silica (Cotton and Wilkinson 1988). Because of its reactivity, aluminum is not found as a free metal in nature (Bodek et al. 1988). Aluminum exhibits only one oxidation state (+3) in its compounds and its behavior in the environment is strongly influenced by its coordination chemistry. Aluminum partitions between solid and liquid phases by reacting and complexing with water molecules and anions such as chloride, fluoride, sulfate, nitrate, phosphate, and negatively charged functional groups on humic materials and clay. 

Strongly Alkaline, Confined Environments, Because Si is most soluble in the form of silicate anions, it is not surprising that smectites form most rapidly in dilute alkaline solutions of sodium silicate and magnesium chloride. The layer silicate structure evolves by the precipitation of a planar Mg(OH)2 (brucite) sheet, on which monomeric silicate ions condense. Silica polymers are unable to reorganize into layered structures. It seems that the metal hydroxide sheet must be layered to begin with, a fact that necessitates 6-coordination of the metal ion that is to form the octahedral sheet of the 2 1 layer. Smectites can be formed from hydroxide sheets of the following cations (listed in order of increasing radius) ... 

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