Anisodesmic structure

In the case of chain and sheet silicates having typically anisodesmic structures, interpretational difficulties arise. Isodesmic structures are distinguished by the absence of very compact, clearly discernable components, whereas anisodesmic structures contain components with internal bonds stronger than those between the components. Consequently, the hardness of compounds of this type depends chiefly on the resistance of the weakest bonds, the variability of the energetic conditions of interlaminar bonds (van der Waals ) and the variety of surface forms of the chains and strata in mineral structures, e.g., smooth, undular, notched, and so on. 

ANISODESMIC STRUCTURE. A type of ionic crystal in which some of die ions tend to form tightly bound groups, e.g.. nitrate and chlorate,... 

The phenomenon of increased hardness occurs principally in minerals of sheet and chain structures, which link together through the cations (silicates and aluminosilicates, as well as hydrated sheet minerals, such as glauconite, melilite and gypsum—M ranging from 0 to about 1.25), and also in minerals of skeletal structures (borates, phosphates, sulphates, nitrates, carbonates, such as calcite, dolomite and others—Ah from 0 to about 1.15). For this reason, the hardness analysis of minerals with weak bonds demands consideration of the fact that just as the basic crystallo-chemical factors, so is hardness influenced by the form of domains (component parts of structures) in all anisodesmic minerals of chain, sheet or skeletal structure. Depending on the form of domain (and also according... 

A proper interpretation of the structural specificity of anisodesmic minerals for calculating Ah may prove difficult, and therefore discrepancies between calculated and experimental values are possible. For this reason, the anisodesmic minerals, for which the differences can be considerable... 

The non-adamantine lattice of nickel arsenide, NiAs, is another common AB structure (Fig. 91) it is also displayed by FeS, FeSc, CoS, CoSe, CoTe, NiS, NiSe, NiTe, NiSb, MnSb, CoSb. Each metal atom is surrounded by a slightly distorted octahedron of six Gp. 5B or Gp. 6B atoms, which themselves form nearly regular tetrahcdra without metal atoms, the structure being anisodesmic. 

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