Atomic and Metallic Linkages

If we proceed further in the direction towards the atomic linkage corner of the tetrahedron, we come to lattices which cohere in two directions by ionic forces but in one by atomic forces they belong to a definite group of silicates whose structure will be discussed later in another connection (see p. 169). Particularly well known representatives are enstatite MgSiOs, diopside CaMg (8103)2 and asbestos. The latter at once suggest by their very marked fibrous habit that in one direction a particularly strong link prevails, namely, the atomic it is produced in the lattice by O-Si-0 chains, while in the other two directions weaker ionic forces prevail. 

The next step leads to substances with one ionic and two atomic linkages again silicates provide particularly suitable examples in the large groups of micas and chlorites. Their essentially laminar structures show very definitely that strong atomic forces are effective in two directions whereas, in the third, cohesion is produced by weaker ionic linkages. 

Finally, we arrive at a class of compounds in which the lattice as such is welded together in all three directions by atomic forces, while ionic linkage is present in the individual lattice cells. Examples of such substances which, following Grimm, will be denoted by the symbol 3a (3i) are CaTiOs, KMgFs and most of the feldspars. Reis has applied the term honeycomb lattice to such lattices, which also exhibit very interesting tensile properties. 

After them we come along the tetrahedron edge 3i—3a to the true atomic lattices which have their typical representative in the diamond and about which everything necessary has been said previously. 

Substances in which lattice structure is provided for in all three directions by homopolar primary valences, while the cohesion of the lattice cells is due to the metallic bond, may be present in many intermetallic compounds although nothing definite can be said here regarding the exact type of linkage involved. 

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