Mineralogy. Principle of a close packing of atoms and ions.

For an explanation of the nature of crystalline structures of substances in a crystallography the principle of a close packing of atoms and ions in crystals is used, agrees which is accepted, that first, the shape of all atoms and ions spherical and, secondly, all volume of a crystal or its separate structural blocks is filled by densely adjoining atoms and ions. On the basis of this principle it was possible to characterise simply and geometrical figuratively many features of a crystalline structure of minerals.

Let's view to begin with possible methods of the close packing of balls of equal diameter. We will figure against each other two stratums of densely adjoining balls, having designated the inferior stratum letter A, upper - B. The third stratum it is possible to figure on stratum B differently. In one case - in the same way as stratum A, in other - balls of the third stratum will take of not iterated position C, they then can be overlapped the fourth stratum of balls which will iterate a standing of stratum A.

Packing of the first type is characterised by repeatability AB AB AB... It term two-layer (and on character of symmetry - hexagonal). For packings of the second type repeatability ABC ABC ABC is characteristic... It term triplex * (cubic). There are many other orders of repeatability of stratums in the close packing of balls, but all of them will be combinations of first two packings.

Densely laid balls occupy only 74 % of volume filled with them, and 26 % are necessary on cavities between balls. Their two types. One cavities, smaller on the size, settle down between four balls. Them term tetrahedral. Other, major cavities on the size are restricted by six balls - octahedral. In the perpetual crystalline construction on n balls it is necessary 2n tetrahedral and n octahedral cavities.

Example of build-up of crystalline structure of substance it is almost exact by a close packing principle corundum Al2O3 can be. In it (radius of 0.132 nanometers on V.Goldshmidtu) form large ions of oxygen a two-layer close packing, 2/3 octahedral cavities are occupied by ions Al (radius of 0.057 nanometers, on V.Goldshmidtu), tetrahedral positions are free.

If to consider crystalline substances constructed on a principle of the ideal close packing, all variety of structures of minerals should be spotted by three factors:

Close packing type, in the size and valence of the atoms forming this packing;

Gang of the atoms, filling a close packing;

Pattern of settling of cavities. Variety of combinations of these factors obviously. However the number of minerals with an ideal close packing of atoms is rather insignificant. It speaks first of all that such crystalline constructions are possible for minerals with directional chemical bonds - metal or ionic. Really, for example, native metals (Au, Cu, Ag) have structures with a triplex (cubic) close packing, native iridium and zinc - with double-layer (hexagonal) packing. On the contrary, the crystalline structure of native sulphur is far from a close packing. In sulphur are shown guided - the covalent chemical bonds, eight-nuclear difficult molecules on a configuration with zero net charge S80 are thus formed, they are joined by residual communications in differently oriented columns.

From the substances spread in the nature the close packing is characteristic for few minerals - for example for corundum Al2O3 and spinels MgAl2O4. It is indicative, that at small molecular quantities of these substances they possess concerning raised in the density. Are close enough to a close packing of structure of some orthosilicates - it is olivinic, garnets, etc.

The majority of minerals has the difficult crystalline constructions, in them only the structure of separate blocks answers a principle of a close packing of atoms. This principle - only the model, helping to interpret a reality.