ENERGY MINIMUM CRITERIA IN MODELING STRUCTURES AND PROPERTIES OF MINERALS

The leading principle in modeling procedures is the minimization of st ructural energy. It is assumed that the calculated minimal energy has to be compared with the experimental estimate of cohesion energy of a crystal. The reference state for structure energy depends on bonding t ype: the lattice energy for purely ionic crystals consisting of cation s and anions, the atomization energy for covalent and metallic crystal s consisting of atoms, the sublimation energy for molecular crystals c onsisting of molecules etc. As is well-known, a majority of minerals c an not be correctly described as purely ionic crystals. Moreover, for these crystals lattice energy cannot be determined empirically because free anions, such as O2-, S2-, As3-, etc., do not exist. In order to describe crystal structures and properties in a better approximation i t is usually proposed that the bonding character of such crystals is i ntermediate between ionic and covalent, so that effective charges inst ead of formal ones, and an appropriate covalent contribution, are invo lved in energy calculations. However, the corresponding calculated str ucture energies are not comparable to any experimental values. Moreove r, they decrease with increasing effective charges and a purely ionic structure seems, as before, to be most stable from energetic point of view. To avoid this ''energetic catastrophe'' the so-called ''charge-t ransfer energy'' has to be taken into account. Estimations of charge-t ransfer energies were sometimes faced with difficulties because of lim itation of knowledge about valence-state energies of atoms. Now it is possible to refine this approach using new data on the average one-ele ctron energies of the valence-shell electrons in ground-state free ato ms. It is demonstrated for the examples of Si and O atoms. The success ive ionization of mixed s, p valence-shell of silicon atom from Si-0 t o Si4+ states is reconstructed and a new extra-polation procedure to o btain the electron affinity of O2- is applied. The structure energy of stishovite SiO2 as a function of the ionicity degree parameter f is c alculated with the aid of pair potential consisting of the effective i onic, the covalent Morse type contributions and the charge transfer co rrection. The structure energies change from - 6.32 eV for the ionic s tructure (f=1) to -11.91 eV for the covalent structure (f=0) with mini mum value of -15.41 eV for f=0.49. The latter can be compared to the e xperimental value of atomization energy (-18.9 eV). The corresponding oxygen atomic coordinate x varies from 0.310 (f=1) to 0.300 (f=0) bein g equal to the true value of 0.306 at f=0.75. At less than 0.7 the imp rovement of the simulated elastic and dielectric constants, when compa red to the ionic model, is sufficient.