THE VARIANCE AND THERMODYNAMIC EQUILIBRIUM OF THE FACE SYSTEM OF CRYSTAL

The thermodynamic elements of a face system of crystal are reticular faces, divided into two uniform groups: basic faces (which are systematic in a face system) and supplementary faces (which shorten and disappear during crystal growth). The existing methods for estimating the equilibrium of a face system and crystal faces are analyzed. It is shown that the Gibbs equilibrium condition is not satisfied for a face system consisting of basic and supplementary faces; it is valid for only a basic crystal. The functions of intensive parameters of a face system are free coordination bonds and temperature. Pressure does not affect the formation of reticular faces. The chemical potentials of the crystal components are invariant. The extensive parameters are the areas of reticular faces and entropy. The variance function of a face system is derived.
An extended form of the Gibbs condition is obtained by expanding the internal intensive parameters in isolated groups of faces, and it is shown that the basic faces, being invariant, d not affect the crystal equilibrium. An equilibrium of a face system is determined by supplementary faces. An example of calculating the variance of two shapes of zircon crystals is presented.