THE CRYSTAL ENERGY OF PYRITE

The crystal energy of iron pyrite, ideally of composition FeS2, is evaluated with an ionic model, assuming atoms in the solid to be ionized as Fe2+ and S-. To calculate the proper crystal energy, terms accounting for the static electric dipole moments of the sulphur ions have to be included. For this purpose, so-called electrostatic lattice constants are introduced, with Madelung's constant being the first term of a Taylor expansion for the electrostatic interaction energy. Following these considerations, a theoretical crystal energy of -2835 kJ per mole of pyrite is calculated. The formalism gives a first estimation of the strength of the sulphur dipole moment in the FeS2 lattice: mu(S) = 12.3 x 10(-30) C m (3.7 D). The thermodynamic Born-Haber cycle is generalized to include polarized species, yielding an 'experimental' value for the crystal energy of -2893 kJ per mole of pyrite, which differs only by 2% from the theoretical one.