FURTHER EXPERIMENTAL-EVIDENCE FOR A CRYSTALLITE SIZE EFFECT IN THE FES2-COS2 SYSTEM

The experimental study of the miscibility gap in the FeS2-CoS2 system at different particle sizes with previously reported theoretical data on the miscibility in small-crystallite binary systems gives evidence for a crystallite size effect that is the dependence of phase relations (particularly, phase boundary position) upon the crystallite size. The pyrite-cattierite solvus for phases consisting of not very small particles (greater-than-or-equal-to 1-2 mum) has a critical point of approximately 740-degrees-C and 13 mole% CoS2, whereas the critical point for small crystallites (approximately 0.1-0.2 mum) occurs at approximately 690-degrees-C and 15 mole% CoS2. Although it appears to be very difficult to prove the equilibrium relations in the systems consisting of small particles, certain criteria are proposed to show that the change in binodal position presents the equilibrium phenomenon rather than kinetic effect. The data obtained support the possibility of the effect of crystallite size on the solid state miscibility in mineral systems.