The surface charge of fluorite in the absence of surface carbonation

Abstract

The electrokinetic behavior of 16 different samples of fluorite particles, synthetic and natural, was determined by microelectrophoresis and/or by packed particle bed streaming potential (PPBSP) measurements. All samples demonstrated a high positive zeta potential in the absence of surface carbonation as expected from lattice ion hydration theory. Most interestingly, an increase in the fluoride ion concentration resulted in a decrease in the positive zeta potential but no reversal in the sign of the surface charge. In contrast to these results, flat plate streaming potential (FPSP) measurements revealed fluorite to have a small negative zeta potential. This anomalous behavior may be due to the fact that chemical equilibrium between the fluorite plate and the aqueous solution is not achieved even after day(s) of contact. In fact, when the solution is saturated with respect to CaF 2 prior to streaming potential measurements, positive zeta potentials are found for the flat plate of CaF 2. These results are discussed in terms of surface crystallography, lattice energy, hydration energy, and the kinetics of equilibration.