Calcium carbonate mineral specimens were transformed to fluorite in NH₄F solutions. With calcite single crystals, the combination of epitaxy and the anisotropy of the reaction led to the formation of elongated hexagonal etch pits. The reaction rate was shown to be directly proportional to the F^-concentration and the surface area. The initial reaction rates were used to obtain estimates of the effective surface area, as contrasted to the geometrical surface area, for porous specimens. Comparison of the experimental rate curves with theoretical rate curves showed that the rate of reaction at the phase boundary was an important, if not the only, rate determining step.