THE DYNAMIC NATURE OF CALCITE SURFACES IN AIR

The {(10(1) over bar 4)} surfaces of optical quality calcite (Iceland spar) were examined in air using scanning force microscopy (SFM) immed iately after cleavage and during the hours and days that followed. In the absence of a visible contacting solution phase, spontaneous rearra ngement of the surface was observed. Images with nanometer-scale resol ution showed the formation of hillocks and holes on terraces and cleav age steps; thickness or depth varied from one to ten calcite monolayer s (3-30 Angstrom). The rate of surface change varied with geochemical system parameters such as humidity and partial pressure of N-2 and CO2 , but instrument parameters such as imaging force, tip composition, sc anning rate, and reimaging frequency had almost no effect. On the basi s of previous work documenting the existence of surface-hydration spec ies and a layer of molecular water on samples exposed only to air, we interpret that the observed process results from dissolution and repre cipitation within an invisible layer of water that is adsorbed from ai r following cleavage. The dynamic nature of calcite surfaces has impor tant implications in conceptual models for the behavior of adsorbed tr ace metals in unsaturated porous media or arid climates and also for a ttack on ''dry'' statues and building stones under acidic atmospheres.