Fresh cleavage surfaces of chemically pure Iceland spar calcite from two separate specimens were examined using Scanning Probe Microscopy (SPM). Initially flat terraces were separated by monolayer cleavage steps. Surface rearrangement took place during the first days after cleavage, as was described by Stipp et al. (1996). The samples were left exposed to air in a clean, covered, plastic box for eight months. Reexamination with SPM showed that the surface of both samples had recrystallized but on one sample, there was an accumulation of bumps along one of the {1014} cleavage directions. The height and width of the bumps ranged from fractions of a micrometer to nanometers. Chemical maps of the surface were made using time of flight secondary ion mass spectroscopy (TOF SIMS); the bumps were depleted with respect to Ca and O, but enriched with respect to Na, K, F, Cl and C2. Some bumps were selectively enriched in one or another of the monovalent trace elements, and all
were elongated in the second of the cleavage trace directions. A fresh surface was prepared by cleaving the sample parallel to the observed surface. It was flat, as the original surface had been, with monolayer high terraces. TOF-SIMS showed a
homogeneous surface of Ca, O and H, with the only enrichment, an increase in H and OH along terrace edges. Na, K, F, Cl and C2 were below background on the freshly cleaved sample.
These results suggest that trace amounts of monovalent elements, that were originally present in the bulk calcite, moved out to the surface during exposure to air, by a process like exsolution. There, the rearrangement of surface material concentrated the trace elements as crystallites located at defects at the intersections of the three cleavage planes. This process of material moving from the bulk to the surface is opposite in effect to a similar process identified recently, where cadmium and zinc, that had adsorbed on calcite surfaces, moved into the bulk by apparent solid-state diffusion (Stipp et al., 1992; Stipp, 1994).
I wish to thank the members of the Environomental Surface Analysis Collaboration, Ecole Polytechnique Fédéral and Université de Lausanne, for interest and support. In particular, I thank Andrzej Kulik and Willy Benoit of genie atomique, physique and Klaus Franzreb and Hans Jörg Mathieu of métalurgie chimique, matériaux for assistance and access to equipment.
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Stipp, S.L., Hochella, M.F., Jr., Parks, G.A. & Leckie, J.O., Geochim. Cosmochim. Acta 56, 1941-1954 (1992).