SURFACE STRUCTURAL CONTROLS ON COMPOSITIONAL ZONING OF SO4(2-) AND SEO4(2-) IN SYNTHETIC CALCITE SINGLE-CRYSTALS

Coprecipitation experiments show that structural characteristics of growth surfaces on synthetic calcite single crystals are a primary control on SO4(2-) and SeO4(2-) incorporation. Electron probe microanalyses of sections through sectorally zoned crystals show that SO4(2-) concentrations in 1014BAR sectors are 50% higher than in 0112BAR sectors. Electron probe and synchrotron X-ray fluorescence microanalyses within 1014BAR sectors document a twofold to threefold difference in SO4(2-) and SeO4(2-) contents between subsectors associated with nonequivalent vicinal faces of growth hillocks. This differential incorporation of SO4(2-) and SeO4(2-) documents path-dependent, nonequilibrium partitioning behavior. Models of nearest-neighbor and second-nearest-neighbor coordination environments of nonequivalent kink sites reveal differences in site sizes and geometries. Vicinal faces that consist of steps with larger and geometrically less constrained kink sites have greater SO4(2-) and SeO4(2-) incorporation, whereas vicinal faces with smaller, more constrained kink sites always have less SO4(2-) and SeO4(2-) incorporation. This correlation requires surface-structural, i.e., crystallographic, controls on 4- and SeO4(2-) incorporation and, therefore, is consistent with SO4(2-) and SeO4(2-) substitution for CO3(2-). Our results clearly show that consideration of surface structural Controls on trace ion partitioning is essential for a better understanding of trace ion studies in carbonate geochemistry and of crystal surface processes.