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

Coprecipitation experiments show that structural characteristics of gr owth surfaces on synthetic calcite single crystals are a primary contr ol on SO4(2-) and SeO4(2-) incorporation. Electron probe microanalyses of sections through sectorally zoned crystals show that SO4(2-) conce ntrations in 1014BAR sectors are 50% higher than in 0112BAR sectors. E lectron 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 partitio ning behavior. Models of nearest-neighbor and second-nearest-neighbor coordination environments of nonequivalent kink sites reveal differenc es in site sizes and geometries. Vicinal faces that consist of steps w ith 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-) in corporation. This correlation requires surface-structural, i.e., cryst allographic, 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 Control s on trace ion partitioning is essential for a better understanding of trace ion studies in carbonate geochemistry and of crystal surface pr ocesses.