Morphology changes caused by seawater ions in templated calcite crystals.*

It has been shown that the presence of Mg in ratios approximately equal to those found in seawater, during the organic-monolayer-templated nucleation of CaCO$_{3}$, significantly changes the morphology of the nucleating crystals. Crystals nucleated from supersaturated subphases of CaCO$_{3}$ under floating arachidic sulfate monolayers grow as (001) oriented tetragonal pyramids [1]. We have found that crystals nucleating under arachidic sulfate from supersaturated solutions of CaCO$_{3}$ containing approximately 5:1 Mg:CaCO$_{3}$ grow as (001) hexagonal prisms, which express (100) or (110) faces. The crystal-water surface energy of these faces is almost twice that of the (104) face expressed on the tetragonal pyramids [2]. The growth of two morphologies displaying different surfaces but the same (001) orientation suggests that epitaxy, when present, plays a larger role than surface energetics alone. Our studies of Mg-concentration-dependent changes in morphology will also be presented. \newline \newline [1] Kewalramani, S. et. al.'', Langmuir, 24, 10579, 2008 \newline [2] Duffy, D.and Harding, J, Langmuir, 20, 7630, 2004