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