EFFECT OF SUBSTRATE ROUGHNESS ON CALCITE EPITAXIAL GROWTH MODE AND SR INCORPORATION

The micro-topography of a crystal substrate influences epitaxial nucleation and growth dynamics and trace element incorporation. Growth on chemically and physically modified surfaces is common in nature, such as in diagenetic environments where growth occurs on physically weathered surfaces, and in hydrothermal and magmatic systems where cycles of dissolution and precipitation can be frequent. We characterize the role of calcite substrate roughness on epitaxial growth mode and subsequent Sr incorporation. Three analogue substrates for chemically weathered surfaces were prepared by acid attacking a cleavage face with dilute HCl for 10, 20, and 30 seconds. Three analogue substrates for physically weathered surfaces were prepared by abrading a cleavage face with 400 grit, 800 grit, and 1000 grit sandpaper until the surfaces were visibly dull and scratched. The analogue surfaces, along with a fresh cleavage surface, were characterized with a focused ion beam (FIB) - scanning electron microscope (SEM). The SEM images were used to make 3D reconstructions and to quantify the roughness and fractal dimensions of the surfaces. Sr-doped calcite was grown on the characterized surfaces at room 25 °C, 1 atm, and a pH of 8.3.

SEM images of the overgrowths show that physically weathered analogue substrates were prone to island growth and crystallite formation. Overgrowth on cleavage and chemically weathered analogue substrates formed via layer-by-layer growth and island growth. Island coalescence was inhibited on the physically weathered analogue substrates but occurred readily on the chemically weathered substrates. These differences manifest in overgrowth morphology, as overgrowths on physically weathered analogue substrates are shingled and overgrowths on chemically weathered analogue substrates and cleavage surfaces are characterized by meandering, widely spaced terraces. Morphological differences are reproducible and persisted throughout experiment durations. Microprobe measurements show a weak positive correlation between fractal dimension of the substrate and overgrowth Sr concentration.