| The lack of dolomite in modern marine setting is a kinetic problem, yet relatively few details about dolomitization are understood. Because the geometries observed on crystal surfaces are dictated by growth and dissolution, and such mechanisms are theoretically related to kinetics, the current study utilizes ex situ atomic force microscopy to investigate nanometer-scale features on synthetic and natural dolomites in order to better understand dolomitization.; Following a relatively long induction period, high-temperature synthetic dolomite forms very rapidly. Initial dolomite products are poorly ordered (i.e., nonideal), whereas stoichiometry (61-50 mole% CaCO3) is dependent on the initial Mg2+:Ca2+ ratio in solution (R2=0.97). Following initial reactant depletion, dolomite products are stoichiometric and well-ordered (i.e., ideal).; Two distinct nanometer-scale growth features - islands and layers - characterize dolomite growth surfaces. Islands are rounded positive relief features. Layers are broad, flat surfaces with steps. Islands occur on nonideal synthetic dolomite prior to reactant depletion, whereas layers form only after calcite reactant depletion. Counter to theoretical predictions, dolomite nanotopography is independent of the Mg2+:Ca2+ in solution. Surface nanotopography does respond, however, to changing carbonate flux at the growth interface following reactant depletion, therefore suggesting that carbonate plays a major role in dolomitization kinetics.; Following chemical etching, ideal synthetic dolomite surfaces exhibit flat layers with deep euhedral pits, whereas nonideal synthetic dolomite surfaces are covered with islands identical to the islands observed on growth surfaces. Chemically etched ideal and nonideal natural dolomites are also characterized by etch pits and islands, respectively. These features are indistinguishable from islands and etch pits observed on synthetic dolomite. Based on models of crystal growth and dissolution, nonideal dolomite surface nanotopography is most consistent with polynuclear growth. Conversely, ideal dolomite is more consistent with spiral growth. These observations indicate that dolomite initially forms by polynuclear growth and is nonideal. Because it is metastable, nonideal dolomite may later be replaced by ideal dolomite.; Due to the similarities between natural and synthetic dolomites, high-temperature experimental findings can serve as a model for interpreting observations from natural low-temperature settings. Therefore, a long induction period followed by rapid growth is the best model to explain the absence of dolomite in modern marine environments. |
