A Comparison of Nanometer-Scale Growth and Dissolution Features on Natural and Synthetic Dolomite Crystals: Implications for the Origin of Dolomite

Abstract

Abstract This paper describes experiments in which mechanisms of crystal growth are inferred from the surface nanotopography of natural and synthetic dolomite using atomic force and scanning electron microscopy. Synthetic dolomite was formed by replacement of calcite at 218°C in Mg–Ca–Cl solutions. The first products to form during dolomite synthesis experiments, as detected with X-ray diffraction (XRD), are invariably poorly ordered (nonideal) dolomite. As the reaction proceeds, the degree of cation order increases, yet the products remain relatively disordered. Upon examination, the surfaces of nonideal dolomite are covered with mounds 10–200 nm wide and 1–20 nm high. Following depletion of the calcite reactant, dolomite becomes relatively well ordered and stoichiometric (ideal). The surfaces of ideal dolomite are covered with broad, flat layers separated by steps that measure tenths to tens of nanometers high. Comparison of natural and synthetic dolomite surfaces after etching in 0.5% H2SO4 for 10–20 seconds indicates that high-temperature synthetic and low-temperature natural nonideal dolomite surfaces are covered with mounds identical to the growth features found on unetched nonideal synthetic dolomite. In contrast, synthetic and low-temperature ideal dolomite surfaces, when etched, are characterized by flat layers with deep, euhedral etch pits. These results suggest that despite the wide range in formation conditions, natural and synthetic dolomites form by the same growth mechanisms. Furthermore, the two different surface nanotopographies described here are consistent with a model in which nonideal dolomite forms by precipitation of amorphous mounds that quickly crystallize to nonideal dolomite while ideal dolomite later replaces nonideal dolomite and grows by spiral growth.