ATOMIC FORCE MICROSCOPE OBSERVATIONS OF NATURAL AND SYNTHETIC DOLOMITE CRYSTALS

Atomic force microscopy was used to investigate surface nanotopography on natural and synthetic dolomite crystals. Natural samples were chemically etched prior to analyses in order to accentuate surface features and defect microstructures. Stoichiometric dolomite crystals (Ca/Mg=1) reveal euhedral pits with flat inter-pit areas. Individual pits have sharp edges and range in size from 10-300 nm wide and 5-30 nm deep. Nonstoichiometric dolomite crystals (Ca/Mg > 1) display a pervasive mound-like nanotopography devoid of euhedral etch pits. Mounds are rounded topographical highs that measure 20-300 nm in diameter and 2-15 nm high.

Dolomite crystals synthesized by replacement of calcite at 218°C were initially Ca-rich, but became increasingly stoichiometric as the reaction progressed. Observations of synthetic dolomite crystals reveal mounds and layers. Individual mounds are 20-200 nm in diameter and 1-20 nm high. Layers are broad, flat surfaces that measure a few tenths to tens of nanometers high. Mounds were observed throughout most of the dolomitization reaction, whereas layers appear only at reaction completion. At the end of the reaction the degree of supersaturation, with respect to dolomite, must decrease as the reactant is consumed. This suggests that mounds occur on nonstoichiometric dolomite crystals during relatively high degrees of supersaturation and layers occur on stoichiometric dolomites under relatively low degrees of supersaturation.

Empirical observations suggest nanotopography is indicative of the dominant crystal growth mechanism. Spiral, layer and island nanotopographies have been interpreted to form by spiral, mononuclear, and polynuclear growth, respectively. Natural and synthetic dolomite crystals exhibit nanotopography that is directly related to stoichiometry. Stoichiometric crystals have flat surfaces and Ca-rich crystals have mounds. These observations suggest that stoichiometric and Ca-rich dolomites form by different crystal growth mechanisms, under different conditions. Specifically, stoichiometric dolomites grow by a spiral or mononuclear mechanism under relatively low degrees of supersaturation, whereas Ca-rich dolomites grow by a polynuclear mechanism under relatively high degrees of supersaturation.