Nanotopography of synthetic and natural dolomite crystals

Examination with scanning electron microscopy (SEM) and scanning force microscopy (SFM) revealed etch pits, layers and islands on dolomite crystal faces synthesized from calcite in Ca-Mg-Cl solutions at 200 degrees C and a wide variety of natural dolomites. Layers are broad, flat structures bounded by steps less than 100 nm high and greater than 1 mu m wide. Islands are rounded topographic highs < 20 nm high and < 200 nm wide. The nanotopography of synthetic dolomite changed from islands throughout most of the reaction to layers at 100% dolomite. Island nanotopography formed on both Ca-rich and near-stoichiometric dolomite. Analyses of reaction products from dolomite synthesis indicates that there are no SFM-detectable products formed in < 10 h. SEM-detectable products formed in 15 h. X-ray diffraction (XRD)-detectable products formed in approximate to 18 h, and the reaction went to completion in approximate to 40 h. Based on SFM analyses, the induction period for dolomitization in these experiments accounts for approximate to 20% of the total reaction time necessary to dolomitize CaCO3 completely under the experimental conditions used here. Island nano- topography is inferred to occur at higher degrees of supersaturation than layer nanotopography for three reasons. First, island nanotopography on synthetic calcite and gypsum forms at higher supersaturations than layer nanotopography. Secondly, island nanotopography formed in solutions with higher degrees of supersaturation with respect to dolomite. Thirdly, the greater surface roughness of a crystal face composed of islands compared with layers indicates that island surfaces have higher surface energy than layer surfaces. Therefore, the stability of island surfaces requires a higher degree of supersaturation. Because islands and layers form under a wide range of conditions, their presence provides broadly applicable criteria for evaluating relative degrees of supersaturation under which ancient dolomite formed. Comparison of synthetic dolomites with natural dolomites demonstrates (1) similar nanotopography on natural and synthetic dolomites and (2) both natural planar and non-planar dolomite may have island nanotopography.