Nanotopography of synthetic and natural dolomite crystals

Abstract

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 °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 μm 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 ≈18 h, and the reaction went to completion in ≈40 h. Based on SFM analyses, the induction period for dolomitization in these experiments accounts for ≈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.

Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula, Mexico

“…Unless proved as altered syndepositional hydrocarbon seeps to the floor of the Basin, the bitumen in the Eocene of the Yax-1 core strongly suggests a higher-thannormal geothermal gradient during organic matter diagenesis. Replacive and vug-filling dolomite crystal surface topography data [compared to crystal microtopography data of Lasaga (1998), Arvidson and Mackenzie (1999), and Kessel et al (2000)] indicate that the dolomitizing fluid was at near-saturation with respect to dolomite and that dolomitization was a surface-controlled process. Polyhedral dolomite could have precipitated from a warm, hypersaline fluid.…”

Self-limiting growth on dolomite: Experimental observations with in situ atomic force microscopy

“…Laboratory preparation of synthetic dolomite, from calcite for example, generally requires hydrothermal conditions. (e.g., Kessels et al, 2000).…”

Surface structures, stabilities, and growth of magnesian calcites: A computational investigation from the perspective of dolomite formation

“…Investigations of structural aspects of dolomite include Ross and Reeder's (1992) X-ray intensity data refining the structural parameters of stoichiometric and ferroan dolomites and high pressure X-ray diffraction studies by Fiquet et al (1994) to obtain bulk moduli for dolomite and other carbonates, while Williams et al (1992) measured vibrational spectra of magnesite and dolomite. More recent studies of dolomites have investigated surface growth processes, for example the transmission electron microscopy (TEM) studies by Paquette et al (1999) of growth hillocks and surface microtopography, and scanning microscopy investigations of etch pits, layers, and islands on synthetic and natural dolomite surfaces by Kessels et al (2000).…”