PROFILOMETER-BASED STUDY OF NANOMETER- TO MILLIMETER-SCALE GROWTH KINETICS OF CALCITE FROM CO2-RICH FLUIDS

Two distinctively different methods have been used in the literature to study the precipitation and dissolution of calcium carbonates in aqueous solutions; a) bulk, macroscopic method using powdered carbonates or single crystals with linear growth rates calculated either from the amount of titrants injected or from weight gains, and b) in situ, real-time atomic force microscopy (AFM) methods, which can provide atomic- or nanometer-scale understanding of nucleation and growth of calcite. Thus, these techniques are on the two ends of a spectrum in terms of spatial and temporal resolutions.

We have developed a novel method for accurate determination of linear growth rates of calcite with height profiles on the order of tens of nanometers and a linear scanning range of several millimeters, employing a computer-driven stylus-based profilometer. By scanning the same line on the surface of single crystals of calcite before and after experiments, 2-dimensional growth patterns and rates can be obtained as a function of time and solution compositions. Our results at 25°C, pCO2=0.1 bar, and degree of supersaturation of 4.5 to 9.3 showed that a) numerous rhombic pits in size ranging from a few to tens of µm occurring randomly across the crystal surface bounded by [441] and [481] (AFM), b) uneven growth of calcite on the cleaved surface over a mm range with an average growth rate of 29±17 nm/hr (profilometer), and c) non-uniform coverage by new layers of calcite even within the same cleaved surface (microscope). This study demonstrated the need and utility of 2D and 3D growth rate measurements of calcite and other minerals from an atomic to macroscopic scales. *Research sponsored by Division of Chemical Sciences, Geosciences, and Biosciences, U.S. DOE under contract DE-AC05-00OR22725, Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.