In Situ AFM Study of Crystal Growth on a Barite (001) Surface in BaSO4 Solutions at 30 °C

Abstract

Abstract:The growth behavior and kinetics of the barite (001) surface in supersaturated BaSO 4 solutions (supersaturation index (SI) = 1.1-4.1) at 30 • C were investigated using in situ atomic force microscopy (AFM). At the lowest supersaturation, the growth behavior was mainly the advancement of the initial step edges and filling in of the etch pits formed in the water before the BaSO 4 solution was injected. For solutions with higher supersaturation, the growth behavior was characterized by the advance of the <uv0> and [010] half-layer steps with two different advance rates and the formation of growth spirals with a rhombic to bow-shaped form and sector-shaped two-dimensional (2D) nuclei. The advance rates of the initial steps and the two steps of 2D nuclei were proportional to the SI. In contrast, the advance rates of the parallel steps with extremely short step spacing on growth spirals were proportional to SI 2 , indicating that the lateral growth rates of growth spirals were directly proportional to the step separations. This dependence of the advance rate of every step on the growth spirals on the step separations predicts that the growth rates along the [001] direction of the growth spirals were proportional to SI 2 for lower supersaturations and to SI for higher supersaturations. The nucleation and growth rates of the 2D nuclei increased sharply for higher supersaturations using exponential functions. Using these kinetic equations, we predicted a critical supersaturation (SI ≈ 4.3) at which the main growth mechanism of the (001) face would change from a spiral growth to a 2D nucleation growth mechanism: therefore, the morphology of bulk crystals would change.

Barite Growth Rates as a Function of Crystallographic Orientation, Temperature, And Solution Saturation State

“…This result is consistent with the observations reported by Jindra et al, 33 who showed that solutions supersaturated in barite with SI ≥ 1 were unstable with respect to barite nucleation. This limitation illustrates that classical kinetics experiments based on the sole evolution of the fluid composition would be poorly informative to deconvolve the nucleation from the growth steps under such conditions and further justifies the use of surface-sensitive techniques to probe growth kinetics, such as AFM [15][16][17]33 and VSI. 19 The rapid nucleation of barite complicated the conduction of experiments at a fixed value of SI, and the duration of the experiment had consequently to be adjusted to (i) yield an appreciable thickness of the grown layer of barite while (ii) limiting the intrinsic evolution of the solution composition resulting from barite nucleation.…”

Studies of Mineral Nucleation and Growth Across Multiple Scales: Review of the Current State of Research Using the Example of Barite (BaSO4)

“…After nucleation, growth of barite surfaces can generally be described using classical crystal growth theory, which predicts barite growth will be dominated by spiral growth at screw dislocations near equilibrium conditions , and formation and spreading of 2-D islands at more supersaturated conditions. ,− Similar to the nucleation mechanisms as discussed above, it is possible that initial attachment is limited by the dehydration of the ion approaching the surface and/or the surface itself. Desolvation of surface sites is likely only slightly faster than desolvation of the equivalent aqueous ion, based on computational estimates of water exchange rates of the surface .…”

Quantitative 3D Visualization of the Growth of Individual Gypsum Microcrystals: Effect of Ca2+:SO42– Ratio on Kinetics and Crystal Morphology

“…Recognizing that crystal growth (and dissolution) is inherently spatially heterogeneous, scanning probe techniques such as atomic force microscopy (AFM), − among others, , have proven valuable for the investigation of processes at the microscopic and nanoscopic level. , However, such studies have tended to employ macroscopic crystal surfaces, with the probe only investigating the behavior of a tiny portion of the crystal surface − which may not necessarily be representative , and where the region imaged can be greatly influenced by processes outside that region . There are notable examples where there is a significant mismatch in nanoscale/microscale kinetic measurements, determined via AFM and macroscopic (bulk) kinetics. , …”