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

Yoshihiro Kuwahara,Wen Liu,Masato Makio,Keisuke Otsuka

doi:10.3390/min6040117

Abstract

The growth behavior and kinetics of the barite (001) surface in supersaturated BaSO4 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 BaSO4 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 SI2, 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 SI2 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.

Highlights

Barite (BaSO4 ) is the most abundant mineral of barium and occurs in a wide variety of geological environments that span geologic times from the Early Archean era to the present [1,2]
We examined the growth behavior of the barite (001) surface in supersaturated
The aims of this study were to reveal the microscopic growth behavior on the barite (001) surface at the step or site level and to estimate the growth kinetics of 2D nucleation and spiral growth that occur at the barite surface–aqueous solution interface, as well as the lateral advance rates of the initial steps, steps of the

Summary

Introduction

Barite (BaSO4 ) is the most abundant mineral of barium and occurs in a wide variety of geological environments that span geologic times from the Early Archean era to the present [1,2]. Barite is one of the few marine authigenic minerals that are reported to form in the water column, as well as in marine sediments and locations around hydrothermal vents and cold seeps [3]. Due to its diverse modes of formation, barite can be utilized for paleoenvironmental, hydrogeological, and hydrothermal studies [3]. Ba in the ground and surface water, due to its low solubility (Ksp = 10−9.99 at 25 ◦ C) in water [4,5]. The common scale mineral barite is almost inevitable in industrial water, oil, and gas production systems due to its low solubility [4,5,6,7,8,9]. The uptake of radioactive Ra ions during barite formation [10]

Objectives

Methods

Results

Discussion

Conclusion