The role of magnesium in the growth of calcite: An AFM study

Abstract

The mechanisms that determine the inhibition of calcite growth by magnesium have remained unclear and subject to controversy over decades. Although it has been long apparent that the inhibition mechanisms take place at the crystal-solution interface, the molecular phenomena occurring at calcite surfaces in contact with Mg-bearing solutions are still not completely understood. The main goal of this work is to contribute to further clarify those phenomena. With this aim, we carried out in situ atomic force microscopy (AFM) observations of the growth behaviour of calcite {10 over(1, -) 4} surfaces in contact with supersaturated aqueous solutions (ß = 5) bearing different amounts of Mg (ranging from 0.05 to 4.00 mmol dm- 3). Under the conditions considered, growth occurred by monolayer spreading. Our observations revealed that only the first elementary growth layer advancing on the original calcite surfaces grow normally, showing characteristics nearly identical to the growth of pure calcite. However, subsequent monolayers behave differently. Thus, as soon as one of these monolayers reaches areas of the surface that have grown incorporating Mg and whose composition can consequently be described as MgxCa1-xCO3, the rate at which this step advances significantly decreases. Moreover, the step becomes progressively rougher. A clear relationship between the extent of the inhibition effect and the concentration of Mg in the aqueous solution exists. Furthermore, our observations allow us to conclude that each newly formed monolayer exerts a certain control on the development of the growth of subsequent monolayers. Such a control causes the reproduction of the nanotopographic features of the original surface, producing the so called "template effect". This behaviour cannot be easily incorporated within the general framework of the currently accepted impurity crystal growth models, which are based on either the pinning of elementary step motion by impurities or changes in the solubility of the newly formed layers as a result of the incorporation of the impurity into the lattice of the growing crystal. We discuss our results on the basis of the solid solution-aqueous solution model and provide a complementary explanation for the development of "dead zones" in the case of the growth of calcite {10 over(1, -) 4} surfaces from divalent cation-bearing aqueous solutions. © 2009 Elsevier B.V. All rights reserved.