Friction characteristics of Cd-rich carbonate films on calcite surfaces: implications for compositional differentiation at the nanometer scale

The study of ion sorption onto mineral surfaces has received considerable attention in the last several decades. Sorption is, in the broad sense, the change of mass of a chemical in the solid phase as a result of mass-transfer between fluid and solid, which includes, 1) true adsorption (chemical or electrostatic), 2) absorption or diffusion into the solid, and 3) surface precipitation to form an adherent phase that may consist of chemical species derived from both the aqueous solution and dissolution of the solid [1]. Thus sorption plays an important role in processes such as the cycling of elements, diagenesis, and the removal of heavy metals from contaminated waters. Furthermore, the interaction of ions with mineral surfaces can influence the crystallization behavior of secondary minerals, affecting the formation of polymorphs, crystal morphology, and inhibiting nucleation and growth as well as dissolution. In this context the study of the interactions between carbonate minerals and metal ions has been the topic of numerous investigations [2-8] since these minerals are some of the most ubiquitous rock forming minerals in the Earth and they are often present in aquifers as well as in sediments and soils [9]. Biogenic calcium carbonates are also very common and the incorporation of elements such as Mg and Sr has been related to particular formation parameters such as temperature, water chemistry, and nutrient levels [10].When dealing with mineral-ion interactions, it is important to consider both the saturation states of possible precipitating solids with end-member composition and of the solid-solutions. Certain solid-solution compositions can theoretically form even when their end members remain undersaturated [7,11]. This is especially true when dealing with interactions between calcite and Cd2+(aq), since this ion is known to substitute for Ca2+ in the calcite structure [12]. Past studies have focused on understanding ion partitioning and other fundamental aspec