Abstract
The reactivity of a mineral surface is determined by the variety and population of different types of surface sites (e.g., step, kink, adatom, and defect sites). The concept of "adsorbed nutrient" has been built into crystal growth theories, and many other studies of mineral surface reactivity appeal to ill-defined "active sites." Despite their theoretical importance, there has been little direct experimental or analytical investigation of the structure and properties of such species. Here, we use ex-situ and in-situ scanning tunneling microcopy (STM) combined with calculated images based on a resonant tunneling model to show that observed nonperiodic protrusions and depressions on the hematite (001) surface can be explained as Fe in an adsorbed or adatom state occupying sites different from those that result from simple termination of the bulk mineral. The number of such sites varies with sample preparation history, consistent with their removal from the surface in low pH solutions.
Highlights
Mineral surfaces are the medium upon which the Earth’s solids and fluids interact
It has been shown that transient spikes in dissolution rate occur in response to pH changes in a way consistent with the formation and dissolution of adsorbed Fe at the hematite surface,[3,4,5,6] and an isotopic exchange and aElectronic mail: carrick@uwyo.edu bElectronic mail: astack@ucdavis.edu cElectronic mail: Kevin.Rosso@pnl.gov
We have presented calculated and experimental STM images showing that nonperiodic sitesboth protrusions and depressionsobserved on hematite001͒ surfaces by STM, both ex-situ and in-situ, are consistent with Fe in an adsorbed or adatom state on the hematite surface
Summary
Mineral surfaces are the medium upon which the Earth’s solids and fluids interact. Their reactivity in the fundamental processes of adsorption, dissolution/growth, and electron transfer is directly tied to their atomic structure.[1] In addition to two-dimensionally periodic surface structures, there are one-dimensional periodic and zero-dimensional structures such as steps and kink sites that play important roles in mineral reactivity. It has been shown that transient spikes in dissolution rate occur in response to pH changes in a way consistent with the formation and dissolution of adsorbed Fe at the hematite surface,[3,4,5,6] and an isotopic exchange and Mossbauer study by Rea et al.[7] concluded that a population of kinetically labile sites characterizes the ferrihydrite surface
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