Mineral Surfaces in Geochemical and Environmental Processes: Direct Observation of the Microscopic Machinery

Carrick M. Eggleston Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006, USA



Minerals communicate chemically with Earth's atmosphere, hydrosphere and biosphere through their surfaces. Therefore, minerals influence geochemical and environmental processes almost exclusively through their surface chemical role as sorbents, catalysts, and sources-sinks of solutes and electrons.

The role of mineral surface chemistry in the broader chemistry of the Earth's surface stems from two basic processes: atom and electron transfer between the solid and adsorbed states, and between the solid surface and species in aqueous or gas phases. Adsorption removes species from surrounding solutions, helping to sequester them in sediments or to retard their transport. Adsorbed species can interact with eachother, leading to surface-catalysis. Dissolution and precipitation are integral to weathering, diagenesis and global elemental cycling. Minerals such as pyrite and magnetite act as electrodes in heterogeneous
electron transfer, and as semiconductors in heterogeneous, abiotic photochemistry.

There are many ways to study mineral surfaces and surface chemistry. This presentation will take a microscopic point of view, using data from scanning probe microscopy in combination with data from other surface spectroscopies to illustrate direct, often in-situ observation of surface structures and reactions. This approach gives a unique and fascinating window on the microscopic mechanisms operating in a wide variety of overall geochemical and environmental processes. An underlying theme is that direct observations provide key constraints on and tests of surface-chemical predictive models.

Examples to be covered

Adsorption: Direct probing of the structure of the electrical double layer through use of the atomic force microscope as a force measurement device; binding energetics and surface diffusion of Cr(III) adsorbed to hematite (001) surfaces from aqueous solution.

Dissolution and precipitation: Direct, in-situ observations of calcite, sulfates, chromates, chromic hydroxide, hematite, and potassium hydrophosphate. Relevance to rate vs. saturation state and inhibition/acceleration of rate by adsorbed impurities. Using direct observation to experimentally determine thermodynamic properties such as formation energies of step and kink sites.

Electron transfer: Oxidation processes on pyrite surfaces: the role of oxidation products in mediating electron transfer, and application of a Marcus-based electron transfer kinetic model constrained by direct observations. Photochemistry and mineral oxides: breakdown of organic adsorbates and pollutants, and driving of redox reactions of trace metals.