MSA PRESIDENTIAL ADDRESS: MINERALS AS KEY ENVIRONMENTAL MATERIALS

Selected examples of the roles played by minerals as key environmental materials will be considered, beginning as primary sources of toxic elements, particularly in mine wastes. The importance of mineral surface chemistry in the breakdown of primary minerals such as the metal sulphides will be emphasised, as will the role of microbial processes in oxidation. Experimental data and molecular modelling has enabled the mechanisms of aqueous oxidation of major sulfides such as pyrite and arsenopyrite to be elucidated.

In aerobic environments, breakdown products include a major role for iron oxyhydroxides; suspended in acid mine waters, they are carriers of toxic elements sorbed to their surfaces or incorporated in these rapidly precipitated (‘nano’) minerals. Toxic elements can be released if the oxyhydroxides break down, a process facilitated by bacteria which include iron-reducers. One pathway for mineral matter suspended in acid mine waters takes them back into the anaerobic conditions found in estuarine or near-shore sediments, where sulfate-reducing bacteria supply reduced sulfur for the neoformation of sulfides, notably mackinawite (FeS). The extent and mechanisms of interaction of a wide range of toxic elements (As, Cu, Ni, Np, U, Zn) with the surface of ultrafine particle FeS have been explored using bulk uptake experiments and spectroscopic studies. For materials such as FeS, direct observation of surfaces and surface reactions by imaging methods is not possible; for minerals available as well developed single crystals, such as magnetite, imaging is possible at ‘atomic’ resolution.

The roles played by minerals in lithospheric and hydrospheric systems are well known. This is not the case for minerals in the atmosphere. Mineral dusts have an important and, thus far, poorly understood influence on global climate; they can act to either absorb or reflect solar radiation, or to nucleate clouds. A particular example concerns the influence of biomass burning on solar radiation. In this case, we have studied samples collected in West Africa so as to determine the nature of the interaction between minerals and carbonaceous matter from such burning. Whether the carbonaceous materials occur as discrete particles or intimately associated with the minerals is important for global climate models.