The Weathering of Oxides
The Weathering of Oxides
Earth’s crust is largely composed of oxides, so the biosphere we inhabit is dominated by interactions between oxides, water, and living things. Part Six of this book, on environmental geochemistry, focuses on these interactions and serves as a review of many of the chemical concepts that form the basis for the rest of the book. As such, the final two chapters frequently refer back to previous chapters for more in-depth discussions of specific chemical phenomena. In this chapter, however, we highlight how the diverse environments on the surface of Earth modify the structure, composition, and chemistry of oxide minerals by weathering phenomena. Conversely, in Chapter 18 we explore how oxide minerals and their weathering products modify the structure, composition, and chemistry of the environments they inhabit. These environmental interactions are influenced by life, and are critical to the health and well-being of all living things. Minerals have a natural life cycle on the surface of Earth. Most oxides emerge from Earth’s interior in the form of igneous rocks that form and are stable at the high temperatures and pressures of subsurface environments (see Chapter 18). These minerals usually do not represent phases that are thermodynamically stable in ambient-temperature water. As a result, any pristine rocks exposed to air and water are subject to the physical and chemical degradation processes we call weathering (Fig. 17.1 and Plate 20). Weathering processes facilitated by water convert anhydrous oxides formed at high temperatures into hydrous oxides, oxyhydroxides, hydroxides, and dissolved by-products. It has been estimated that volcanic rocks represent only 8% of the rocky outcrops on Earth’s surface whereas 26% are more coarsely grained plutonic rocks of igneous origin. The remaining 66% of rocky outcrops represent the decomposition products of these igneous parents, including sandstone (16%), claybased rocks such as shale (33%), and simple ionic salts such as limestone (16%) and evaporates (1.3%). The focus of this chapter is on the physical and chemical processes that form and affect these decomposition products under ambient-temperature conditions.
Keywords: Mesabi Iron Range, Paragenesis, Tambora volcanic eruption, acids in the environment, carbonate chemistry and speciation, geologic eras, mechanical denudation rates, oxoanions, phase diagramsfor weathering, redox cycling
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