The Structure of Hydrous Species in Nominally Anhydrous Minerals: Information from Polarized IR Spectroscopy

Abstract

Hydrogen is a major constituent in a wide variety of minerals in the Earth’s crust. Usually H is bonded to oxygen forming H2O molecules or OH− groups. In rare cases, H3O+, H3O2− and H5O2+ units are also formed. The occurrence of NH4+, CHx, etc. in minerals will not be considered here. Hydrogen occurs stoichiometrically in hydrous compounds such as hydrates (e.g., gypsum), (oxy)hydroxides (e.g., goethite), and in many rock-forming silicates (e.g., micas), as well as in nonstoichiometric major amounts in microporous minerals such as zeolites and clay minerals, which are of considerable economic and ecologic importance. Hydrogen also occurs as a minor or trace constituent in minerals that by definition (and by their formulae) do not contain hydrogen at all, i.e., the so-called nominally anhydrous minerals (NAMs). NAMs include common rock-forming minerals in the Earth’s crust (e.g., quartz, feldspars) and upper mantle (e.g., olivine, pyroxene and garnet), but also high- P and high- T phases (e.g., wadsleyite, ringwoodite, and majorite garnet) stable in the mantle transition zone (410–660 km depth). The aim of the present chapter is to review the use of polarized infrared (IR) spectroscopy as it applies to detecting traces of hydrogen in minerals and to characterizing its speciation and structural environment in nominally anhydrous minerals. The basic theoretical background will be supplemented by a number of examples from recent research. It is important to note that many of the concepts described here for NAMs can also be applied to synthetic compounds of importance in the materials sciences. ### The importance of hydrous species in NAMs Hydrogen may be incorporated as defects in nominally anhydrous minerals of the Earth’s mantle. Due to the large volume of rock in the Earth’s mantle, even trace concentrations of H …