- © 2000 Mineralogical Society of America
Hydrogen is the simplest and most abundant element in the universe. It is the third most abundant element on the surface of the planet. Studies and interpretation of the spectroscopic properties of hydrogen, and those of its isotopes, have been intertwined with the development of experimental and theoretical physics and chemistry. It is now realized that hydrogen forms more chemical compounds than any other element, including carbon, and a survey of its chemistry would encompass the whole periodic table. In mineralogy hydrogen is also ubiquitous. The range of minerals capable of hosting hydrogen range from ice to nominally anhydrous silicate phases implicated in water storage in the mantle. The later observation has led to proposals of hydrogen content equivalent to several oceans in Earth’s deep interior and has provided new impetus for the study of hydrogen and its role in stabilizing high-pressure hydrous phases. This chapter will largely deal with this issue and will concentrate on the structures of high-pressure phases that are potential hosts for water, the properties of these materials, and the hydrogen bond in these and model systems.
THE HYDROGEN BOND
The properties of many substances suggest that, in addition to the normal “strong” interactions attributed to chemical bonding between atoms and ions, there exists some further interaction (10–60 kJ per mole) involving a hydrogen atom placed between two or more other groups of atoms. The definition of this interaction, the hydrogen bond, and its history is well described in chapter one of Jeffrey’s (1997) book. The concept is introduced in Pauling (1939; Nature of the Chemical Bond): “Under certain conditions an atom of hydrogen is attracted by rather strong forces to two atoms instead of only one, so that it may be considered to be acting as a bond between them. This is called a hydrogen bond.” He …