The role of OH and H2O in oxide and oxysalt minerals

Abstract

Abstract Hydrogen plays an extremely important role in the structure and chemistry of the oxide and oxysalt minerals. The characteristics of this role can be profitably analyzed in a simple and intuitive fashion using bond-valence theory. For any crystal structure, the structural unit may be defined as the strongly bonded part of the structure; structural units are linked together by interstitial species, usually alkali or alkaline earth cations and (H2O)0 groups that are involved in much weaker bonding. This scheme gives a binary representation of even the most complex structure. The interaction between the structural unit and the interstitial species can be quantitatively evaluated using the valence-matching principle (Brown, 1981). As components of the structural unit, (OH)− and (H2O)0 play a major role in dictating the dimensional polymerization of the structural unit because of the very asymmetric nature of the donor-hydrogen and hydrogen…acceptor interactions. As an interstitial component, (H2O)0 can play three different roles. Interstitial (H2O)0 may bond to an interstitial cation, essentially forming a complex cation. In this role, (H2O)0 acts as a bond-valence transformer, moderating the Lewis acidity of the interstitial cations such that it matches the Lewis basicity of the structural unit and the valence-matching principle is satisfied. Interstitial (H2O)0 need not bond to an interstitial cation to occupy well-ordered atomic positions; a stable hydrogen-bonded network can occur in the interstitial regions between structural units. The role of such (H2O)0 is to satisfy the bond-valence requirements of H atoms that are part of the structural unit, propagating the bonding across the interstitial space to other parts of the structural unit. Occluded (H2O)0 may occur in some minerals. Such (H2O)0 is not bonded to interstitial cations and does not participate in a static ordered hydrogen-bond network. However, this type of (H2O)0 will still affect many of the physical properties of a mineral.