MSA PRESIDENTIAL ADDRESS: CRYSTAL CHEMISTRY IN THE 21st CENTURY

In the 20^th century, important gains were made in developing an experimental and theoretical basis for understanding the crystal structures, the bonded interactions and the crystal chemistry of oxide and silicate minerals, under ambient conditions as well as high pressures and temperatures. Bond strength calculations, for example, allow us to predict reactions on mineral surfaces, the partitioning of elements between coexisting phases and the high-pressure evolution of framework minerals. However, substantially less gain has been made in our understanding of the crystal chemistry of sulfides. Sulfide anions tend to adopt asymmetric coordinate polyhedra about M atoms, in contrast to those found in silicates. Metal excess sulfides display non-chemical formulae that do not appear to charge balance. In addition, they display metal-metal (M-M) contacts similar to those displayed by the bulk metal. Disulfides such as pyrite contain S-S bonded interactions displayed by S₂ molecules whereas O₂ molecules are unknown in silicates. It is noteworthy that iron sulfides are among the few compounds that adopt the post-perovskite structure recently found in MgSiO₃ at very high pressures and temperatures (Murakami et al. 2004) under ambient conditions.

One promising approach to understand the crystal chemistry of sulfides is to determine the topological properties of their electron density distributions (EDDs) as described by Bader (1990). The EDD provides a quantitative basis for characterizing the bonded interactions and the crystal chemistry in sulfides at the atomic level. By charting the bond paths of the EDD, bonded interactions can be established and the strengths and the relative stabilities of the interactions can be determined together with the bonded radii and the coordination numbers of the bonded atoms. A determination of the EDD properties not only sheds light on the relative strengths and properties of the M-S bonded interactions but also establishes whether M-M (and S-S) bond paths exist. Thus the experimental and theoretical strategies for studying EDDs for silicates and oxides may not only provide the basis for understanding the more complex bonded interactions in sulfides, but may also provide a unifying basis for crystal chemistry in the 21^st century.