STRUCTURE AND CHEMISTRY OF PHOSPHATE MINERALS

For complex rocks in which the structure of minerals, rather than thei r chemical composition, changes with progressive evolution of the syst em, it makes sense to try and monitor such an evolving system through the progressive change in the crystal structures of the constituent ph ases. In effect, the paragenetic sequences of minerals in such complex environments should be related to the crystal structures of the const ituent minerals. In order to consider variations in structure topology , we need to organize crystal structures into hierarchical schemes, us ing the hypothesis that structures may be hierarchically ordered accor ding to the polymerization of the coordination polyhedra with higher b ond-strengths. Structural units are organized according to the mode of polymerization: unconnected polyhedra, clusters, chains, sheets and f rameworks. The bond-valence structure of (OH) and (H2O) shows that on one side, (OH) and H2O are strong Lewis bases; on the other side, they are weak Lewis acids. As a result, a very important role of both (OH) and (H2O) is to prevent polymerization of the structural unit in spec ific directions. Thus, the dimensionality of the structural unit is co ntrolled primarily by the amount and role of hydrogen in the structure . The way in which we have formulated these ideas also allows developm ent of a predictive framework within which specific aspects of the che mistry and structure of phosphates can be considered. This approach to mineral structure, applied via the idea of a structural unit, can pla y a major role in developing structural hierarchies in order to bring about some sort of order to the plethora of hydroxy-hydrated-phosphate structures. Furthermore, by combining the idea of binary structural r epresentation with bond-valence theory, we see the eventual possibilit y of predicting stoichiometry and structural characteristics of these minerals, particularly those in complex low-temperature hydrothermal e nvironments.