CRYSTAL-CHEMICAL COMPLEXITY IN NATURAL GARNETS - STRUCTURAL CONSTRAINTS ON CHEMICAL VARIABILITY

A crystal-chemical database for natural garnets has been developed at the CSCC, Pavia. It presently consists of 281 garnets representative o f common chemical compositions and petrogenetic conditions. For most s amples, X-ray single-crystal structure-refinement and SEM-WDS analysis were done on the same crystal. Ion-microprobe analysis for detection and evaluation of H, Li, Be, B was also done in some cases; spectrosco pic techniques were used to confirm the presence of unusual substituti ons. This combined approach allowed detection and characterization of substitutions occurring in natural garnets. Plots of atomic coordinate s and geometrical parameters as a function of the unit-cell edge show that garnet crystal-chemistry is characterized by the presence of stro ng inductive effects and non-linear behaviour, the quantitative aspect s of which are discussed in the companion paper. There are stringent s tructural constraints on chemical variability due to the extensive edg e-sharing between adjacent polyhedra. These phenomena, which make the correct determination of site populations a non-trivial task, have now been analyzed in detail and fully understood. We present here an auto matic procedure which allows the determination of reliable site-popula tions starting from the results of the structure refinement. It is bas ed on multiple regression equations for the independent estimates of e ach cation content and on their subsequent normalisation according to crystal-chemical constraints.