Original paper
Crystal-chemical complexity in natural garnets: structural constraints on chemical variability
Merli, Marcello; Callegari, Athos; Cannillo, Elio; Caucia, Franca; Leona, Marco; Oberti, Roberta; Ungaretti, Luciano
European Journal of Mineralogy Volume 7 Number 6 (1995), p. 1239 - 1250
32 references
published: Dec 22, 1995
manuscript accepted: Jun 1, 1995
manuscript received: Jan 12, 1995
DOI: 10.1127/ejm/7/6/1239
Abstract
Abstract A crystal-chemical database for natural garnets has been developed at the CSCC, Pavia. It presently consists of 281 garnets representative of common chemical compositions and petrogenetic conditions. For most samples, X-ray single-crystal structure-refinement and SEM-WDS analysis were done on the same crystal. Ionmicroprobe analysis for detection and evaluation of H, Li, Be, B was also done in some cases; spectroscopic techniques were used to confirm the presence of unusual substitutions. This combined approach allowed detection and characterization of substitutions occurring in natural garnets. Plots of atomic coordinates and geometrical parameters as a function of the unit-cell edge show that garnet crystal-chemistry is characterized by the presence of strong inductive effects and non-linear behaviour, the quantitative aspects of which are discussed in the companion paper. There are stringent structural constraints on chemical variability due to the extensive edge-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 automatic procedure which allows the determination of reliable site-populations starting from the results of the structure refinement. It is based on multiple regression equations for the independent estimates of each cation content and on their subsequent normalisation according to crystal-chemical constraints
Keywords
garnet • crystal-structure refinement • crystal chemistry • site population