Topological analysis of the electron density of the clinopyroxene structure by the maximum entropy method : an exploratory study

In the framework of a series of methodological studies to test the reliability of the reconstruction of the electron density distribution (EDD) by the maximum entropy method (MEM), we have attempted to check the possibilities of getting further information about the structural changes as a function of the chemical composition occurring in low-Ca clinopyroxenes by the analysis of the topology of the MEM EDD. The results of this study show that the topological analysis of MEM EDD yields results comparable to the bond critical points characteristics obtained in ground state EDD in minerals. Notable differences related to solid solutions have been found only at the M2 site of clinopyroxene structure. An attempt to describe the bond character at this site in the presence of significant solid solutions has shown differences related to chemical substitution that can be described in terms of variable ionicity. This result could be ultimately related to the different thermodynamic behaviour of the P21/c ↔ C2/c phase transitions shown by chemically different clinopyroxenes. Since MEM can deal with experimental data on solid solutions, which cannot be treated with other available methods to reconstruct the experimental electron density, this approach opens new possibilities for the study of changes in the EDD topology of minerals with complex chemical substitutions.