Charge Distribution and Bond Valence Sum Analysis of Sulfosalts—The ECoN21 Computer Program
Gheorghe Ilinca
Abstract
The charge distribution (CD) and the bond valence sum (BVS) methods are used to calculate the charges assignable to atomic positions in crystal structures, based on the distribution of bond lengths. Discrepancies between calculated and formal charges may point to errors in the determination of atomic coordinates, in the initial allocation of oxidation numbers, occupancies, or site populations. Unlike the BVS method, which has been frequently used for the validation and interpretation of sulfosalt crystal structures, the CD method has been scarcely and limitedly employed for this group of minerals. In this paper, the applicability of the CD method to sulfosalts is practically tested for the first time. The calculation is made using ECoN21—a novel software tool designed for CD, BVS, and general coordination geometry analysis of crystal structures. The program addresses normal valence compounds with distorted homoligand or heteroligand polyhedra in both cation- and anion-centered descriptions. The program is also able to calculate a comprehensive set of parameters describing the internal and external distortion of coordination polyhedra. The paper presents the background of the CD, BVS, and coordination geometry calculations, as well as several case studies focusing on various applications of these methods to sulfosalts.