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Modern mineralogical nomenclature is based on the principle that each new mineral species within a structural type is identified by the predominance in at least one structural site of a different chemical element compared to the known minerals of that type. Until very recently, a significant challenge to realizing this principle was presented by structurally complex multicomponent members of isodimorphous series (such as those in the eudialyte and labuntsovite groups) that are nonstoichiometric compounds with a varying number and composition of atoms in the structural sites. Our studies indicate that the entire diversity of such minerals observed in nature can be satisfactorily described as combinations of a limited set of stoichiometrically complete structural and chemical end members, which might be denoted by the terms "structals" and "chemals," respectively. Each structal corresponding to a particular experimentally determined structure type is characterized by an idealized positional formula, and each chemal by an idealized chemical formula. The crystal chemical characteristics of each structurally determined member of an isodimorphous series can be described by partial fractions of structals and molar fractions of chemals. Mineral species are identified by the combination of structural and chemical end members that are dominant in the solid solution. Chemals and structals that are subordinate in the composition and structure of such solutions may be taken into account when identifying the chemical and structural varieties of mineral species. One structural end member that is dominant in different solutions can be matched by combinations with different chemals that are dominant in the same solutions; as a consequence, one structure type can be matched by a set of several different mineral species. Analysis of available data on the 24 eudialyte-group minerals identified so far using these approaches allowed the author to assign them to 15 unique structure types (see the Abstract: Eudialyte-group minerals as an example of solid solutions with a varying number and composition of atoms in the structural sites).
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