Classification of eudialyte-group minerals

According to the rules accepted by the Commission on New Minerals, Nomenclature and Classification, International Mineralogical Association, the definition of a new mineral species within the eudialyte group should be based on the general crystal chemical principle of the predominance of a new element at one or several key sites. To date, 25 minerals of the eudialyte group have been approved in line with this recommendation. Additionally, 50 samples (including potential new species of the eudialyte group) have been investigated using IR spectroscopy and X-ray diffraction. The new data require the modification and complementation of the current systematics of the eudialyte group. The eudialyte-type structures contain unique fragments that involve species-forming key sites. The crystal chemical classification that develops in this study is based on the hierarchical principle that takes into account the following features (listed in order of their lowering classification rank): (i) simple or modular character of the crystal structure and value of c parameter (about 30 and 60 for 12- and 24-layer minerals, respectively) symmetry determined by cation ordering in the octahedral ring, i.e., the possibility of dividing site M(1) into M(1a) and M(1b) the distribution of different components into five M(2) square-based sites the water content (subdivision of the eudialyte-group minerals into low-and high-hydrous species with Na > (H2O + H3O) and Na < (H2O + H3O), respectively. The 12-layer minerals are subdivided into eudialyte- and oneillite-type species. The further subdivision of these two types into subtypes is based on the predominance of octahedra, tetrahedra, or vacancies at sites M(3) and M(4) in the centers of nine-membered Si,O-rings and different combinations of these variants. Both type and subtype terms are used here in the crystal chemical sense. Among 24-layer minerals, the first subdivision is based on the degree of hydration; the second subdivision into subtypes takes into account combinations of predominant components (Zr + Zr, Ti + Ti, or Zr + Ti) in Z octahedra of different modules (floors). Various eudialyte-group minerals that belong to different crystal chemical taxons are briefly characterized.