Abstract
Eudialyte-group minerals (EGMs) are Na-Ca zirconosilicates typical for peralkaline plutonic rocks. In the zeolite-like crystal structure of these minerals, there are many sites of different volumes and configurations, and therefore EGMs can include up to one-third of the periodic table. Although there are preferred sites for many elements in the crystal structure of eudialyte-group minerals, the same element can appear in several sites. In addition, many sites may be partially or fully vacant. Currently, 30 mineral species are established in the eudialyte group. However, this diversity is, in fact, limited to holotype specimens. To name any mineral from the eudialyte group, you need to solve its crystal structure and compare it with holotypes. Meanwhile, the composition (and, therefore, the name) of any mineral of the eudialyte group is an excellent indicator of the composition of the mineral-forming media, which is very important to petrological and mineralogical studies. In this article, we propose a diagnostic scheme for minerals of the eudialyte group, based only on the chemical composition. The scheme includes five consecutive steps, each of which evaluates the content of a species-forming element (or the sum of such elements). This scheme can be supplemented by new members without changing its hierarchical structure.
Highlights
Eudialyte-group minerals (EGMs) are alkaline zirconium-calcium silicates that accommodate many different elements in their complex zeolite-like structure
The crystal structures of EGMs are based on a heteropolyhedral framework (Figure 1) formed by three types of layers: the T-layer containing three- and nine-membered rings of SiO4 -tetrahedra [(Si3 O9 ) and (Si9 O27 ), respectively]; the
The value of c parameter (~30 Å or ~60 Å for “12-layer” and “24-layer” members, respectively); Cation ordering in octahedral six-membered [M(1)6 O24 ] rings, i.e., ring of six calcium atoms vs. ring with alternating atoms of different elements (Ca + Mn, Ca + Fe, Mn + Na); What elements prevail in centers of [Si9 O27 ] rings: Si, Nb, Ti, W, Mn; What elements prevail M(2)On polyhedra; What elements prevail N(3) and N(4) polyhedra; The water content [Na > (H2 O + H3 O+ ) or Na < (H2 O + H3 O+ )]. Based on this classification scheme and taking into account the data on the site occupancy in the crystal structures of different EGMs (Table 2), we propose a new algorithm that makes it possible to determine minerals based on chemical data only without a direct site assignment procedure
Summary
Eudialyte-group minerals (EGMs) are alkaline zirconium-calcium silicates that accommodate many different elements in their complex zeolite-like structure. The value of c parameter (~30 Å or ~60 Å for “12-layer” and “24-layer” members, respectively); Cation ordering in octahedral six-membered [M(1) O24 ] rings, i.e., ring of six calcium atoms vs ring with alternating atoms of different elements (Ca + Mn, Ca + Fe, Mn + Na); What elements (or vacancies) prevail in centers of [Si9 O27 ] rings: Si, Nb, Ti, W, Mn; What elements prevail M(2)On polyhedra; What elements prevail N(3) and N(4) polyhedra; The water content [Na > (H2 O + H3 O+ ) or Na < (H2 O + H3 O+ )] Based on this classification scheme and taking into account the data on the site occupancy in the crystal structures of different EGMs (Table 2), we propose a new algorithm that makes it possible to determine minerals based on chemical data only without a direct site assignment procedure. Zr or Ti in Z site; Division according to the assignment of the M(1) site; Division according to the occupation of M(3) and M(4) sites; Division according to population of the M(2), N(3), N(4) sites
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