Crystal Structure of Cl-Deficient Analogue of Taseqite from Odikhincha Massifстатья
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Аннотация:An eudialyte group mineral, found in pegmatites of the Odikhincha massif (the northern part of the Siberian platform), has been investigated using X-ray diffraction, IR spectroscopy, and Raman spectroscopy. The mineral is characterized by a high strontium content and a low chlorine content. It has a trigonal unit cell with the following parameters: a = 14.2700(6) A and c = 30.057(1) A; V = 5300.6(1) A(3); sp. gr. R3m. The structure has been refined to R = 0.047 in the anisotropic approximation of atomic displacements using 1697F > 4 sigma(F). The idealized formula (Z = 3) was found to be Na12Sr2Ca6Fe32+Zr3NbSi25O72(OH,O)(4)Cl(H2O)(0.2). The chemical composition and structure of this mineral are close to those of taseqite; however, it differs from the holotype sample by a low chlorine content and peculiarities of cation distribution over basic structure sites. A comparative analysis of strontium-rich eudialytes has revealed their important crystallochemical feature: selective concentration of strontium in the N4 site. Thus, taseqite, along with heterophyllosilicates, may play a role of strontium concentrator in agpaitic pegmatites.