New Data on the Isomorphism in Eudialyte-Group Minerals. IХ: Blocky Isomorphism at Key Sites. Crystal Structure of Fe-Deficient Eudialyte from the Khibiny Massif

Abstract

The crystal structure and crystallochemical features of Fe-deficient manganese-rich eudialyte from the Khibiny massif (Kola Peninsula) have been investigated by X-ray diffraction analysis, electron probe microanalysis, and IR spectroscopy. The trigonal unit-cell parameters of the mineral are found to be a = 14.199(1) A, c = 30.305(1) A, and V = 5291.2(2) A3; sp. gr. R3m. The crystal structure is refined to the final reliability factor R = 4.46% in the anisotropic approximation of atomic displacements, using 1937 reflections with F > 3σ(F). The idealized crystallochemical formula of the mineral (Z = 3, Roman numerals indicate the coordination numbers (CNs) of cations in the key sites) is N1–5(Na,H3O) $$_{{15}}^{{{\text{V}}-{\text{X}}}}$$ M1(Ca,Mn) $$_{6}^{{{\text{VI}}}}$$ M2(FeIV,NaVI,NaV,MnVII,MnV)3 M3(SiIV,NbVI) M4SiIV Z(Zr,Ti) $$_{3}^{{{\text{VI}}}}$$ [Si24O72] (OH,Cl,SO4)3 (H2O,OH) ⋅ 1.5H2O. The regularities of block isomorphism in eudialyte-group minerals are discussed.