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) Å, c = 30.305(1) Å, and V = 5291.2(2) ų; 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(Fe^IV,Na^VI,Na^V,Mn^VII,Mn^V)₃ ^M3(Si^IV,Nb^VI) ^M4Si^{IV 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.