New Data on the Isomorphism in Eudialyte-Group Minerals. VIII: Crystal Structure of the Fe2+-Analog of Feklichevite, a Potentially New Mineral from the Khibiny Massif

Abstract

A potentially new calcium-rich eudialyte-group mineral from the Khibiny massif has been investigated by X-ray diffraction (XRD) analysis. The trigonal unit-cell parameters are found to be as follows: a = 14.221(1) A, c = 30.04(1) A, and V = 5261.3 A3; sp. gr. R3m. The crystal structure is refined to the final reliability factor R = 2.8% in the anisotropic approximation of atomic displacements, using 1714 reflections with F > 3σ(F). The idealized formula of the mineral (Z = 3) is (Na12Ca3)Ca6Fe $$_{3}^{{2 + }}$$ Zr3NbSi25O75(H2O,OH,F,Cl)3. The dominance of divalent iron form is confirmed by Mossbauer spectroscopy data. The closest analog of the mineral under study is feklichevite from the Kovdor alkaline massif; the main difference is the dominance of divalent iron over trivalent iron in the former.