Crystal structure of magnesioferrikatophorite

The crystal structure of the Na,Ca-amphibole magnesioferrikatophorite found in carbonatites from the Turiy Cape (Kola Peninsula) was refined (Siemens P4 diffractometer, λMoKα radiation, 1481 independent reflections with |F|>4σ(F), anisotropic refinement, R(F) = 0.039). The parameters of the monoclinic unit cell are a = 9.875(5) Å, b = 18.010(8) Å, c = 5.309(3) Å, β = 104.39(5)°, sp. gr. C2/m, Z = 2. The distribution of the cations over the crystallographically nonequivalent M(1–4)-positions was revealed by Mössbauer spectroscopy and X-ray diffraction analysis. The character of splitting of the A-position correlates with the characteristic features of the magnesioferrikatophorite composition. The resulting structural formula (Na0.87K0.13)Σ = 1 · (Na1.18Ca0.82)Σ = 2(Mg1.41Fe0.423+Ti0.174+)Σ= 2 Fe1.313+Mg0.69)Σ = 2(Mg0.60Fe0.382+Mn0.02)Σ = 1(Si3.16Al0.84)Σ = 4 · Si4O22(O1.05OH0.66F0.29)Σ= 2 agrees well with the electron microprobe analysis data. Based on the zonal character of the crystal and high Fe 3+ content, the conditions of crystallogenesis are defined as oxidative against the background of a decrease in the Na potential in the course of the evolution of a mineral-forming system.