Кристаллохимия нептунита Хан-Богдинского щелочного массива (Монголия)
Abstract
Crystal chemical characteristics have been studied for neptunite from alkaline granites of the
Khan Bogdo massif (Southern Mongolia). The mineral occurs to be the practically clear Fe-variety of
its species. Refinement of its crystal structure was carried out in two space groups: centric C2/c and
acentric Cc. It is indicated that correct space group is the acentric Cc due to ordering of octahedral cations
Ti and Fe. The structure was refined to R1 = 0.025 (wR2 = 0.057) for 4889 reflections with
|Fo| 4sF. Unit cell parameters: a = 16.4542(7), b = 12.5115(4), ñ = 9.9980(4) Å, b = 115.542(5)°,
V = 1857.10(15) Å3, Z = 4. Ti octahedra show strong distortions: one bond Ti—O 1.75 Å, another
bond 2.20 Å, and four Ti—O bonds around 2.0 Å. Fe octahedra have a more regular shape with
an average bond length Fe—O 2.13—2.14 Å. Empirical formula calculated on the basis of chemical
analysis is (K0.86Cs0.02Zn0.02Ca0.01)S0.91Na1.90Li0.82(Fe1.54Fe Mn Mn
2
0.22
3
0.16 0.04
+ + )S1.96Ti2.1(Si7.76Al0.25)O24.
Structural formula is K0.97Na1.90(Li0.94Na0.06)S1.00(Fe1.80Mn0.15Mg0.05)S2.00Ti2Si8O22(O,OH)2. Idealized
formula is KNa2LiFe2Ti2Si8O24.
Key words: Fe-dominant neptunite, refined crystal structure, equivalent and anisotropic displacement
parameters, inter-atomic distances, pyroxene type and octahedral chains, mangan-neptunite.