Crystal chemistry of neptunite from the Khan Bogdo alkaline massif (Mongolia)
Zolotarev Α. Α., Vladykin N. V., Krivovichev S. V., Panikorovskii T. L.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2016. V. 145. N 2. P. 112-127
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| 4F. Unit cell parameters: a = 16.4542(7), b = 12.5115(4), c = 9.9980(4) Å, β = 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)∑0.91Na1.90Li0.82(Mn0.16Mn0.04)∑1.96Ti2.1(Si7.76Al0.25)O24. Structural formula is K0.97Na1.90(Li0.94Na0.06)∑1.00(Fe1.80Mn0.15Mg0.05)∑2.00Ti2Si8O22(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.