Tantalum-free niobium analogue of ixiolite from the Eifel paleovolcanic region, Germany, and its crystal structure. On the problem of “ashanite”

Zubkova N. V., Chukanov N. V., Pekov I. V., Ternes B., Schuller W., Pushcharovsky D. Yu.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 4. P. 125-134

https://doi.org/10.31857/S0869605520040073

Language: Russian

Abstract

The crystal structure of a Ta-free Nb-dominant analogue of ixiolite with the empirical formula (Nb_1.55Ti_1.11Fe³⁺_1.01Mn²⁺_0.19Cr³⁺_0.04Mg_0.04Al_0.03Zr_0.02Mn³⁺_0.01)_S₄O₈ (Z = 1) from the Laach Lake area (Eifel, Germany) was solved based on single-crystal X-ray diffraction data and refined in the space group Pbcn to R = 4.47%. The unit-cell parameters are: a = 4.6578(6), b = 5.6230(7), с = 5.0182(5) Å, V = 131.43(3) Å³. The mineral belongs to the α-PbO₂ structure type. It was found in volcanic nosean sanidinite, in a late (presumably, pneumatolytic) mineral association including sanidine, nosean, magnetite, ilmenite, biotite, corundum, and columbite-(Fe). The crystal chemistry of the ixiolite-like niobate is discussed, including the aspect related to the definition of the discredited mineral species “ashanite”.

Keywords: ixiolite, ashanite, niobate, α-PbO₂ structure type, crystal structure, sanidinite, Laach lake, Eifel volcanic region