Single-crystal X-ray diffraction, EMPA, FTIR and X-ray photoelectron spectroscopy study of narsarsukite from Murun Massif, Russia

A crystal chemical study of narsarsukite from the Murun alkaline massif, Russia has been carried out combining single crystal X-ray diffraction, electron microprobe analyses, micro-Fourier Transform infrared spectroscopy and X-ray photoelectron spectroscopy. The narsarsukite single crystals are tetragonal (space group I4/m) with unit cell parameters: 10.7140(1)  a  10.7183(2) Å and 7.9478(1)  c  7.9511(1) Å. The XPS analysis showed that Fe occurs in the mineral as Fe3+, whereas the FTIR spectrum evidenced that the studied sample is anhydrous. The Murun narsarsukite has average crystal chemical formula: Na2.04K0.01(V5+0.01Ti0.74Zr0.01Al0.01Fe3+0.22Mg0.01)1.00Si4.00(O10.74F0.23OH0.03) 11.00. Structural disorder at octahedral and interstitial sites was modeled and discussed also in consideration of the main substitutional mechanism Ti4+ + O2- ⟷ Fe3+ + (F-, OH-) active in the structure of the mineral.