Pekov I. V., Zubkova N. V., Chukanov N.V., Yapaskurt V. O., Turchkova A. G., Ksenofontov D. A., Pushcharovsky D. Yu.

Natisite, Na2TiSiO5, an indicator mineral of hyperagpaitic hydrothermal assemblages in Lovozero and Khibiny alkaline intrusions (Kola Peninsula): occurrence, crystal chemistry and genetic features

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 3. P. 1-26

https://doi.org/10.31857/S0869605521030084

Full text is available on eLIBRARY.RU

Language: Russian 

Abstract

Natisite, a natural tetragonal (P4/nmm) modification of Na₂TiSiO₅ = Na₂TiO(SiO₄) is a widespread, indicator mineral of relatively low-temperature (not higher than 300–350 °C) peralkaline (hyperagpaitic) hydrothermal assemblages in the Lovozero and Khibiny alkaline intrusions (Kola Peninsula, Russia). Natisite amount in the hypersodic (with Na >>K) hydrothermal veins in Lovozero is up to 30 vol. % and in the hydrothermally altered zones of potassium-rich in Khibiny it is up to 10 vol. %, i.e. natisite can be a rock-forming mineral, the major concentrator of Ti in the rock. In Lovozero natisite is a primary mineral directly crystallized from the hydrothermal solution, whereas in Khibiny natisite mineralization has secondary character: the major amount of natisite occurs in pseudomorphs after earlier Ti minerals, namely ilmenite, titanite, aenigmatite, lamprophyllite and rinkite. Natisite demonstrates stable chemical composition typically close to Na₂TiSiO₅. A very bright blue to bluish-white fluorescence in shortwave ultraviolet light (λ = 245 nm) is an important diagnostic feature of natisite. The crystal structure of natisite was first solved for the natural sample (Mt. Rasvumchorr, Khibiny), R1 = 1.97%. The comparative study of natisite and other titanosilicates with tetragonal pyramids TiO5 showed the clear negative correlation between length of essentially covalent bond Ti–O in titanyl group and frequency of its stretching vibrations in the IR spectrum.

Keywords: natisite, sodium titanosilicate, crystal structure, IR spectrum, titanyl bond, hyperagpaitic pegmatites and hydrothermalites, Lovozero alkaline intrusion, Khibiny alkaline intrusion, Kola Peninsula