Extremely fluorine-rich fluoborite from fumarolic exhalations of the Tolbachik volcano (Kamchatka)

Bulakh M. O., Pekov I. V., Chukanov N. V., Yapaskurt V. O., Koshlyakova N. N., Britvin S. N., Sidorov E. G.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 2. P. 42-56

https://doi.org/10.31857/S0869605521020040

Full text is available on eLIBRARY.RU

Language: Russian

Abstract

Fluoborite extremely close to the fluorine end-member of the fluoborite Mg₃[BO₃]F₃–hydroxylborite Mg₃[BO₃](ОН)₃ series, was found in exhalations of the Arsenatnaya fumarole (Tolbachik volcano, Kamchatka, Russia). The amount of the Mg₃[BO₃](ОН)₃ component in it does not exceed 1 mol %. This borate is associated with halite, sylvite, langbeinite-group and aphthitalite-group sulfates, anhydrite, krasheninnikovite, sanidine, fluorophlogopite, hematite, tenorite, chubarovite, alluaudite-group arsenates, lammerite and urusovite. Fluoborite forms here fine acicular or hair-like crystals up to 5 mm long assembled in radial, near-parallel or chaotic open-work aggregates up to 10 cm across. Its chemical composition determined by the electron microprobe is: Mg 38.71, B 5.90, O 25.52, F 30.83, total 100.96 wt %; the empirical formula is: Mg_2.97B_1.02O_2.98F_3.03. The unit-cell parameters calculated from the powder X-ray diffraction pattern are: a = 8.808(4), c = 3.098(2) Å, V = = 208.1(3) Å³. The IR and Raman spectra of borates of the fluoborite–hydroxylborite series are given and discussed. Fluoborite in the Arsenatnaya fumarole crystallized at a temperature not lower than 450 °C. The fumarolic genetic type is novel for this borate.

Keywords: fluoborite, hydroxylborite, natural fluoroborate, IR spectrum, Raman spectrum, fumarole, Tolbachik volcano, Kamchatka