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) ų. 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.