Abstract: Fluoborite extremely close to the fluorine endmember of the fluoborite Mg3[BO3]F3–hydroxylborite Mg3[BO3](ОН)3 series has been found in exhalations of the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. The proportion of the molecule Mg3[BO3](ОН)3 in it does not exceed 1 mol %. This borate is associated with halite, sylvite, langbeinite- and aphthitalite-group sulfates, anhydrite, krasheninnikovite, sanidine, fluorophlogopite, hematite, tenorite, chubarovite, alluaudite-group arsenates, lammerite, and urusovite. Fluoborite occurs as tiny acicular or hairlike 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 electron microprobe is, wt %: 38.71 Mg, 5.90 B, 25.52 O, 30.83 F, total 100.96; the empirical formula is Mg2.97B1.02O2.98F3.03. 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 occurring in the Arsenatnaya fumarole crystallized at a temperature no lower than 450°C. The fumarolic genetic type is novel for this borate.